Introduction
Most gas species in the atmosphere are transformed by their reaction with the
hydroxyl radical (OH). These processes lead to the formation of
oxidised, secondary pollutants such as ozone and aerosol. Due to the large
number of organic OH reactants , several methods
have been developed in order to measure OH reactivity (the inverse
OH lifetime). OH reactivity (kOH) is the sum of
OH reactant concentrations ([X]) weighted by their reaction
rate coefficient with OH (kOH+X):
kOH=∑ikOH+XiXi.
Predicting trace gas loadings and lifetimes requires a comprehensive
understanding of the atmosphere's chemical cycling and oxidative capacity,
which is aided by the measurement of total OH reactivity. Measurements
can be compared to calculations from OH reactant concentrations in
order to quantify unexplained reactivity. In addition, the total loss rate of
OH can be calculated if OH concentrations are concurrently
measured in order to analyse the OH budget by comparing the total
OH loss rate with the sum of OH production rates.
The measurement of OH reactivity has been shown to be extremely useful
. Up to several tens per second unexplained reactivity was identified in biogenic-dominated environments such as in a
forest in Michigan , in the Amazonian rainforest and in a boreal forest in Finland
. The magnitude of missing reactivity appears to be
dependent on the biogenic source, time of the day and season
. However, the agreement between measured
and calculated reactivity is also a valuable result, because it indicates that
all trace gases that are relevant for the photochemistry were measured. This
was the case in environments that were influenced by anthropogenic OH
reactants as in New York and in the North China Plain
, in isoprene-dominated environments during daytime in a
Mediterranean forest and in a chamber study
. In addition, the gap between measured and calculated
OH reactivity could be closed in some field studies if oxygenated
VOCs (volatile organic compounds) derived from model calculation were additionally taken into account
e.g.. First attempts
were also made to measure OH reactivity fluxes .
The application of OH reactivity measurements for the analysis of the
OH budget also provided new results. A gap in the understanding of
OH recycling processes was found in a field study in Nashville in 1999
, in China in 2006 , in Borneo
in 2008 and in chamber experiments investigating the
oxidation of isoprene by OH . Because of the close
connection between oxidation of organic compounds by OH and ozone
production, OH reactivity can help to calculate local ozone production
rates .
Several methods to measure OH reactivity have been developed since the
first measurements were made by Penn State University (PSU)
. The different methods fall into two categories. One
method determines the OH reactivity directly from the time-dependent
decay of measured OH that is artificially produced. The other method
determines kOH indirectly from the concentration change of a
reference species, which competes with atmospheric reactants in their reaction
with artificially produced OH.
In the instrument developed by , the decay of OH is
measured in a flow tube through which ambient air is drawn by the direct
detection of OH using laser-induced fluorescence. OH is
continuously produced by water photolysis. The time-resolved OH decay
is measured by varying the reaction time using a movable injector to produce
OH. A compact aircraft instrument was later developed by PSU and
deployed for the first time in 2006 . Similar instruments were
built at Indiana University and at the University of Leeds
. The latter apparatus was recently replaced by an
instrument applying a pump-probe technique (see below).
In an alternative instrument a flow-tube set-up is combined with a chemical
ionisation mass spectrometer (CIMS) which detects sulfuric acid
(H2SO4) following the chemical conversion of OH to
H2SO4 . In this instrument
developed by the German Meteorological Service (DWD), OH is produced
by water photolysis in the flow tube. The reaction of OH with ambient
OH reactants is terminated by chemically removing OH by its
reaction with sulfur dioxide, which is injected at two positions within the
flow tube, giving one reaction time for the OH decay. The remaining
OH concentrations for the two injection positions are measured to
calculate the OH reactivity.
developed an instrument that uses a pump-probe
technique, called laser photolysis – laser-induced fluorescence (LP–LIF).
OH is produced by ozone photolysis using radiation of short laser
pulse at 266 nm at a low repetition rate of 1 to 2 Hz. The
OH decay is observed by laser-induced fluorescence with a high time
resolution. The pump-probe technique has the advantage that the flow
conditions do not need to be exactly known in order to determine a reaction
time. This technique is now used by several groups such as Tokyo Metropolitan
University , the University of Leeds ,
the University of Lille and Forschungszentrum Jülich (FZJ)
.
The indirect technique for the measurement of OH reactivity was
pioneered by . The comparative reactivity method (CRM) is
based on the detection of pyrrole that reacts with artificially produced
OH in clean or ambient air. The pyrrole competes with the ambient
OH reactants, so that the pyrrole concentration depends on the ambient
OH reactivity. In most CRM instruments, pyrrole is detected by
proton-transfer-reaction mass-spectrometry (PTR-MS) but can also be detected
by gas chromatography (GC) . The CRM method is more
commonly used than the direct OH measurement techniques because of the
commercial availability of PTR-MS instruments. It is applied by the
Max Planck Institute Mainz (MPI) , IMT Lille Douai, formally
called Mines Douai (MDOUAI) , Laboratoire des
Sciences du Climat et de l'Environnement (LSCE) , Indian
Institute of Science Education and Research Mohali , the Finnish
Meteorological Institute , Peking University
, the University of Leicester and University of California, Irvine .
Only two side-by-side comparisons have been performed between two CRM
instruments in a remote environment and between a CRM and
a pump-probe instrument in an urban environment . Both
comparisons show generally good agreement between measurements within 20 to 50 %.
In 2014 a workshop was held at the Max Planck Institute for Chemistry in Mainz in
order to assess the current status and future of OH reactivity
measurements . At the workshop, a comparison campaign was
suggested to investigate the performance of instruments under different
atmospheric chemical conditions. Large environmental chambers are ideal for
this purpose, as they ensure that all instruments sample air with the same
chemical composition. In addition, chemical conditions can be systematically
varied. This was demonstrated in several comparison exercises in the
atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich
e.g. as well as in the EUPHORE chamber
e.g.. Here, we report the results of two kOH
comparison campaigns that were conducted in the SAPHIR chamber. The two
comparisons were not blind: quick-look data were presented from some groups
during the campaigns. After first data submission without the knowledge of
the final results from other participants or OH reactant concentrations,
data were allowed to be revised. Only final data are presented in this paper,
but changes after the first data submission are described.
A large number of OH reactivity instruments applying different
techniques were successfully used in these campaigns (CRM instruments of
MPI, IMT Lille Douai and LSCE; a flow-tube LIF instrument from PSU; a CIMS
instrument from DWD and LP–LIF instruments from Lille, Leeds and FZJ,
Table ). The CRM instrument from the Finnish Meteorological
Institute was also used in the campaign, but measurements by this instrument
failed due to technical problems and no valid data could be acquired.
Experiments in the SAPHIR chamber
The SAPHIR chamber
The outdoor atmospheric simulation chamber SAPHIR is made of a double-wall
Teflon (FEP) film and has a cylindrical shape (5 m diameter,
18 m length). The Teflon chamber is mounted inside a steel frame that
is equipped with a shutter system that allows for experiments in the dark or
in the presence of sunlight. The space between the inner and outer Teflon
film is continuously purged with nitrogen (Linde, purity > 99.9999 %) to
prevent contamination from outside. In addition, the pressure inside the
chamber is 45 Pa higher than ambient pressure. Small leakages and air
sampling by instruments require the air to be replenished to maintain the pressure
difference. This leads to a small dilution of trace gases by 3 to 5 % per
hour. The dilution can be as high as 60 % per hour if the replenishment
flow needs to be high.
Specification of instrument parameters of OH reactivity instruments
in these campaigns.
Instrument
Technique
Time res./
1σ LODa/
1σ accur.a
Flow rate/
[OH]/
Inlet res.
Inlet line
Reference
s
s-1
(@ kOH)
L min-1
1010 cm-3
time/s
MDOUAIb
CRMc
600
1
18 %
0.37d
72e,f
5
1/4′′ PFA
LSCEb
CRMc
600
1
35 %
0.23d
78e,f
6
1/4′′ PFA
MPIb,g
CRMc
900
1.6b
37 %
0.38
75e,f
6
1/4′′ PFA (heated)b
1.3g
1/2′′ PFA (heated)g
PSUb
FT-LIFi
30
0.5j
23 % (< 2 s-1)
100
0.5e
1
1′′ PFA
8 % (10 to 100 s-1)
7 % (> 100 s-1)
Lilleb
LP–LIFh
30 to 60
0.4
8 %
9.5
0.6k
4
1/2′′ PFA
Leedsb
LP–LIFh
100
0.4 to 1.0
6 %
16
3k
2.5
1/2′′ PFA
FZJMb
LP–LIFh
40 to 160
0.2
10 %
15
0.7k
0.5
10 mm steel
(Silconert coating)
FZJSb,g
LP–LIFh
60
0.1
10 %
20
0.8k
0.5
10 mm steel
(Silconert coating)
DWDg
FT-CIMSl
60 to 300
0.5
1 s-1 (< 30 s-1)
2280m
0.01e
no additional inlet line
2 s-1 (30 to 40 s-1)
a Limit of detection/accuracy as stated by the
operator. b October 2015. c Comparative reactivity
measurement. d Faster flow of 1 L min-1 in inlet line.
e Produced continuously by water photolysis (185 radiation of a
Pen-Ray lamp). f Derived from the difference in the C1 and
C2 measurement. g April 2016. h Laser flash photolysis
and laser-induced fluorescence. i Flow-tube and laser-induced
fluorescence. j Limit of detection without the dilution, which
amplifies this number by a factor of 5. k Peak value produced by
flash ozone photolysis (266 nm of a quadrupled Nd:YAG laser).
l Flow-tube and chemical-ionisation mass-spectrometry.
m Sampling rate from the chamber.
Specification of instruments measuring OH reactant concentrations in
the two campaigns.
OH reactant
Measurement
1σ accuracy
1σ precision
Reaction rate constant
CO
Piccarro CRDS, GC (RGA)
15 ppbv, 8 %
5 ppbv
NO
chemiluminescence
5 %
4 pptv
NO2
chemiluminescence
5 %
4 pptv
O3
UV photometer
5 %
1 ppbv
CH4
Piccarro CRDS
1 ppbv
1 ppbv
n-pentane
GC-FID
13 %
20 pptv
1-pentene
GC-FID, PTR-TOF-MS
13, 4 %
20, 19 pptv
toluene
GC-FID, PTR-TOF-MS
13, 7 %
20, 7 pptv
o-xylene
GC-FID, PTR-TOF-MS
13, 2 %
10, 3 pptv
isoprene
GC-FID, PTR-TOF-MS
13, 6 %
20, 33 pptv
MVK
GC-FID
13 %
30 pptv
MACR
GC-FID
13 %
30 pptv
MVK + MACR
PTR-TOF-MS
6 %
22 pptv
α-pinene
GC-FID
13 %
10 pptv
limonene
GC-FID
13 %
10 pptv
myrcene
GC-FID
13 %
10 pptv
β-pinene
GC-FID
13 %
10 pptv
camphene
GC-FID
13 %
10 pptv
Δ3-carene
GC-FID
13 %
10 pptv
β-ocimene
GC-FID
13 %
10 pptv
β-phellandrene
GC-FID
13 %
10 pptv
sum monoterpenes
PTR-TOF-MS
4 %
5 pptv
β-caryophyllene
GC-FID, PTR-TOF-MS
13 6 %a
10, 15 pptv
HCHO
Hantzsch monitor
5 %
18 pptv
CH3CHO
GC-FID, PTR-TOF-MS
13, 6 %b
200, 40 pptv
a PTR-TOF-MS measurements 40 % lower than GC-FID
in 2016.
b PTR-TOF-MS measurements 50 % higher than GC-FID in 2016.
Ultra-pure air (Linde, purity > 99.9999 %) is used to purge the chamber
with a high flow (up to 250 m3 h-1) in order to clean the chamber. The
high flow rate is also required to humidify the chamber air with steam from
boiling water that is supplied by a Milli-Q water device. Ozone produced by a
silent discharge ozoniser can be added to the chamber air. Two fans ensure
that the air is well mixed, so that all instruments always sample the same
air mass e.g..
The use of high-purity air ensures that there are no measurable gaseous
OH reactants present in the chamber after the purging procedure. Small
amounts of mostly unidentified organic compounds and nitrogen oxide
compounds like HONO (< 100 pptv) can be observed in some
cases during the humidification. The total OH reactivity measured by
the OH reactivity instrument that is permanently installed at the
chamber shows that the reactivity is typically well below 1 s-1
after humidification. In fact, instruments measured on average no significant
OH reactivity in these campaigns in the clean chamber (see below).
If the chamber is exposed to sunlight, well-characterised sources for
HONO, formaldehyde and acetaldehyde lead to an increase in OH
reactivity (production rates are typically 200 pptv h-1). Photolysis of
HONO is also the dominant source of OH and
nitrogen oxides in the chamber. The source strength depends on the relative
humidity, temperature and radiation. The overall increase in OH
reactivity is of the order of 0.2 s-1 per hour, and is much
smaller than the reactivity from added OH reactants in these campaigns.
OH reactants were added either from gas mixtures via calibrated flow
controllers or as liquids that were injected into a heated inlet line with a
syringe. The vapours were transported by a flow of synthetic air into the
chamber. In addition, a recently built plant chamber allows for the
quantitative transfer of mixtures of biogenic organic compounds from up to
six trees into the SAPHIR chamber . Environmental
parameters in the plant chamber can be fully controlled.
Calculated OH reactivity
A number of instruments for the detection of OH reactants took
measurements
concurrently with the OH reactivity instruments
(Table ). Nitrogen oxides (NO and NO2) were
detected by a chemiluminescence instrument (Eco Physics TR 780). CO
was measured using a Piccarro cavity ring-down instrument (Picarro G2301) and
by gas chromatography (GC, Trace Analytical RGA 3). Both measurements
agreed within 5 %. Data from the cavity ring-down instrument were used here
for calculations of the OH reactivity due to its higher accuracy. This
instrument also measured methane and water vapour concentrations. Organic
compounds were detected by PTR-TOF-MS (proton-transfer-reaction
time-of-flight mass-spectrometry, Ionicon) and GC (Agilent 7890N).
Measurements agreed for those species that could be detected by both
instruments, such as isoprene better than 20 % with some larger
discrepancies for acetaldehyde and β-caryophyllene in the second set of
experiments in 2016 (Table ). Differences between
measurements need to be regarded as additional uncertainties in the calculation
of OH reactivity.
PTR-TOF-MS measures the sum of methyl vinyl ketone (MVK) and methacrolein (MACR)
and the sum of monoterpenes. In order to calculate OH
reactivity, PTR-TOF-MS measurements were used taking the relative
distribution of MVK and MACR and monoterpenes as measured by GC, because
PTR-TOF-MS has a high time resolution. Formaldehyde was additionally measured
by a Hantzsch monitor (Aero Laser AL 4001). All reaction rate constants used
for the calculation of OH reactivity are taken from IUPAC (International Union of Pure and Applied Chemistry)
recommendations if not stated
differently in Table . Temperature and pressure are
assumed to be the same in the instruments and the SAPHIR chamber. This
approach is applicable as indicated by temperature and pressure measurements
in the instruments. The overall 1σ uncertainty of the calculated
OH reactivity is around 20 % in most experiments but can be higher
(e.g. 40 % in case of the experiment with sesquiterpenes) depending on the
uncertainty in the OH reactant measurements, the agreement between
simultaneous measurements by different instruments and the uncertainty in
reaction rate constants.
Experiments performed in 2015
Two campaigns were conducted for this comparison. The first one took place in
October 2015. All instruments listed in Table were used
in this campaign with the exception of the CIMS instrument.
In the experiments, the chamber was flushed with high-purity air before each
experiment, until trace gas concentrations were below the limit of detection
of instruments (Table ). The chamber air was humidified to
approximately 75 % relative humidity (RH) at the beginning of each
experiment, except for the experiment on 6 October 2015, when the experiment
was started with 25 % RH and the humidity was increased to 90 % RH in
three steps. Relative humidity typically dropped to 40 to 50 % during the
experiment due to temperature changes and dilution. Ozone was also added at
the beginning of the experiments to allow OH production in the LP–LIF
instruments if ozone was not expected to affect the chemical composition of
the chamber air (e.g. by ozonolysis reaction or by the conversion of
NO to NO2). Initial ozone mixing ratios were typically between 50 and 80 ppbv.
OH reactivity was typically increased in several steps to maximum
values of approximately 50 s-1 at the end of the experiment
(maximum 150 s-1). The time between two injections of trace gases
was approximately 45 min. In addition, chemical conditions were
changed during the course of some experiments, such as opening or closing the
chamber roof or adding nitrogen oxides or water vapour. Chemical conditions in
the different experiments are summarised in Table .
Conditions during the experiments. Maximum concentrations during the
experiments are given. Maximum values for OH reactivity are approximate values
that do not refer to a specific instrument. Photolysis reactions were possible if the chamber roof was open (mostly only part of the experiment). A mixture of
aromatic compounds, alkenes and NOx species is summarised as “urban”
conditions.
Date
Added OH reactants
kOH/
CO/
∑VOC/
NO2/
NO/
O3/
jNO2/
s-1
ppbv
ppbv
ppbv
ppbv
ppbv
10-3 s-1
5 October 2015
CO
200
33500
< LOD
< 0.01
< 0.1
85
dark
6 October 2015
CO, CH4, NO2
25
3300
11 000
12
< 0.03
85
dark
7 October 2015
CO, CH4, NO
60
4100
13 500
< 1
120
< LOD
dark
9 October 2015
monoterpenesa
25
< LOD
11
< 0.01
< 0.05
295b
dark
11 October 2015
CO, isoprene
35
1650
9
0.06
0.7
50
3.0
MVKc, MACRc
2.3
3.2
12 October 2015
urband, NO2
40
< LOD
50
55
2.8
50
3.2
13 October 2015
CO, urband, NO2
35
900
48
12.5
1.2
50
1.6
14 October 2015
monoterpenese
15
< LOD
6
< 0.08
< 0.01
65b
dark
15 October 2015
CO, TMEf, NO
100
5400
40
15
30
45
dark
16 October 2015
β-caryophyllene,
30
< LOD
1.5
< 0.04
< 0.04
43b
dark
CH3CHO,
1.9
MVK, MACR
11
7 April 2016
CO
60
16 000
< LOD
< 0.05
< 0.01
80
dark
8 April 2016
n-pentane, NO
35
< LOD
60
< LOD
15
< LOD
dark
9 April 2016
isoprene, NO2
45
< LOD
15
8
0.5
115
4.5
MVK, MACR
7
11 April 2016
urband, NO2
50
< LOD
96
28
6
110
4.5
CH3CHO
45
12 April 2016
urband, NO
60
< LOD
94
< LOD
32
< LOD
dark
CH3CHO
67
13 April 2016
monoterpenesa
40
< LOD
19
< 0.05
< 0.02
185b
dark
14 April 2016
monoterpenesa, NO2
40
< LOD
16
29
2.8
70
4.3
15 April 2016
CO, β-caryophyllene
65
11 500
4
< 0.5
< 0.04
45b
dark
a α-pinene, limonene, myrcene (liquid volume ratio
0.68 : 0.11 : 0.21). b Added at later times for ozonolysis.
c Photochemically formed. d o-xylene, toluene, 1-pentene
(liquid volume ratio 1 : 1 : 1). e Identified compounds of plant
emissions: α-pinene, β-pinene, limonene, myrcene, camphene,
Δ3-carene, ocimene, β-phellandrene. f 2,3-dimethyl-2-butene.
Some experiments aimed to primarily test the instruments' performances:
linearity with CO (5 October 2015), the influence of humidity
(6 October 2015) and the presence of NO (7 October 2015).
The last test was repeated on 15 October. However, due to an operational
error, ozone was added at the beginning of the experiment, so that a mixture
of NO and NO2 was present. In order to reduce the ozone
concentration, 2,3-dimethyl-2-butene (TME), which reacts rapidly with ozone,
was injected twice.
The other experiments focused on the instruments' performances in the
presence of specific OH reactants and atmospheric mixtures of
reactants. In part of these experiments, OH reactants were also
oxidised by either OH or ozone. In five experiments, biogenic
reactants were present: isoprene (11 October 2015), isoprene oxidation
products MVK and MACR (16 October 2015), a mix of monoterpenes
(α-pinene, limonene, myrcene, 9 October 2015) and a sesquiterpene
(β-caryophyllene, 16 October 2015). In another experiment with biogenic
reactants, emissions from plants (3 pine and 3 spruce) were transferred into
the chamber at a flow rate of 11 m3 h-1. In two experiments, an urban
environment was simulated by a mixture of 1-pentene, o-xylene and toluene
together with NO2 (12 and 13 October 2015). On 16 October 2015,
acetaldehyde was injected.
Experiments performed in 2016
In the second campaign in 2016, only three instruments measured OH
reactivity: a CRM instrument (MPI), a LP–LIF instrument (FZJS) and the CIMS
instrument from DWD. The CRM and LP–LIF instruments were the same as in the
2015 campaign. The CIMS instrument sampled air with a high flow rate
(2280 L min-1), requiring the chamber to be operated with a high
replenishment flow. As a consequence, all trace gases were diluted at a high
rate of approximately 60 % per hour. Oxidation products could not
accumulate. Accordingly, the experimental procedure was different in these
experiments compared to those in 2015: humidification was done two to four
times over the course of an experiment in order to maintain a sufficiently
high water vapour concentration for the production of OH in the LP–LIF
and CIMS instruments (typical range of relative humidity between 25 and
80 %). If ozone was present in the experiment, ozone was also injected
several times. Initial ozone concentrations were around 100 ppbv and
dropped to 20 ppbv before re-injection. Similar chemical conditions
as in the first campaign were tested in order to achieve comparable results.
Tests were done with single, anthropogenic OH reactants (CO,
pentane) in the presence (8 April 2016) and in the absence (7 and 15 April 2016)
of NO, with biogenic reactants (isoprene, MVK, MACR on 9 April 2016,
a monoterpene mixture on 13 and 14 April 2016, β-caryophyllene on
15 April 2016) and with a mixture of anthropogenic reactants (11 and 12 April 2016).
The same monoterpene and urban reactant mixtures were used as in the
experiments in 2015 and in 2016.
Data coverage
The CRM instrument from the Finnish Meteorological Institute (FMI) was used in
the first campaign, but no valid measurements could be acquired due to
technical problems of this instrument. Data were submitted for all other
instruments for nearly all experiments and are included in the comparison. A
leak in the OH injection system of the MDOUAI CRM instrument was found
after the third experiments and this leak possibly led to systematic errors
in the measurements in this experiment. Data from the experiment on
7 October 2015 were therefore rejected for this instrument. The sampling system
of the CRM instrument from LSCE was changed on the second day of the campaign
(6 October 2015). Measurements from this experiment were rejected. On
12 October 2015, the flow-tube instrument from PSU did not take measurements, except for
the last 2 h due to technical problems. All other instruments
took measurements at all times during the campaign.
Procedure of data comparison
The measurement comparison was not strictly blind, but some rules were
applied to which all participants had agreed prior to the campaign. The
general outline of the campaign was as follows:
Before the official campaign started, a test experiment with CO was
performed in the SAPHIR chamber. During this experiment, the participants
were informed about the added CO concentrations in order to test the
functionality of their instruments (4 October 2015, not included in the
comparison, and 7 April 2016).
During the campaign, participants were informed about the
types of trace gases which were planned to be added to the chamber air before
the experiments. Concentrations of reactants, however, were not disclosed to
the participants.
During the campaign, participants had the opportunity to present quick-look
data of measured values at daily meetings, but data were not exchanged or distributed.
After the campaign, all participants independently submitted their evaluated
data to a neutral person at Forschungszentrum Jülich who was not involved
in reactivity measurements. Only after all data were received, the measured
trace gas concentrations and the kOH data on all participants
were made available.
After data disclosure, some participants applied corrections to their data
and submitted a revised data set together with an explanation for the correction.
The comparison in this paper is based on the final data versions.
Changes of data that were made as a result of the comparison are described in
the next section for each instrument.
Instruments for the detection of OH reactivity
Comparative reactivity method (CRM)
The comparative reactivity method (CRM) is an indirect method for the
measurement of OH reactivity developed by . The
measurement principle relies on the competition of the reaction of OH
with either a known pyrrole concentration or ambient OH reactants.
Pyrrole acts as a reference species that is typically not present in ambient
air. A small flow of humidified, ultra-pure nitrogen (flow rate approximately
240 cm3 min-1) passes over a Pen-Ray lamp, leading to formation of
OH by water photolysis at 185 nm with concentrations of
approximately 1 to 3 × 1012 cm-3
(Table ). Water photolysis, however, not only produces
OH but also HO2 radicals. The higher reactivity of OH
compared to HO2, also towards surfaces, may lead to HO2
concentrations exceeding the concentration of OH in the reactor.
Correction applied to the raw data. Some corrections are non-linear
and depend on several parameters (such as the pyrrole and OH concentrations).
Values are given for typical atmospheric conditions. The instrument zero for
the MDOUAI CRM was due to a contamination in the inlet system that was only
present in this campaign. Corrections due to deviations from a pseudo first-order
reaction assumption depend on the actual OH reactivity value and specific VOC
(see text for details). Typical numbers are given for 10 and 60 s-1.
Interferences are present from NO, NO2 and O3 in some instruments.
The correction often depends on the concentrations of the interfering species
in a non-linear way. Therefore, only typical values can be given here.
Instrument
Instrument
Humiditya
Deviation from
10 ppbv NOa/
10 ppbv NO2a/
Dilution
Other/
zeroa/s-1
pseudo first-ordera
s-1
s-1
factorb
s-1 (@ kOH)
MDOUAI
-15c
< 1 s-1
-0.5 (@ 10 s-1)
10
3
1.32
-3 (@ 60 s-1)
LSCE
0
< 1 s-1
-1.6 (@ 10 s-1)
17.5
1.9
1.45
-9.6 (@ 60 s-1)
MPI
0
< 1 s-1,d
-2 to -2.5 (@ 10 s-1)
20
1.6
1.3 to 1.9
O3 interference
-12 to -14 (@ 60 s-1)
6 to 8 s-1,a,e (@ 50 ppbv O3)
PSU
-3.4 ± 0.3
0
0
3f
0
5g
Lille
-4.1 ± 0.4
0
0
0h
0
0, < 1.7i
non-linearity
-30 to -35 %b,j
Leeds
-2.3 ± 0.4
0
0
0
0
1.07k
∼ -2.5 s-1,a,j
FZJM
-2.7 ± 0.2
0
0
0h
0
1.01k, < 3i
FZJS
-1.3 ± 0.2
0
0
0h
0
1.01k, < 7i
DWD
-8.3 ± 0.2
0
0
> 8
0
0
NO contaminationl
a Absolute change. b Relative
change.
c Determined from periods of experiments without reactants,
contamination in the inlet line only in this campaign. d Up to
4 s-1 during fast humidity changes in 2016. e Absolute change
due to recycling of OH by ozone. f Correction of the decay
. g Only applied in this campaign to reduce
sampling flow rate from the chamber. h Bi-exponential fit for
NO > 20 ppbv – only applied on 7 October (Lille, FZJS, FZJM) and
15 October 2015 (FZJS, FZJM). i Only applied for dilution for high
reactivity (Lille: > 150 s-1, FZJS/FZJM: variable dilution for
> 60 s-1) on 5 October 2015. j Deviations from single-exponential OH decay, likely due to misaligned photolysis beam in this
campaign.
k Correction for added flow with O3, when no O3 was present
in the experiment. l Presence of 0.14 ppbv NO contamination within
the instrument corrected for kOH > 2.5 s-1.
Ambient OH reactants and/or pyrrole react with OH in a reaction
volume (94 cm3) made of glass, with the inner surface covered by
Teflon. The instrument is alternately switched between two measurement modes:
the small flow of pyrrole (approx. 2 to 3 cm3 min-1) is mixed into a
flow of purified air (C2-mode) (approximately 300 cm3 min-1) or into a
flow of ambient air (air sampled from the chamber in these experiments)
(C3-mode). As OH exclusively reacts with pyrrole in the C2-mode,
maximum reduction of the pyrrole concentration is achieved, whereas the
pyrrole concentration is higher in the C3-mode, when ambient OH
reactants are also present. In order to calculate the OH reactivity, the
initial pyrrole concentration needs to be known (typically 1 to 2 × 1012 cm-3).
Because a small fraction of the radiation of the
Pen-Ray lamp enters the reaction volume, a small fraction of the pyrrole is
photolysed (typically less than 10 %). Therefore, the pyrrole concentration
is measured when zero air is sampled and when the light of the Pen-Ray lamp
is turned on (C1-mode). This is typically done once a day.
The design of the reaction volume is identical for all instruments, because
they were all manufactured by the Max Planck Institute for Chemistry in
Mainz. Three CRM instruments are included in this comparison, by MPI, IMT Lille Douai (MDOUAI) and LSCE. The instruments differ mainly in
the exact operational conditions such as flow rate, pyrrole and OH
concentration and the inlet lines (Table ). The
transformation of raw data into kOH values requires corrections
that have been characterised for each CRM instrument
(Table ). These corrections, described below, can
significantly differ between instruments due to the different operating conditions.
The pyrrole concentration is monitored by proton-transfer-reaction
mass-spectrometry (PTR-MS) in nearly all instruments but can also be
detected by GC . This is done for
the instrument from the Finnish Meteorological Institute.
A number of corrections need to be applied to the signals measured in the
different modes due to a variety of factors :
The OH production rate in the two measurement modes can be different if the water vapour concentration is not the same in both
modes.
OH can be significantly reformed by the reaction of HO2 that
is present at high concentrations in the reactor with ambient NO.
The reaction deviates from pseudo first-order conditions.
Ambient OH reactant concentrations are diluted due to the additional
nitrogen flow. The dilution factor is calculated from measured flow rates.
Corrections are usually determined from instrumental characterisation in the
laboratory and in the field, with the assumption that they are representative
of ambient air measurements. Typical values of corrections are listed in Table .
All groups operating a CRM used empirical functions to correct for deviations
from the pseudo first-order decay for the final data evaluation
. The exact value, however, depends on the chemical
composition of OH reactants (see below). Different representative
mixtures are taken to characterise of this correction for the various
CRM instruments and operating conditions are optimised to reduce the
correction dependence on the chemical composition. The error associated to
this correction can then be factored into the measurement uncertainty .
Additional instrument-specific corrections are described in the following section.
MPI CRM instrument
The correction of measurements taken with the CRM by MPI for deviations from a
pseudo first reaction uses results from numerical
simulations. However, this can only be applied if the relative importance of
the most abundant reactive compounds in the sampled air is known
. The model correction was not applicable in this
comparison,
because no data on the concentration ratios of the main OH reactants
were known in contrast to typical situations in field campaigns. In this
campaign, the empirical correction procedure was also chosen as an
alternative correction procedure that was shown to be advantageous by
. Tests with isoprene, methanol, ethane, propane, propene
and toluene were done to determine the correction factor.
In addition to the corrections applied by all groups operating a CRM
instrument, measurements by the MPI CRM were corrected for the presence of
ambient ozone. The necessity of this correction was recognised after the
first comparison of results from the 2015 campaign. This correction was not
applied in the first version of submitted data. The procedure to correct data
was then determined in laboratory characterisation experiments. The
correction was applied to data from the 2016 campaign, from the beginning.
OH is reformed in the reaction of HO2 with O3 in the
reaction volume of the CRM, where O3 is present in sampled ambient
air but is also produced in the photolysis of oxygen by the 185 nm
radiation of the Pen-Ray lamp. The assumption is that the effect of OH
reformation on the measurement is typically insensitive to the exact
concentration of ambient ozone, which is not present in all modes of the
measurement cycle. If this assumption is not true, the OH reactivity
is underestimated depending on the ambient ozone concentration. This was
observed for the MPI CRM instrument in this campaign. Therefore, measurements
were corrected by an empirical function derived from laboratory measurements
after the first data submission. Although the ozone concentration in the
reaction volume was similar to the concentration in the other two CRM
instruments, no ozone dependence was seen for the MDOUAI and LSCE
instruments. The exact reason is not clear but might be related to different
HO2 concentrations in the instruments. The insensitivity of other CRM
instruments to the ozone interference indicates that operating conditions
exist for which the interference is negligible. Further investigations should
be performed to characterise these conditions.
In addition to the ozone correction, errors in the calculation of the
dilution factor and the calibration of the pyrrole sensitivity were noticed
for the MPI CRM instrument after the first data submission in 2015. Although
corrections were made after knowledge of OH reactant concentrations
and measurements of other instruments, unreasonable data already suggested
the need for these corrections before. Also, the correction for the presence
of NO2 was again characterised for conditions when also O3
was present and slightly changed in the final data. Furthermore, the
correction due to the deviations from pseudo first-order decay were changed
in the final data because a reanalysis of the concentration of test
compounds used for the characterisation revealed higher impurities than
certified by the manufacturer.
The total increase in OH reactivity measurements between the first and
final submission was typical within the range of a factor of 1.5 to 3 but
was a factor of 4 to 5 for low OH reactivities around 10 s-1 in the
presence of ozone mixing ratios of 40 ppbv.
MDOUAI CRM instrument
The performance of the MDOUAI CRM instrument was worse in this campaign than
previously observed due to additional sources of noise from the PTR-MS
instrument and the inlet system. It was recognised that the pump (Teflon
surfaces) in the sampling line upstream of the CRM instrument, which is
necessary to avoid a pressure drop between ambient pressure and the CRM
reactor , released contaminants which caused an additional
OH reactivity of 15 s-1 on average during measurements.
This instrument zero was subtracted from all measurements. The value was
determined daily in each experiment between the humidification of chamber air and
the injection of OH reactants. Deviations from pseudo first-order behaviour
of the kinetics were characterised by tests with isoprene, propene, ethene,
ethane and propane. Data were not revised after the first data submission.
LSCE CRM instrument
At the beginning of the campaign, a pressure change was observed for the two
measurement modes of the CRM instrument at the exit of the reactor that could
affect the measurements. The total flow rate in the sampling line was
increased and only a small part was sampled into the reaction volume in order
to avoid a change in pressure. Therefore, the sampling flow was not directly
injected into the CRM reactor, but it was first pulled through a pump with
Teflon surfaces. The flow was restricted by a valve (Teflon surfaces) before
the air entered the reactor. This sampling flow system was used for the first
time during this campaign and may have reduced the performance of the instrument.
Corrections were applied to the raw data as described by
to obtain the OH reactivity values. Specifically, the correction for
the deviation from the pseudo first-order conditions was determined from
laboratory and field tests using certified concentration of gas standards
containing propane and isoprene. The same procedure was applied in a previous
field campaign in an isoprene-dominated environment .
Previous field deployments of the same instrument were conducted in
environments with low NOx concentrations, for which a correction for
OH recycling by NO was not needed. For this reason, a
correction for high NOx concentrations was determined in laboratory
tests after the campaign in SAPHIR and data from the experiments from LSCE were
revised after the first submission for experiments, when NOx was injected.
Instrument operators decided to use the part of the experiments before
OH reactants were added to subtract a background signal, when positive,
non-zero values were on average measured (5, 7, 9 and 16 October 2015) in
their first data evaluation. However, this correction was not applied in the
final data set, because it was agreed not to use knowledge of the chemical
conditions for the data evaluation if it is not required.
Changes in the revised OH reactivity measurements were smaller than
±20 % except for measurements at high NO mixing ratios on 7 October 2015,
when changes were up to 80 s-1 as no correction for the NO interference
was applied in the first submitted data.
Direct OH loss rate measurement by laser-photolysis – laser-induced fluorescence (LP–LIF)
All other instruments measured the decay of OH in the presence of ambient OH
reactants in a flow tube. In most of the instruments, OH radicals are detected
by LIF .
All methods measuring the OH decay have a higher time resolution compared to
the CRM instruments (Table ), because no time is used up
when switching between ambient and purified air. In general, fewer corrections are
required to derive the OH reactivity from the measured OH decay.
Four LP–LIF instruments were used in the campaigns: instruments from
University of Lille and University of Leeds and two instruments from FZJ, one
of which is permanently installed at the SAPHIR chamber (FZJS) and the other
of which is used for mobile field deployment (FZJM).
In the laser-photolysis instruments, ambient air passes (flow rate 10 to
20 L min-1) through a flow tube. Part or all of the air is drawn into
an OH fluorescence detection cell. The exact position and design of
the flow tube and the fluorescence cell differ among the instruments.
OH is produced by flash photolysis of ozone with subsequent reaction
of O(1D) with water vapour. Radiation is provided by a quadrupled
Nd:YAG laser pulse at 266 nm, which is operated at a low repetition
rate of 1 to 2 Hz. The OH concentration is measured with a
high frequency of 3 to 8.5 kHz, so that the decay of OH can be
observed with a high time resolution between two photolysis shots. Tens of
consecutive decays are summed up to increase the signal to noise ratio.
The OH radicals decay in a pseudo first-order reaction with ambient OH reactants,
so that the time-resolved OH signal can be fitted to a single-exponential function
that directly gives the OH reactivity. Differences between the fitting procedures
of the instruments are described in the Supplement. The accuracy
of the time basis of the OH decay is only determined by the accuracy of the
photon-counting electronics.
Measurements need to be corrected for an instrument zero that is subtracted
from all measurements. This zero loss rate in the flow tube is due to the
wall loss reactions and likely limited by the diffusion of OH. Values
are typically of the order of a few s-1 (Table ) and
are regularly determined by sampling high-purity zero air.
Conversion of HO2 to OH in the presence of ambient NO
can influence the measurements. As there is no concurrent production of
HO2 in the ozone photolysis, LP–LIF instruments are less affected by
this recycling process compared to instruments that use water photolysis for
OH production. It is expected that this recycling process only becomes
relevant for NO mixing ratios higher than 20 ppbv for typical
atmospheric chemical compositions of ambient air . In this
case, the single-exponential decay of OH turns into a bi-exponential
decay that can clearly be identified in the summed decays. If a
bi-exponential decay is observed, the faster decay time can be attributed to
the OH reactivity. The underlying assumption is that the timescale of
OH formation is slow compared to the OH loss. This is
reasonable for typical atmospheric conditions but may not be applicable in
all cases, specifically in artificial air mixtures. In field experiments,
bi-exponential behaviour in the OH decay due to OH recycling at
high NO concentrations in ambient air measurements has been observed
by the FZJS and Lille instruments. A bi-exponential function was applied to
measurements in a campaign in China for the FZJS instrument .
Measurements of the Lille instrument that showed bi-exponential behaviour
during a campaign on the campus of the University of Lille were evaluated by
applying a single-exponential function. Measurements were evaluated by only
using the first part of the decay curve that contained information on the
faster decay . No significant difference between this
procedure and the results from a bi-exponential fit was found. The fitting of the
data using a single- or bi-exponentially decay function is discussed later
in the paper, as differences were observed in the returned value of the
OH reactivity in this campaign at high NOx (> 20 ppbv) depending on
the type of fit used.
In the normal operational procedure, no dilution or only a small amount needs to
be taken into account for most of the instruments. If there is insufficient
ambient ozone to generate a measurable OH signal, then an addition of
O3-containing flow needs to be added to the flow tube and a small
dilution correction needs to be made. This was required in experiments
without the presence of ozone in the chamber air.
The number of data points on the decay curve that are above the noise level
decreases with increasing OH reactivity, so that the accuracy and
precision of the measurements start to decrease for exceptionally high OH
reactivity values (for example higher than 60 s-1 for the FZJ
LP–LIF instrument). In addition, initial inhomogeneities in the OH
distribution in the flow tube due to inhomogeneities of the laser photolysis
beam can impact the shape of the observed OH decay curve for these
high reactivity values. For this reason, an additional dilution flow can be
applied in order to reduce the OH reactant concentrations and improve
the data quality. This was done in some instruments (FZJ, Lille) in this
campaign, when the measured reactivity exceeded a threshold
(e.g. > 150 s-1 for the Lille instrument) but is not required as
indicated by measurements by the Leeds LP–LIF instrument.
Imperfect alignment of the photolysis laser can enhance the inhomogeneities
in the initial OH distribution, so that deviations from a single-exponential OH decay can also occur at low reactivity values. This was
observed in this campaign in the Lille and Leeds LP–LIF instruments but
recognised only at the end of (Lille) or after (Leeds) the campaign. As a
consequence, the evaluation procedures were changed for measurements in this
campaign in order to account for this effect.
Data from FZJS and FZJM instruments were not revised after the first data
submission and no instrument-specific description is required here. The Lille
and Leeds instruments required a campaign-specific data evaluation that was
applied before (Lille) or after (Leeds) the first data submission.
Lille LP–LIF instrument
Quick-look data presented from the LP–LIF instrument from Lille systematically
deviated from measurements of the other instruments. The overestimation of
approximately 30 % was confirmed by determining the reaction rate constant
of the reaction between CO and OH in test experiments, in which
a mixture of CO in synthetic air was sampled. This overestimation was
due to two reasons: (1) misalignment of the photolysis laser leading to
deviations from single-exponential behaviour of the OH decay curve,
likely due to an inhomogeneous initial OH concentration (see above);
(2) the procedure of analysing the decay by adapting the length of the decay
curve used for the fit. The length is limited to 15 times the first estimate
of the inverse reactivity in order to avoid noise from the
background signal over longer periods of time. As a consequence, the fitted zero decay
time appeared to change if the length of the curve used for the fit was
shortened for zero-air measurements like done for the high reactivity values.
This was then used to account for the deviations in the reactivity
measurements by determining an artificial zero decay time as a function of
the fit length. In the final data, this zero decay time, which depends on the
fit length and therefore reactivity value, was subtracted from the
measurements (Fig. S1 in the Supplement). With this method,
correct reactivity values could be calculated for the laboratory test
experiments with CO. The drawback is that the accuracy is lower for
high reactivity values due to the decreasing number of points used for the fit.
This correction would not be needed for a good alignment of the photolysis
laser. It is therefore only needed for the data evaluation of this campaign but could be used to deal with similar alignment problems in the future.
Leeds LP–LIF instrument
Similar behaviour of the decay curves to that observed for the Lille LP–LIF
instrument was recognised for the Leeds LP–LIF instrument after the campaign.
In the decay, a fast component was followed by a slower component rather than
single-exponential behaviour. This behaviour was also apparent during the zero
decay measurements conducted with zero air for this campaign. As a
consequence, the fit of the single-exponential function was started after the
fast section of the decay curve for the data evaluation (fit range between
150 and 400 ms) for low reactivity values
(kOH < 10 s-1). An accurate determination of the
OH reactivity was more difficult for high reactivity values
(kOH > 10 s-1), when values became similar to the fast
component of the decay. A single-exponential function between 100 and
200 ms was fitted to the measured decay curve in this case.
Similar to the procedure that was applied to the data from the Lille LP–LIF
instrument, zero-air measurements were evaluated using the same fit ranges as
for evaluating low and high reactivity values. A decay time of
(2.3 ± 0.4) s-1 was obtained for the low reactivity case. This
is close to the real loss rate of OH in the instrument without
OH reactants (instrument zero). A higher value of
(4.8 ± 0.6) s-1 was determined if the fit range was shifted to
an earlier start as it is for evaluating decays for high reactivity values. These
two values were subtracted as instrument zeros when either one of the fit
ranges were used. The higher value acts as a correction for the overlap of
the faster instrumental component and the OH decay due to chemical
reactions. For decays taken on the 7 and 15 October 2015 when NO was
present, a fit range between 105 and 150 ms was chosen, giving an
instrument zero of (5.1 ± 1.1) s-1.
The difference between the revised data, when this evaluation scheme was
applied, and the initially submitted data is mainly due to the higher
instrument zero that was subtracted for kOH > 10 s-1,
so that these values are 2.5 s-1 lower than before. Deviations
of the OH decay from single-exponential behaviour for conditions
without OH recycling in the instrument were not observed in other
field campaigns in the past. This correction is specific for the data from this campaign.
Direct OH loss rate measurement by flow-tube technique with laser-induced fluorescence (PSU instrument)
The flow-tube LIF instrument from PSU also measures the decay of OH
radicals. In contrast to LP–LIF instruments, OH is continuously
produced by water photolysis at 185 nm in this instrument using a
Pen-Ray lamp with concurrent HO2 production as in the CRM instruments.
In the PSU instrument, the reaction time is varied by a movable injector,
which is used to change the distance between OH injection and the
point of OH detection . The reaction time
is calculated from the velocity measured with a hot-wire anemometer and the
known distance travelled for each position of the injector. Within each scan,
more than 100 data points were used to calculate the decay. Finally, during
normal operation in the field, the PSU instruments sample ambient air with a
high flow rate (> 100 L min-1). This exceeds the flow rate which
can be consumed during operation of the SAPHIR chamber; therefore the PSU
instrument had to apply a high dilution flow in this campaign. Only
20 L min-1 were sampled from the chamber, to which
80 L min-1 of high-purity synthetic air provided by the SAPHIR air
supply system was added. The dilution factor was determined from monitored
flow rates and was verified in several tests during the campaign, in which
the ratio of flows was varied. Using a dilution flow has two drawbacks.
Firstly, the calculated OH reactivity is very sensitive to the exact
ambient and dilution flows. Secondly, any error in the instrument zero decay
due to wall loss or trace impurities in the dilution air is amplified by the
ratio of the dilution flow to the ambient flow, in this case a factor of 5.
Thus the typical limit of detection of 0.5 s-1 becomes 2.5 s-1.
As for the CRM instruments, measurements by the PSU instrument can be
affected by OH recycling from the reaction of ambient NO with
HO2, which is concurrently produced with OH by water photolysis.
The correction of OH recycling in the PSU instrument is based on
correcting each point in the decays for the recycling calculated from
measured NO and HO2 before applying the fit to determine the
OH reactivity .
Changes made after the first data submission in the data by the PSU
instrument were mostly smaller than ±10 %. These small changes were due
to improvements in the data evaluation algorithms that were made between the
first and final submissions. These included improvements in the procedure,
how data on measurements from instruments that were used for the corrections were
synchronised to the OH reactivity measurements and refinement of
instrument parameters such as air velocity and location of the injector.
In addition, the change in the correction procedure for OH
regeneration due to the reaction of HO2 with NO led to the
final data being 2.5 times higher than the first data submission at the
highest NO mixing ratios on 7 October 2015. Initially a new
optimisation fitting procedure was developed and used for the first data
submission, but laboratory and modelling studies showed that the method in
was superior and less uncertain. Thus, the method in
was used for the revised final data submission.
These changes were specific for this campaign because the instrument was not
exactly the same instrument as used in previous and future campaigns. It was
assembled from parts of the original PSU instrument and parts (mainly the
laser system for the OH detection) provided by the Max Planck Institute for
Chemistry in Mainz and the University of California, Berkeley.
Direct OH loss rate measurement by flow-tube technique with chemical ionisation mass spectrometry (DWD instrument)
The measurement scheme of the CIMS instrument by DWD is similar to that of
the flow-tube LIF instrument by PSU. However, only one reaction time is
currently realised to measure the OH decay .
Excess OH (108 cm-3) is produced by water
photolysis in front of the flow tube with concurrent production of
HO2. The reduction of its concentration by reacting with ambient
OH reactants is measured at two set time periods. This is achieved by
terminating this reaction by chemical conversion of OH after a
specific reaction time. For this purpose, a high concentration of sulfur
dioxide is added at two injection points, so that OH is converted to
sulfuric acid. After the OH titration, a high concentration of propane
is injected to scavenge any OH present. The injection of sulfur
dioxide is alternately switched between these two points. The reaction time
is determined by adding known amounts of OH reactivity (e.g. propane)
in front of the flow tube. OH wall losses from the flow tube are
quantified by using humidified synthetic air. If the OH lifetime in
the instrument is of the order of the travel time between the two injection
points, no reasonable measurement is possible. In the current set-up, an
upper limit of OH reactivity values of 40 s-1 is achieved.
Additionally, measurements by the CIMS instrument can also be affected by
OH recycling from the reaction of ambient NO with HO2
that is concurrently produced with OH by water photolysis. Corrections
for OH recycling in the CIMS are based on laboratory characterisation
at the Hohenpeissenberg station (ambient pressure ∼ 900 hPa). An
empirical function corrects for the systematic underestimation seen in CIMS
OH reactivity measurements, which is dependent on both the magnitude
of OH reactivity and the levels of NO present. The function has
been derived for propane, isoprene and ethene for NO concentrations up
to 15 ppbv . Under the assumption that any complex
mixture in the SAPHIR chamber behaves like the three OH reactant
mixture above, the NO correction was applied to the SAPHIR campaign
data set for kOH larger than 2.5 s-1. The fit
function optimised for OH reactivity up to 40 s-1 and
NO ranging from 0 to 15 ppbv leads to a systematic
overestimation of OH reactivities below 2.5 s-1 (Fig. S3), not representing laboratory observations.
Therefore no correction is applied to kOH < 2.5 s-1.
The OH recycling efficiency is partly dependent
on the reaction time between the two injection zones. As the NO
correction was determined at the laboratory at Hohenpeissenberg Observatory
at a pressure of 900 hPa, an uncertainty of +10 % exists for its
application at the SAPHIR chamber, as a result of lower flow rates
(i.e. longer reaction time in CIMS) at 1000 hPa.
In addition to ambient NO, the CIMS measurements were influenced by an
NO impurity in the SO2 cylinder, leading to the presence of
0.14 ppbv NO in the CIMS flow tube at all times in this
campaign. The presence of the NO impurity became evident from the
inspection of the CO experiments (7 and 15 April 2016) where a
systematic, repeatable underestimation in OH reactivity was found for
reactivities above 20 s-1. Therefore, an NO correction
function was applied to the whole data set, also for experiments without
NO in the chamber, e.g. in experiments with monoterpenes (13 April 2016) and
sesquiterpenes (15 April 2016).
The DWD CIMS instrument is a relatively new instrument that had only been
used in a remote environment at the monitoring station at Hohenpeißenberg.
Therefore, the correction procedure had been developed for chemical
conditions experienced in this campaign and were further refined after the
first data submission. They would also be required if the instrument
took measurements in similar environments.
The wall loss of OH in the instrument and the time in which the air
travelled between the two titration points were initially determined from zero-air phases of the experiments on each day. In order to provide data which are
fully independent from the experiments, measurements were revised after data
from the other instruments were known. The parameters were determined by an
external flow tube with propane and synthetic air concentrations only once
before the start of the campaign. This resulted not only in a constant change
in the data due to the change in the zero decay time (wall loss) but also a
scaling of data due to the change in the calculated reaction time (Table S1 in
the Supplement). Final data are on average 10 % lower than initially submitted.
Results and discussion
A summary of OH reactivity measurements of all instruments together
with calculated OH reactivity is shown in Figs.
to and the results are discussed in detail in the following
subsections. For comparing data, the calculated reactivity is taken as the
reference value if no oxidation products were formed during the experiment.
In all other cases, one of the LP–LIF instruments (FZJS) is taken as
reference. This instrument was chosen because its measurements have a high
precision and time resolution. Regression lines were determined using the
fitexy procedure by . This method takes into account the
measurement errors of both instruments and is symmetric, i.e. the fitted
parameters are independent of whichever of the two instruments is assigned to be
the dependent or independent variable.
Measured (dots) and calculated (coloured areas) OH reactivity during
experiments in the SAPHIR chamber 2015. Error bars are omitted for the
clarity of the plot but are within the range of the scatter of data.
Vertical dashed lines with labels give points in time of the injection of
trace gases. Vertical dashed blue lines indicate addition of water,
horizontal blue lines the presence of O3 and horizontal dashed yellow
lines illumination of the chamber by sunlight.
Same as Fig. . For the experiment on
16 October 2015 the subsequent addition of various VOC reactants is shown, so that
differences among measurements that only occur for a specific VOC reactant
impact the visual agreement at later times, when other VOC reactants were
injected.
Measured (dots) and calculated (coloured areas) OH reactivity during
experiments in the SAPHIR chamber 2016. Vertical dashed lines with labels
give points in time of the injection of trace gases. Vertical blue lines
indicate addition of water, horizontal blue lines the presence of O3 and
horizontal yellow lines illumination of the chamber by
sunlight.
Measured (dots) and calculated (coloured areas) OH reactivity during
experiments in the SAPHIR chamber 2016. Vertical dashed lines with labels
give points in time of the injection of trace gases. Vertical blue lines
indicate addition of water, horizontal blue lines the presence of O3 and
horizontal yellow lines illumination of the chamber by
sunlight.
OH reactivity measurements with zero air
Ultra-pure air was present in the chamber at the beginning of each
experiment. As discussed above, it can be assumed that there was no OH
reactivity present in this case. For normal operation of the LIF instruments,
ozone and water vapour need to be present. A small contamination from
OH reactants could appear during the humidification process of the
chamber air. Measurements from previous experiments in the chamber indicate
that OH reactivity introduced together with water is most often below
the limit of detection of the reactivity instrument (approximately
0.2 s-1, e.g. ) but always less than
1 s-1. This is likely due to either contaminants in the water or
contaminants coming off the Teflon film of the chamber with increasing
humidity. Therefore, these periods are ideal for testing the instrument zeros and
the precision of the measurements.
If an instrument zero needed to be taken into account, it was independently
determined from the zero-air phase of the experiments for all instruments
except for the MDOUAI CRM instrument. The instrument zeros were typically
measured on a daily basis. No systematic change in the value was observed
over the course of the campaign for these instruments. The instrument zero of
the Leeds LP–LIF instrument was determined only once at the end of the
campaign and the zero of the CIMS instrument once at the beginning of the
campaign. The same air supply as for the chamber was used in these
experiments, except for the CIMS, for which bottled air was used (Linde,
purity 99.999 %). The derived values were used to correct all data. No
instrument zero is expected for the CRM instruments (except for the
contamination in the MDOUAI instrument), because only differences between
measurement modes are used to calculate the OH reactivity.
Figure shows the histogram of measurements during all zero-air
parts in the two campaigns. A Gaussian fit function is fitted to the
distribution in order to determine a potential bias in measurements and to
estimate the precision of the measurements (Table ). Overall, the
distributions of zero measurements give a Gaussian shape. If all data are put
together, none of the instruments exhibit a significant bias. Some exceptions
are observed for specific experiments for some instruments. This result also
demonstrates that no significant OH reactivity was present during
these parts of the experiments.
Partly due to the small number of data points, the distribution is noisier
for the measurements by the CRM instruments compared to the distribution for the
LP–LIF instruments. No bias of the MDOUAI instrument can be determined
because of the use of zero-air phases of experiments to determine an
instrument zero (see above). The bias in the other two CRM instruments varies
between experiments (Fig. ): the day-to-day variability is
between ±3 and ±5 s-1 with maximum values of ±10 s-1.
A small bias is also observed in a few experiments for measurements of the
Leeds LP–LIF instrument (smaller maximum value around 2 s-1). A
small positive bias of approximately 2 s-1 is also seen in the PSU
measurements after 13 October 2015 for unknown reasons. However, this change
is likely affected by the amplification of zero variability and errors due to
the dilution procedure that was used.
Distribution (bars) of OH reactivity measurements before
OH reactants were injected into the chamber. MDOUAI, LSCE and MPI
measurements are binned to 1 s-1 intervals, other measurements to
0.1 s-1 intervals. Solid lines shows the fit to a Gaussian
distribution.
The width of the distribution can only be regarded as an upper limit for the
precision because of the deviation of zero measurements from a Gaussian
distribution for some instruments (Table ). Alternatively, the
width of the distribution was calculated for a distribution of data after
subtraction of the bias observed for each instrument for each individual
experiment. The width of this distribution gives a more realistic estimate of
the precision of the measurements (Fig. S5). The
widths of the corrected distributions for the CRM instruments give a
precision of approximately 2 s-1 at a time resolution of 10
(LSCE-CRM, MDOUAI-CRM ) or 15 min (MPI-CRM), slightly higher than the
stated limits of detection of 1 to 1.5 s-1
(Table ). The widths of the distributions give a
precision between 0.1 and 0.3 s-1 for LP–LIF instruments at time
resolutions between 30 and 160 s for the different instruments, and a
precision of 0.4 s-1 for the CIMS (60 to 300 s time
resolution) in agreement with their stated limits of detection
(Table ). The PSU flow-tube instrument gives a
precision of 0.9 s-1. The precision in
Table of 0.5 s-1 is stated for normal
operation of the instrument without the dilution and becomes
2.5 s-1 when corrected for the dilution amplification.
Fit results of the distribution of the measurements to a Gaussian function
when no OH reactants were present in the chamber. The distribution is either
calculated by taking all data as they are measured or by forcing the mean values
on each day to zero for an individual instrument. For the MDOUAI instrument, no
independent instrument zero was determined.
Instrument
Data
Width
Width/
Bias/
points
(daily
s-1
s-1
mean
zero)/
s-1
MDOUAI
46
2.3
N/A*
N/A*
LSCE
39
1.8
3.2
-0.4
MPI
105
2.1
4.6
1.8
PSU
798
0.9
1.1
1.1
Lille
758
0.2
0.5
0.5
Leeds
322
0.3
0.9
0.8
FZJM
585
0.1
0.2
0.2
FZJS
1450
0.2
0.3
0.0
DWD
230
0.3
0.4
-0.5
* Not applicable because measurements during zero-air phases
were used as instrument zero.
OH reactivity measurements in the presence of CO and CH4
During several experiments, only CO and CH4 were present in
the chamber for the entire experiment or part of the experiment. These
experiments were performed in the dark, so that there was no photochemistry. The
linearity of instruments and behaviour for a chemically simple system can be
investigated from these experiments.
As can be seen in Figs. and ,
measurements of all instruments followed the expected changes (expressed
by kcalc) due to the additions of CO and CH4. In
the tested range of up to 150 s-1, the agreement is mostly very
good. Measurements by the DWD instrument show a clear upper limit of
measurable reactivity of 40 s-1 as expected from the measurement
principle (see above). Some instruments exhibited large transient deviations
from the expected values (e.g. MPI on 6 October 2015 between 10:00 and
11:00 UTC) but otherwise agree well during the CO and CH4 experiments.
Results of the correlation analysis (linear correlation coefficient R2
and slope and intercept of a weighted linear fit) for different subsets of the
data. Errors of fit results (not shown here) are not significant within two
digits of the fit parameters. 〈|Δk|/kfit〉 gives
the mean value of the relative difference between measurements and the
regression line.
Data subset
Instrument
No. of data
R2
Slope
Intercept/
〈|Δk|/kfit〉
points
s-1
CO, CH4a
MDOUAI
122
0.79
1.05
0.8
0.33
k(OH) < 60 s-1,b
LSCE
112
0.81
1.31
-0.1
0.54
MPI
178
0.91
1.05
0.3
0.32
PSU
1151
0.97
1.03
1.1
0.19c
Lille
1506
0.97
1.04
1.3
0.10
Leeds
829
0.98
1.06
1.3
0.11
FZJM
795
0.98
1.12
0.2
0.10
FZJS
2255
0.99
1.01
0.6
0.08
DWD
295
0.99
0.87
-0.3
0.05
Urban mixd,
MDOUAI
145
0.85
0.99
0.8
0.17
isoprenee,f
LSCE
155
0.86
0.77
-0.1
0.17
MPI
228
0.80
1.06
-1.1
0.17
PSU
863
0.94
1.01
1.0
0.11c
Lille
1355
0.97
0.94
0.4
0.08
Leeds
677
0.98
1.07
0.1
0.09
FZJM
993
0.98
0.94
0.1
0.06
FZJS
reference
DWD
719
0.91
0.73
-0.3
0.16
Monoterpenes
MDOUAI
69
0.48
0.56
2.4
0.45
no oxidationf,g
LSCE
69
0.68
0.82
-0.3
0.37
MPI
73
0.72
0.70
1.5
0.34
PSU
502
0.93
1.20
0.2
0.18c
Lille
639
0.98
1.08
0.4
0.09
Leeds
318
0.96
1.02
-0.1
0.16
FZJM
585
0.99
0.96
0.2
0.07
FZJS
reference
DWD
142
0.98
1.01
-0.6
0.09
a 5, 6, 7, 11, 13 and 15 October 2015. 7 and 15 April 2016;
b reference: calculated reactivity. c scatter amplified
by the dilution factor of 5 in this campaign.
d 12–13 October 2015.
11 April 2016. e 11 October 2015. 9 April 2016; f reference:
measurements by FZJS. g 9, 14 October 2015. 13–14 April 2016.
Correlation between measured and calculated OH reactivity for
experiments in the SAPHIR chamber if only CO, CH4, O3 and water
vapour were present. Only the subset of the entire data set for values below
60 s-1 is shown. Red lines give the results of a linear regression analysis
(Table ). The grey area indicates the mean relative difference
between measurements and the regression line. Measurements of the MDOUAI
instrument during the first three experiments (5–7 October 2015) have a
higher uncertainty due to technical problems.
Although linearity appears not to be a problem for all instruments, even for
exceptionally high reactivity values (Fig. S6 and Table S2), the discussion of the results focuses on OH
reactivity values below 60 s-1, which are more relevant for
atmospheric measurements. Figure shows the correlation of
measured and calculated OH reactivities and Table gives
the result of a regression analysis for all periods, when only CO
and/or CH4 were present in the dark chamber. High linear correlation
coefficients (R2 > 0.8) are calculated for all instruments. A linear
regression analysis gives slopes between 0.98 and 1.17 for most instruments.
Errors of the fitted slopes were always smaller than 0.01, because the
precision of data are higher than the scatter of data around the regression
line. These results demonstrate the ability of instruments to measure the
correct reactivity values. Only the regression analysis for one CRM
instrument (LSCE) gives a higher slope of 1.31, mostly due to measurements
during the first experiment, whereas better agreement (Table )
is achieved in other experiments. The larger deviation for this instrument is
likely due to the correction for deviations from pseudo first-order
behaviour (Table ). This was determined from
characterisation measurements with a mixture of isoprene and propane, which
might not represent chemical conditions well with only CO.
Although the slopes of the regression lines indicate on average a good
agreement of the measurements for these chemical conditions, the scatter in the
correlation plots (Fig. ) is considerably different for the
instruments. The time series in Figs. and
show that the scatter in the correlation plot is caused
by statistical noise, and for some instruments by irregular systematic
deviations pointing to instrumental instabilities. The mean of the relative
absolute difference between measured and calculated OH reactivity is
32 to 48 % for CRM instruments, 19 % for the PSU LIF instrument and
between 8 and 11 % for LP–LIF and the CIMS instruments. If the PSU
instrument was operated similarly to how it was in the field without the large dilution flow,
measurements would have scattered significantly less (at least a factor of 5).
Thus, for the instruments as configured for this comparison study, the
PSU LIF and LP–LIF instruments appear to have the highest measurement precision.
OH reactivity measurements in the presence of isoprene, MVK, MACR and OH reactants in urban environments
In a second set of experiments, chemical conditions included volatile organic
compounds, NO2 and CO (Table ). The most
abundant biogenic species, isoprene and OH reactants were tested, which are
representative of alkenes and aromatic compounds found in urban
environments (1-pentene, o-xylene, toluene). Oxygenated VOCs from
isoprene oxidation (MVK and MACR) and acetaldehyde were present in separate
experiments in 2015 (Fig. ). In 2016, these species were
present in experiments together with isoprene and the urban OH reactant mixture.
Similar results are obtained for isoprene and urban OH reactants.
Because these experiments partly included oxidation products that were not
measured by instrumentation at the chamber, measurements by the LP–LIF FZJS
instrument are taken as the reference value. Measurements of this instrument
differ less than 10 % from calculations using measured OH reactant
concentrations. This difference is smaller than the 1σ accuracy of the
calculation, so that results would not significantly differ if calculated
OH reactivity was used.
For most instruments (except for LSCE CRM and DWD CIMS instruments), the
agreement between measurements found for these chemical conditions is about
as good as for the experiments with only CO and CH4
(Fig. ). High linear correlation coefficients between
measured and calculated reactivity values are obtained (R2 > 0.80) and
slopes of the regression lines are 0.94 and 1.07, showing good absolute
agreement (Table ).
Correlation between measured OH reactivities and measurements by the
FZJS instrument for experiments in the SAPHIR chamber when either isoprene
(11 October 2015), isoprene, MVK and MACR (9 April 2016) or a mixture of
propene, xylene and toluene (12–13 October 2015, 11 April 2016) were present
in the chamber together with O3 and water vapour. Lines give results of a
linear regression analysis of the combined data set (Table ).
The grey area indicates the mean relative difference between measurements and
the regression line.
Change in reactivity measurements when acetaldehyde and a mixture
of MVK and MACR were added to the experiment on 15 October 2015. The box and
whisker plot shows the statistics (median and 10, 25, 75, 90 percentiles) of
20 min time intervals 5 min before and after the injections of OH
reactants. Dashed red lines give values calculated from the measured change
in OH reactant concentrations.
The performance of the LSCE instrument is better than in the experiments with
only CO and CH4, but measurements are lower than the reference in
this case, whereas measurements are higher in the CO and CH4 case.
As discussed above, the correction for deviation from pseudo first-order
kinetics (which is based on a characterisation with propane and isoprene
standards) might better represent chemical conditions during the experiment
with alkenes, aromatics and isoprene compared to the CO and CH4
case. In general, this issue can cause a variability in the agreement between
measured and calculated reactivity in this campaign. This indicates that a
more intensive characterisation of this correction is required for the
specific chemical conditions, specifically if individual OH reactants are studied.
Measurements by the DWD CIMS instrument give larger deviations from
calculated reactivity in these experiments compared to results found in the
CO and CH4 case. The experiments on 9 and 11 April 2016 started
with high reactivities of about 40 s-1, but only 60 % is
measured by the DWD CIMS instrument (Fig. ). The agreement
improves when kOH decreases. For values below
10 s-1, the measurements agreed well with calculated reactivities.
The exact behaviour of the relationship between measured and calculated
reactivity changed for periods of the experiments with different chemical
conditions. In addition, an increase in the measured reactivity with
increasing water vapour concentration after starting humidification of the
chamber air is observed in these experiments. This is less obvious in other
experiments (see below).
Part of this large discrepancy could be the result of an instrumental
instability, which was seen as an intermittent increase in noise in the CIMS
reactant ion counts (NO3-) from 9 April 2016 onwards and coincided
with the periods deviating from the FZJS instrument observations (see
Supplement). This could be relevant because the reactant ion
count is used to normalise the HSO4- counts, thus obtaining the
equivalent OH concentration. At high OH reactivities, when
OH signals are smaller, higher noise in the (comparatively) large
reactant ion concentrations could thus affect the resultant kOH estimation.
The exact reason why there was an increase in noise in reactant
ion concentrations remains unclear. Additionally, these two experiments have
in common the illumination of the chamber by sunlight and presence of
NO2 in the second part of the experiments. Interestingly, measured
and calculated reactivity agree better in these parts of the experiment
compared to the first parts. However, there is no obvious reason why these
conditions would impact the measurements of this instrument. Chemistry
occurring in the inlet system may impact the OH concentration for the
more complex chemical composition of air. In the presence of NO (see
below), any unaccounted OH recycling would affect the accuracy of
the measurements.
In the 2016 experiments, the relationship between measured and calculated
reactivities does not change when oxygenated VOCs (MVK, MACR and acetaldehyde)
were present compared to the part of the experiments when only the parent
VOC was present. This can be seen in the time series of experiments on 9 and
11 April 2016 (Fig. ). In the 2015 campaign, the impact of
the presence of these compounds on the instruments was tested in a separate
experiment (16 October 2015, Fig. ). Because the compounds
were consecutively injected, only the observed change in the measured
OH reactivity for each instrument is calculated for the analysis
(median of 20 min of measurements before and after the injection).
This value can be compared to the expected change in the reactivity that is
calculated from measured reactant concentrations (Fig. ).
Measurements of LP–LIF instruments are not affected by these species and
agree with the change in calculated reactivity. Also, the flow-tube LIF
instrument by PSU and the CRM by MPI give similar values within 10 to 20 %.
The change in reactivity measured by the LSCE instrument agrees in the case
of MVK and MACR but is less in the case of acetaldehyde. Changes measured by
the MDOUAI CRM instrument are up to a factor of 3 lower than observed by
the other instruments. Losses on surfaces in the inlet system may explain
part or all of the discrepancy. Both instruments used an additional pump with
Teflon surfaces in their inlet system (see also the discussion for
monoterpenes/sesquiterpenes).
The largest differences are seen in the presence of acetaldehyde for the
MDOUAI CRM instrument. In addition, measurements by the LSCE and MDOUAI
instruments are more variable compared to those by the other instruments as
indicated by the large difference between 25 and 75 percentile values in this
case. The presence of oxygenated VOCs may cause additional complications in
the reaction system in the CRM that impacts the OH concentration. The
oxidation of aldehyde species by OH proceeds by H-atom
abstraction from the aldehydic group, leading to the formation of acyl peroxy
radicals, RC(O)O2. For instance, the oxidation of acetaldehyde will
lead to the formation of the acetyl peroxy radical, CH3C(O)O2, with
a yield of approximately 95 % . The
reaction of acyl peroxy radicals with HO2 is known to efficiently
recycle OH in the atmosphere. For the acetyl peroxy radical,
and recently reported an OH yield of 0.5.
Also, one reaction pathway in the reaction of MVK with OH forms
an acyl peroxy radical that leads to OH reformation in the reaction
with HO2 with a high yield of 0.64 . These
recycling mechanisms can act as a secondary source of OH in CRM
instruments, which in turn can mask a fraction of the OH reactivity
from aldehyde species for these instruments, leading to a negative bias.
Results of model calculations and laboratory investigations (Fig. S4)
performed for the MDOUAI instrument confirm that
the OH reactivity of acetaldehyde is underestimated by this
instrument, which is consistent with observations during the 16 October 2015
experiment (Fig. ), when acetaldehyde was first introduced
in SAPHIR. However, Fig. shows that the two other CRM
instruments (LSCE and MPI) are less (or not) impacted by OH recycling
from CH3C(O)O2 + HO2. These different behaviours are not well
understood and need more investigation.
It is noteworthy that concentrations of acetaldehyde and other aldehydes in
the atmosphere are typically smaller than in the experiment in this campaign but can constitute a significant fraction (10 to 20 %) of the total reactivity
e.g.. The maximum error that is caused by the
underestimation of the total reactivity measurement by the CRM instrument
would be less than 13 % if the results of the experiment of this campaign are
extrapolated to atmospheric conditions.
OH reactivity measurements in the presence of monoterpenes and sesquiterpenes
The third type of chemical condition tested in the campaigns was the
presence of terpenes. This was done either by injecting a mixture of
monoterpenes, a sesquiterpene or by flushing real plant emissions into the
SAPHIR chamber. These experiments also included ozonolysis reactions of
terpenes. Maximum reactivities (25 s-1) were lower than in other
experiments in 2015. Oxidation products of the ozonolysis reactions were not
measured, so that it is expected that calculated reactivities are
underestimating the real reactivity. Therefore, one of the instruments
(LP–LIF FZJS) is taken as reference for the comparison of the measurements.
As seen in the correlation plots (Fig. ) and the results of
the regression analysis for data without the presence of ozone
(Table ), differences between measurements of the LP–LIF
instruments and the other instruments are largest in these experiments
compared to the other experiments.
High linear correlation coefficients are obtained (R2 > 0.96) and slopes of
the regression lines between 0.96 and 1.08 are calculated for the LP–LIF
instruments. No systematic change in the relationship of measurements is
observed whether ozone and hence ozonolysis products are present or not.
Similarly, measurements between LP–LIF instruments agree in the presence of
the sesquiterpene (with and without the presence of ozone and ozonolysis
reaction products). Because this experiment started with the addition of
other OH reactants (Fig. ), only the measured
difference is compared that due to the injection of β-caryophyllene that is
observed by each instrument (Fig. ), as done for
the oxygenated VOCs (see above).
Correlation between measured OH reactivities and measurements by the
FZJS instrument. This subset of data includes experiments with injections of
a mix of monoterpenes (α-pinene), limonene, and myrcene
(9 October 2015 and 13–14 April 2016) or with the transfer of real plant
emissions, which consists predominantly of monoterpenes (14 October 2015).
Times when O3 was present are indicated by bluish colours. Lines give
results of a linear regression analysis of a subset of data when no ozone was
present (Table ). The grey area indicates the mean relative
difference between measurements and the regression line.
Change in reactivity measurements when β-caryophyllene was
first added three times and ozone in the last step of the experiment
on 16 October 2015. The box and whisker plot shows the statistics (median
and 10, 25, 75, 90 percentiles) of 20 min time intervals 5 min before and
after each addition. Dashed red lines give values calculated from the
measured change in OH reactant concentrations.
Measurements of the flow-tube LIF instrument (PSU) varied more with respect
to the reference measurements in the presence of monoterpenes and
sesquiterpenes compared to the other chemical conditions. The level of
agreement varies among the three experiments (9, 14 and 16 October 2015): when
the monoterpene mixture was injected, measurements by the PSU instrument are
10 to 15 % lower than measurements by the FZJS instrument, but they are
20 % higher when plant emissions are transferred into the chamber. During
the continuous transfer small inhomogeneities cannot be fully excluded, but
the discrepancies between measurements also remain after the injection and
oxidation part of the experiment. The relationship does not depend on the
presence of ozone in these two experiments. In the third experiment, changes
in the OH reactivity measured by the PSU instrument due to the
increase in sesquiterpene concentration (and ozonolysis products) are up to
40 % smaller than the changes observed by the LP–LIF instruments
(Fig. ). The higher and lower values observed in these
experiments may not be related to the chemical conditions but instead to the
instrument problems. This is indicated by higher values of the PSU instrument
compared to measurements by the other instruments in nearly all experiments
after 13 October, independent of the chemical conditions
(Figs. and ). Difficulties in maintaining
consistent operation of the laser and the electronics driving the movable
OH source could have led to much of this variability. As a result,
this comparison exercise probably does not represent the capability of the
PSU instrument to measure OH reactivity in forest environments.
Measurements by the FZJ LP–LIF and DWD CIMS instruments in the experiments with
terpenes (Figs. and ) agree well during
the first experiment with monoterpenes (13 April 2016,
Fig. ) and only a small underestimation is seen during the
second experiment (14 April 2016). Though no obvious explanation can be
provided as to why the CIMS underestimated OH reactivity compared to
LP–LIF up to 12 s-1 on 14 April 2016, the CIMS instrument
performance might have been influenced by unidentified internal chemical
reactions. This also corresponds to observations in the presence of isoprene,
MVK, MACR or a mixture of urban OH reactants (see above).
Because NO was present as an impurity in the CIMS sulfur dioxide
titration gas mixture (see above and Table ), a
NO correction function was also applied in experiments with
monoterpenes (13 April 2016) and sesquiterpenes (15 April 2016). This could
explain some of the smaller, but systematic differences compared to LP–LIF
measurements .
Similarly to the CIMS instrument, the agreement between CRM and LP–LIF
instruments is worse in the presence of monoterpenes and the sesquiterpene
compared to other experiments. Lower linear correlation coefficients (R2)
between 0.48 and 0.72 and a higher scatter of data with relative mean
absolute residuum values between 0.34 and 0.45 (numbers only for periods
without ozone) are observed. The agreement is even worse during the
ozonolysis parts of the experiments, when CRM instruments measure values that
are up to five times smaller than measurements of the LP–LIF instruments.
Similar results are seen in the experiment with the sesquiterpene
(Fig. ). In all these cases, the level of agreement varies
among the CRM instruments and the specific experiment, but the measurements tend
to be significantly smaller than those of the other instruments.
Correlation between OH reactivity measurements by the MPI CRM and
DWD CIMS instruments and measurements by the FZJS instrument for the part of
the experiment on 15 April 2016 when β-caryophyllene was present in the
chamber. Times when ozone was also present are coloured differently.
Relative difference between measured and calculated OH reactivity
depending on the NO mixing ratio in experiments on 7, 12, 13,
15 October 2015. Boxes give median and 25 and 75 percentiles, whiskers give
10 and 90 percentiles. Because the MDOUAI CRM instrument did not measure on
7 October 2015, when NO concentrations were systematically changed, limited
data are available for this instrument. For the Leeds LP–LIF instrument the
OH decays were fitted by a single-exponential decay for all NO, whereas a
bi-exponential decay was used for Lille, FZJM and FZJS for NO > 20 ppbv.
The residence times in the sampling lines of the CRM instruments were
generally longer (5 to 6 s) compared to the sampling lines of the
other instruments (0.5 to 4 s) and the volume to surface ratio was
lower, because CRM instruments used 1/4′′ OD PFA tubing in 2015. In addition,
two CRM instruments (MDOUAI and LSCE) used a sampling pump with Teflon
surfaces to introduce the sample into the CRM reactor. Oxygenated and low-volatility (monoterpene and sesquiterpene) species may adsorb on these
surfaces and the pump may have therefore played a role in the underestimation
seen for these instruments. One instrument (MPI-CRM) used a heated inlet
line. Other instruments used up to 1′′ OD PFA or Silconert-coated stainless
steel tubing (Table ). Results (Figs.
and ) show that MPI measurements are partly significantly higher
than those of the LSCE and MDOUAI instruments, suggesting that the
underestimation of the LSCE and MDOUAI CRM instruments could be partly due to
a loss of OH reactants in the sampling system (unheated inlet
line+pump). However, an impact of the monoterpene or sesquiterpene chemistry
on the CRM measurements cannot be ruled out.
In the experiments with terpenes in 2016 (Figs.
and ), a better agreement between measurements by the MPI CRM
and FZJS LP–LIF is found, specifically for the experiment with
β-caryophyllene, compared to the experiments in 2015. The
reason for this improvement is not clear but could be related to the larger
diameter inlet tube used in 2016 compared to 2015
(Table ), supporting the potential influence of losses in
the inlet system for these compounds.
OH reactivity measurements in the presence of NO
The presence of NO can affect measurements of the OH reactivity in all
instruments due to the recycling of OH by the reaction of HO2 with NO that
is contained in ambient air (see above). These effects are amplified if OH is
produced by water photolysis, because HO2 is concurrently formed with OH.
In 2015, the NO concentration was increased stepwise (up to 120 ppbv) in the
presence of CO on 7 and 15 October 2015 (Fig. ). NO was also
present in the two experiments with urban OH reactants (12 and 13 October 2015).
Figure shows the dependence of the relative difference between
measured and calculated reactivity on the NO mixing ratio in these experiments.
In 2016, the presence of NO was tested in two experiments in combination with
the presence of pentane and in the urban OH reactant mixture (8 and 12 April 2016,
Fig. ). Due to the lack of OH reactant concentration measurements
on 8 April 2016, measurements performed by the FZJS instrument are taken as the
reference with which to analyse the impact of NO on the performance of the other two instruments
(Fig. ).
Discrepancies between calculated and measured reactivity are mostly within
the range of differences observed in other experiments for LP–LIF instruments.
For NO mixing ratios higher than 20 ppbv, median values deviate up to 20 %
from calculated reactivities. The OH production rate from recycling reactions
is within the range of the OH destruction rate in the case of CO at highest NO
mixing ratio in the experiment on 7 October 2015. Nearly all the LP–LIF instruments
applied a bi-exponential fit function in this case except the Leeds LP–LIF
instrument. Only the FZJM/FZJS instruments applied a bi-exponential fit to
measurements on the second experiment with high NO (15 October 2015). However,
separating OH reactivity from OH recycling by applying a bi-exponential fit
function might still lead to some systematic errors for the experiment with
high NO in the presence of only CO because of the OH recycling rate that
was higher than for typical atmospheric conditions. Therefore, the faster decay
rate could deviate from the OH reactivity. The scatter in the measurements
increases with increasing NO mixing ratio as expected from the lower precision
of measurements for high reactivity values.
As with all other LP–LIF instruments, OH reactivity from the Leeds LP–LIF
agrees well with calculated OH reactivities below 20 ppbv NO. However, values are
increasingly lower for higher NO mixing ratios. The lower values of OH reactivity
for NO higher than 20 ppbv are caused by the application of a single-exponential fit to the OH decay data rather than a
bi-exponential fit as used by other LP–LIF groups. Similar behaviour is
achieved if a mono-exponential fit is applied to measurements by the Lille
and FZJS/FZJM LP–LIF instruments during the experiment with CO and
NO (7 October 2015). A bi-exponential fit, although it gives an
OH reactivity closer to the calculated value for this particular
experiment, is not necessarily the correct function to apply to fit
atmospheric data, and so was not used by Leeds (even though a bi-exponential
fit returns a larger value of OH reactivity). Model simulations under
relevant conditions indicate that a bi-exponential fit can return an
OH reactivity that is greater than the true value. Fitting the data
more rigorously requires a modelling approach, similar to that applied when OH
recycling was observed in a laboratory kinetics study .
Hence, although application of a bi-exponential fit improves the agreement
with the calculated value for this experiment, caution is needed when applying
it to atmospheric data at high NO where conditions could be different.
Relative difference between measured OH reactivity and measurements
by the FZJS instrument depending on the NO mixing ratio for the experiment on
8 and 12 April 2016. Boxes give median and 25 and 75 percentiles, whiskers give
10 and 90 percentiles.
Measurements by the PSU LIF instrument, which also uses water photolysis as
an OH source, show a tendency to underestimate reactivity values with
increasing NO mixing ratios. The maximum median of the relative
difference is 20 %. This difference was significantly reduced from 70 to
20 % in the final data compared to the data submitted before reactivity
measurements from all groups and OH reactant concentrations were made
available. The correction procedure for the presence of NO was changed
later from a new procedure to the one described in (see above).
Measurements by the MDOUAI and LSCE CRM instruments do not exhibit a clear
trend in the relative difference between measured and calculated reactivity
with NO. In contrast, measurements by the MPI CRM instrument give
lower reactivity values compared to calculated reactivities with increasing
NO mixing ratios in both campaigns in 2015 and 2016 (Figs.
and ). Measurements by all CRM instruments were corrected by
applying an empirical correction function. The magnitude of the correction is
of the order of the OH reactivity values (Table ),
making results very sensitive to any systematic error in the correction
procedure. The differences in the corrections needed for each CRM instrument
emphasise the necessity for a careful characterisation of the instrument.
In the campaign in 2016, the relative difference between measured reactivity
by the DWD CIMS instrument and the reference (FZJS LP–LIF instrument) is
small for NO mixing ratios lower than 5 ppbv but increases
with increasing NO mixing ratio (Fig. ) to up to a
factor of 1.3 (median value) for 10 to 20 ppbv NO.
This difference demonstrates that the correction applied to the CIMS
measurements leads to systematic errors for NO mixing rations larger than
5 ppbv, in particular for the urban mixture (12 April 2016). The
chemical composition does seem to play a role (Table ):
the correction in the pentane experiment fits well
in contrast to the urban mix experiment, in which it partly
produces inaccurate results. The strength of
OH recycling by the reaction of HO2 with NO is also
dependent on the CIMS internal abundance of HO2 which was not
measured during the SAPHIR campaign. Also, the correction term can become
rather large for high OH reactivity and large NO concentration
(up to 30 s-1), which illustrates the limit of the instrument in
its current configuration (Fig. ).
Influence of humidity on OH reactivity measurements
In experiments in 2015, humidity was similar in most experiments and
only systematically varied in one experiment on 6 October 2015
(Fig. ). In contrast, water vapour concentrations were
highly variable in experiments in 2016 because of the high dilution flow that
was required. Figure shows the dependence of the relative
difference of measurements by the instruments (taking measurements by the
FZJS instrument as reference) for all experiments, in which an overall good
agreement is observed (CO, pentane). A clear trend towards overpredicting
OH reactivity with increasing water vapour can be seen for measurements
by the MPI CRM. This trend is consistent with lower measurements at the
lowest water mixing ratios observed in the experiment on 6 October 2015
(Figs. and S7). In contrast, the results from 6 October 2015
do not indicate that the other CRM instruments are affected in the same way by
water vapour. No clear trend with water vapour is observed for the LSCE and MDOUAI instruments.
Some changes in the relationship between the CIMS instrument and the LP–LIF
instrument are seen after water vapour additions in some experiments in 2016
(for example 9 and 11 April 2016, Fig. ). On 9 and
11 April 2016, the CIMS instrument showed an instrumental instability before the
water addition, which could explain some changes in the relationship (Fig. S2).
No systematic trend in the entire data set is
observed (Fig. ). Also, on 15 April 2016 CIMS measurements
deviate from observations of the LP–LIF instrument, when the humidity was
increased after the addition of sesquiterpenes, but deviations could be due
to errors in either one of the instruments. The LP–LIF instrument observed an
increase in OH reactivity, whereas the decreasing trend of the CIMS
measurements does not change. The increase observed by the FZJS LP–LIF
instrument could be due to desorption of sesquiterpenes inside the
instrument but could also be due to desorption from the chamber wall
increasing the reactivity in the chamber. However, the decrease observed by
the CIMS instrument would be consistent with the dilution of trace gases in
the chamber. Both instruments agree better again after the injection of
ozone, when sesquiterpenes have become small.
Relative difference between measured OH reactivity and measurements
by the FZJS instrument depending on water vapour mixing ratio during
experiments in April 2016 when only CO, pentane, or a mixture of propene,
xylene and toluene were injected (part of the experiments on 7, 8, 11 and
15 April 2016).
In the CRM and CIMS instruments, the concentrations of OH and
HO2 depend on the water vapour concentration as they are produced
together by water vapour photolysis. In CRM instruments, corrections are
applied, when the water vapour concentration changes between the different
measurement modes which are required to calculate the OH reactivity
(Table ). Also, a humidity dependence in the detection
sensitivity of pyrrole is taken into account. Fast changes in the water vapour
concentrations, for example during the humidification procedure of the
chamber air, can therefore cause systematic errors. However, systematic
differences are observed on a longer timescale than the duration of the
humidification (< 30 min) in these experiments. Humidity-dependent
memory effects in the inlet system could be the reason for this behaviour.
Another possibility could be that observations are related to changes in the
OH and HO2 concentrations that depend on the water vapour
concentration. OH recycling processes and correction factors depend on
the radical concentrations and chemical conditions. For experiments in 2015,
the dependence of the correction factor for the deviation from pseudo first-order kinetics was not well characterised for the low OH
concentrations at low water vapour concentrations, so that the deviation from
calculated OH reactivity might be due to a systematic error in this
correction for these conditions.
Further investigations will be necessary to understand the exact influence of
the water vapour concentration on the OH reactivity measurements by the
MPI CRM instrument and the reason for the instrumental instability of the
CIMS instrument. It cannot be fully excluded that the observed effects are
related to the experimental procedure of humidifying the chamber air that
leads to a relatively fast change in the water vapour concentration.
Summary and conclusions
Measurements of OH reactivity were compared in experiments performed
in the atmosphere simulation chamber SAPHIR in two campaigns in 2015 and 2016.
All instrument types presently used for atmospheric measurements were used in one or both of the campaigns. A few additional instruments exist
worldwide e.g., but they are similar to the instruments in these campaigns.
Summary of findings
Not only were many measurements successfully performed in these campaigns but
also a number of findings already led to an improvement in the data quality
during the process from measurements to the final data:
an ozone-dependent background signal was found for measurements of the MPI CRM;
application of the correction of measurements is recommended due to the deviation from
pseudo first-order conditions in CRM instruments by empirical correction
factors ;
misalignment of the photolysis laser beam in the LP–LIF instruments can lead
to a complication in the data evaluation procedure.
These results will also improve the precision and accuracy of measurements in the future.
The findings of the comparison of the final data set are as follows:
Measurement techniques are capable of measuring OH reactivity for a
range of chemical conditions that are relevant for ambient air measurements but with different levels of precision and accuracy. Losses of OH
reactants in inlet lines could be of importance.
Measurements by LIF and CIMS instruments have a higher precision than CRM
instruments leading to a limit of detection better than 1 s-1 at a
time resolution of a few minutes. For chemically complex conditions, the
scatter of the data is within the range of 10 % for LP–LIF instruments and
10 to 20 % for the CIMS and the flow-tube LIF instruments. The precision of
data from the flow-tube LIF instrument was reduced in this campaign compared
to typical operation in the field due to the application of a high dilution
flow. Measurements by CRM instruments exhibit a higher limit of detection of
approximately 2 s-1 at a time resolution of 10 to 15 min.
The scatter of the CRM data for chemically complex conditions ranges from
17 to 45 % (mean relative difference between measurements and linear
regression with reference values). Additional work is needed on the CRM
technique to improve the measurement precision at a level closer to that
observed for other instruments.
Biases in the measurements by the LP–LIF instruments are lower than their
limit of detection with a few exceptions in some experiments for the Lille
and Leeds instruments. The instrument zero in the PSU instrument varied by
1.3 s-1, but this value is amplified by the dilution factor of 5
that is not normally used in field measurements. The smaller number of data
points for the CRM instruments makes conclusions about a day-to-day
variability of a potential bias less accurate. However, the distribution of
measurements during zero-air measurements becomes more compact for the LSCE
and MPI CRM instruments if an offset is subtracted for each individual
experiment (Table ).
Maximum absolute deviations of LP–LIF measurements from calculated
reactivities or measured reactivities from the instrument taken as the
reference (FZJS) are 12 % (mostly less than 5 %). Deviations are smaller
than the accuracy of the calculation from measured OH reactant
concentrations (Table ). Results from this campaign
demonstrate a high accuracy of LP–LIF instruments.
The accuracy of CRM instruments varies with chemical conditions. Whereas
measurements agree on average with calculated reactivities within 5 %
(higher deviations for LSCE CRM 31 %) if only CO, CH4 or
pentane are present, deviation from measurements with the FZJS LP–LIF
instrument are up to a factor of 2 for mixtures containing terpenic
compounds. Also, the scatter of data is larger in these cases. While the
impact of OH recycling in the terpenic chemistry cannot be ruled out,
losses of these compounds in inlet systems can explain the observed
discrepancies. The transmission of low-volatility compounds such as terpenes
and their oxidation products needs to be improved for CRM instruments. Inlet
systems used in this campaign partly differed from deployments in previous
campaigns (for example the use of the additional pump in the inlet of the
LSCE instrument), so that losses could have been different in campaigns in the past.
Even in the presence of up to 120 ppbv NO, agreement with
calculated reactivity within the accuracy of measurements and calculations is
achieved for the MDOUAI and LSCE CRMs, whereas deviations of up to 50 % for
the MPI CRM instrument and a factor of 1.8 for the CIMS are observed. All
these instruments applied large corrections to account for OH
recycling from the reaction of HO2 with NO. The variability in
the accuracy of the correction emphasises the need for a careful
characterisation of the instrument-specific operational conditions.
Measurements by LP–LIF instruments are not affected as much as the other
instruments by OH recycling reactions even for NO mixing ratios
higher than 20 ppbv. In this case, a bi-exponential fit function to
the OH decay curve rather than a single-exponential fit improves the
agreement with the calculated value of the OH reactivity. A
bi-exponential function was not applied to OH decays measured by the
Leeds LP–LIF, so larger deviations were observed for NO mixing ratios
higher than 20 ppbv. Although a bi-exponential fit to the data
generated a closer agreement with the calculated value of the OH
reactivity for the conditions of this particular experiment, it should be
noted that it may not represent the best function to fit to atmospheric data
at high NO (where the composition is different to this experiment),
and careful thought needs to be given as to the optimum function to fit to the data.
Measurements by the flow-tube LIF instrument give larger deviations
(±20 %) in the chemically more complex experiments compared to conditions
with single, anthropogenic reactants (deviations ±3 %), although the
flow-tube LIF measurements are likely affected by instrument issues related
only to the instrument that was assembled for this comparison.
Experiments in 2016 reveal a so far unrecognised effect of the water vapour
concentration on measurements by the MPI CRM instrument (factor of 2
difference at 1 % water vapour mixing ratio), although changes of the
humidity and therefore radical concentrations are taken into account in the
evaluation. The water vapour correction procedure might have not been
applicable here, because humidity changes were faster than typical in the
atmosphere. Water vapour was changed in only one experiment in 2015. Results
do not indicate that the other CRM instruments are affected in the same way
by water vapour.
The accuracy of measurements by the CIMS instrument varied between
experiments. Compared to the calculated OH reactivity, an agreement
is observed within the accuracy of measurements and calculations for the
experiments with CO and pentane (deviation of the regression slope
from 1 : 1 line of 13 %). For the isoprene and urban reactant mixture cases,
lower accuracy is observed with a deviation of the regression slope from 1 : 1
line of 27 %. In contrast to that, the regression slope is 1.01 for the
monoterpene/sesquiterpene cases when measurements are referenced to the FZJS
LP–LIF instrument. On some days a change in the relationship between
measurements by the CIMS instrument and the LP–LIF instrument is observed
with changing water. Overall, the variability in the level of agreement hints
to instrumental instabilities.
Conclusions for future instrument operation and measurements in the past
Overall, the comparison demonstrates that OH reactivity measurements
by LP–LIF instruments are precise and accurate for a wide range of
atmospheric conditions. Instrumental parameters such as laser alignment and
instrument zero are recommended to be regularly checked to achieve a high
accuracy and to avoid additional complications in the data evaluation.
In this campaign, the flow-tube LIF instrument gives slightly less accurate
and precise measurements compared to the LP–LIF instruments, which is related
to the different operational conditions compared to previous campaigns. A
different laser system was used and a high dilution flow was applied, which
reduced the instrument performance. Had it been possible to use the field PSU
instrument without dilution, it is likely that its precision and accuracy
would have been similar to that of the LP–LIF instruments.
The OH reactivity scheme of the CIMS instrument is relatively new.
It has only been deployed at the monitoring station Hohenpeissenberg so
far, where OH reactivity values are typically small (2 to 10 s-1).
Further improvements of the data quality for high NO conditions (> 3 pbbv)
are needed to expand the device ability to measure in more polluted regimes.
The accuracy of the current observations depend on the quality of NO
concentration measurements and the assumption that the OH decay obeys
single-exponential behaviour. All OH recycling processes need to be
well characterised. Any deviation from these assumptions leads to systematic
errors, and needs further investigations to capture other unknown complex
mixtures of OH reactants under polluted conditions. Additional
reaction times (injection points for SO2 and propane) and concurrent
measurements of ROx and HO2 concentrations could help
characterise the OH recycling processes in unknown mixtures of OH reactants in the future.
While CRM instruments are less precise and accurate than other techniques, a
reasonable agreement is usually observed between the CRM instruments and the
other techniques for air mixtures containing simple compounds such as
CH4, CO and isoprene, and for urban air mixtures containing
anthropogenic hydrocarbons and NOx. The correction factors, which
depend on the exact instrumental conditions such as the OH,
HO2 and pyrrole concentrations in the reaction volume, are a
potential source of systematic errors. In order to minimise these errors, the
CRM operating conditions are such that the ratio of pyrrole to OH
concentrations ranges from 1.7 to 2, so that corrections for operating under
non-pseudo first-order conditions can be within 10 % (1σ) for
different air compositions . The error associated to this
correction needs to be propagated to the measurement uncertainty. The largest
correction is for the recycling of OH from
HO2 + NO, which is only relevant for urban atmospheres. This
correction can be of the same order of magnitude as the measured OH
reactivity value and needs to be carefully characterised on each CRM instrument.
The level of agreement is degraded when low-volatility terpenoid compounds
and/or their oxidation products are sampled. Although all CRM instruments use
the same detection scheme and the same reaction tube, measurements differ
between the three CRM instruments and also significantly differ from other
LIF-based techniques. While OH recycling in the CRM reactor cannot be
ruled out when these species are sampled, losses of OH reactants in inlet
lines and sampling pumps (partly different than typically inlet lines in
field campaigns) could have led to additional systematic errors in this
campaign. The quality of the measurements depends on both the instrumental
technique but also the procedure used to transfer the sample into the
instrument. A high flow (short residence time) in the inlet lines and/or the
use of inert inlet line materials like Silconert coated steel might help to
reduce inlet line effects as indicated by the results of LP–LIF instruments.
This improvement is a prerequisite to investigate whether the terpenoid
chemistry inside CRM reactors can lead to an underestimation of ambient measurements.
The CRM method is a younger technique compared to the LP–LIF and flow-tube
LIF method, but the number of instruments has quickly increased due to the
commercial availability of detectors for pyrrole such as the PTR-MS
instrument. Results of this campaign emphasise that careful instrument
characterisation for the specific operational conditions are required in
order to achieve accurate measurements. Future work should focus on improving
its performances in terms of precision and limit of detection. In addition,
the accuracy of measurements would improve if corrections could be lowered.
The results of this campaign demonstrate that all detection schemes that are
currently applied to OH reactivity measurements give reasonable
results for a range of chemical conditions which are relevant for ambient air
measurements. These first comprehensive comparison campaigns were conducted
to assess the current performance of the instruments. The results already led to
the implementation of changes in some instruments to achieve better data
quality. More work will be done in order to improve the instrument
performance for issues that have been identified which currently limit the
precision or accuracy of measurements. More comparison campaigns could help
to further increase the trustability of measurements by conducting them in a
formal, blind way and/or at even more realistic conditions with ambient air.
In the field, OH reactivity measurements are often used to identify
unexplained reactivity from OH reactants that were not measured as
individual species . Large unexplained reactivity (several
10 s-1) was found in several field campaigns in biogenic
environments, such as the boreal forest in Finland and
rainforests , as well as in urban
environments in wintertime e.g..
Results here show that measurements by the LIF instruments are accurate. In
these comparison campaigns, deviations that are seen mostly for CRM
instruments for complex conditions involving large concentrations of terpenic
compounds show the tendency for OH reactivity to be underestimated.
Therefore, results do not indicate that high, unexplained reactivity values
that were measured in previous field campaigns were due to measurement artefacts.