AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-2827-2016Measurement of OH reactivity by laser flash photolysis coupled with
laser-induced fluorescence spectroscopyStoneDanielhttps://orcid.org/0000-0001-5610-0463WhalleyLisa K.InghamTrevorEdwardsPeter M.https://orcid.org/0000-0002-1076-6793CryerDanny R.https://orcid.org/0000-0002-0828-4218BrumbyCharlotte A.SeakinsPaul W.https://orcid.org/0000-0002-4335-8593HeardDwayne E.d.e.heard@leeds.ac.ukhttps://orcid.org/0000-0002-0357-6238School of Chemistry, University of Leeds, Leeds, LS2 9JT, UKNational Centre for Atmospheric Science (NCAS), University of Leeds, Leeds, LS2 9JT, UKnow at: Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, Heslington, York, YO10 5DD, UKDwayne E. Heard (d.e.heard@leeds.ac.uk)7July2016972827284414February201629February20163June20168June2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/2827/2016/amt-9-2827-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/2827/2016/amt-9-2827-2016.pdf
OH reactivity (kOH′) is the total pseudo-first-order loss rate
coefficient describing the removal of OH radicals to all sinks in the
atmosphere, and is the inverse of the chemical lifetime of OH. Measurements
of ambient OH reactivity can be used to discover the extent to which measured
OH sinks contribute to the total OH loss rate. Thus, OH reactivity
measurements enable determination of the comprehensiveness of measurements
used in models to predict air quality and ozone production, and, in
conjunction with measurements of OH radical concentrations, to assess our
understanding of OH production rates. In this work, we describe the design
and characterisation of an instrument to measure OH reactivity using laser
flash photolysis coupled to laser-induced fluorescence (LFP-LIF)
spectroscopy. The LFP-LIF technique produces OH radicals in isolation, and
thus minimises potential interferences in OH reactivity measurements owing to
the reaction of HO2 with NO which can occur if HO2 is co-produced
with OH in the instrument. Capabilities of the instrument for ambient OH
reactivity measurements are illustrated by data collected during field
campaigns in London, UK, and York, UK. The instrumental limit of detection
for kOH′ was determined to be 1.0 s-1 for the campaign in
London and 0.4 s-1 for the campaign in York. The precision, determined
by laboratory experiment, is typically < 1 s-1 for most
ambient measurements of OH reactivity. Total uncertainty in ambient
measurements of OH reactivity is ∼ 6 %. We also present the
coupling and characterisation of the LFP-LIF instrument to an atmospheric
chamber for measurements of OH reactivity during simulated experiments, and
provide suggestions for future improvements to OH reactivity LFP-LIF
instruments.
Introduction
OH radicals dominate atmospheric oxidation chemistry, controlling the
lifetimes of most primary pollutants and greenhouse gases emitted into the
atmosphere, including methane, CO, volatile organic compounds (VOCs),
NO2 and SO2, whilst also contributing to the production of
secondary pollutants such as ozone, sulphuric acid and secondary organic
aerosol (SOA) (Stone et al., 2012). Appreciation of the factors controlling
atmospheric OH radical concentrations is thus essential to understanding the
processing and fate of trace species in the atmosphere, and to our ability
to understand and predict air quality and climate change. Moreover, the
short chemical lifetimes of the OH radical and the closely related HO2
radical make OH and HO2 ideal species for testing the chemical
mechanisms used in atmospheric models since their concentrations are
controlled by in situ chemistry alone and are not influenced by transport
processes. However, model simulations of OH concentrations require
calculation of both OH production and loss rates, and there is potential for
agreement between modelled and observed OH concentrations based on opposing
errors in the production and loss terms. Similarly, when model calculations
show poor agreement with observations, it can be problematic to determine
whether the model discrepancies result from incomplete knowledge of the
total production rate or of the total loss rate.
Observations of OH radical concentrations made in conjunction with
measurements of OH reactivity (kOH′), the total loss rate of OH and the
inverse of the OH chemical lifetime (τOH) thus provide a means
to separate the production and loss terms for OH, enabling a more robust
test of our understanding of OH radical concentrations and of atmospheric
oxidation chemistry. In addition, comparison of measured OH reactivity with
calculated OH reactivity, based on observed concentrations of OH sinks and
known rate coefficients for their reactions with OH (Eq. 3), also
provides an indication of the presence and importance of unmeasured OH
sinks:
-d[OH]/dt=Σkx[X][OH]=kOH′[OH]kOH′=Σkx[X],
where kx is the rate coefficient for reaction of OH with species X and
kOH′ is the OH reactivity (the pseudo-first-order rate coefficient for
reaction of OH with all reaction partners present). Finally, using both [OH]
and kOH′ to determine -d[OH]/dt experimentally, it is possible to evaluate
the completeness of our knowledge of OH sources, which when added together
should equal +d[OH]/dt if the steady-state budget is closed (Martinez et al.,
2003; Whalley et al., 2011; Fuchs et al., 2013; Lu et al., 2013).
Measurements of OH reactivity in the atmosphere have been made by three
different techniques – the flow tube technique, the laser flash photolysis
technique and the comparative reactivity method, with all three methods
relying on production of above ambient concentrations of OH radicals and
monitoring of the OH decay rate, either directly or indirectly. The flow
tube method (Kovacs and Brune, 2001) typically generates OH radicals at the
tip of a sliding injector by photolysis of water vapour (Reaction R1) using a mercury
vapour lamp, also resulting in production of HO2 radicals (Reaction R2).
H2O+hν→H+OHH+O2(+M)→HO2(+M)
The OH radical signal is monitored downstream of the injector after mixing
with a flow of ambient air in the main tube. By changing the position of the
sliding injector relative to the point at which OH is detected it is
possible to vary the contact time of OH with the ambient air, and thus to
determine the total loss rate for OH in the flow tube. However, the
technique has a number of disadvantages. The time resolution of measurements
made by the flow tube method is relatively poor, owing to the need to
measure OH signals at a number of different injector positions to obtain a
kinetic profile, during which time the ambient OH reactivity could show
significant variability, although Mao et al. (2009) overcome this issue for
airborne measurements of OH reactivity by reducing the number of time points
used to determine the OH decay rate. The flow rates of sampled air in the
flow tube method are relatively high (∼ 300–900 standard L min-1), with turbulent flow conditions leading to high wall
loss rates of OH in the flow tube and relatively high uncertainties in
determinations of OH reactivity owing to uncertainties in the wall loss
rates. Knowledge of the flow velocity in the flow tube, requiring direct
measurement or knowledge of the flow regime, total flow rate and
cross-section of the flow tube, is also needed to convert the distance over
which OH and ambient air are mixed to reaction time, and can lead to
uncertainties in the contact time between OH and reactants in ambient air. A
significant disadvantage of the flow tube method is the generation of equal
concentrations of OH and HO2 following photolysis of water vapour at
the tip of the sliding injector (Reactions R1–R2), leading to the potential
for production of OH in the flow tube on the timescale of the experiment
owing to the reaction of HO2 with ambient NO (Reactions R3).
HO2+NO→OH+NO2
The production of OH from Reaction (R3) reduces the observed decay rate of OH in the
flow tube, and measurements of OH reactivity using the flow tube method thus
also require simultaneous measurements of ambient NO concentrations in order
to correct for interferences from HO2+ NO, which can be quite
significant. For example, for 75 ppb NO, a typical rush hour mixing ratio in
Mexico City, a correction factor of ∼ 1.7 was required to
account for the production of OH from HO2+ NO within the flow tube
(Shirley et al., 2006).
In the comparative reactivity technique, a reactive molecule not usually
present in air, typically pyrrole, is entrained in a gas flow and the rate
of its decay owing to reaction with artificially high concentrations of OH
is measured in “zero” air and ambient air by proton transfer quadrupole mass
spectrometry (PTR-QMS) (Sinha et al., 2008), proton transfer time of flight
mass spectrometry (PTR-ToFMS) (Michoud et al., 2015) or photoionisation
detection (GC-PID) (Nölscher et al., 2012). Comparison of the rates of decay
of the molecule in “zero” air and ambient air enables determination of the
competition between the reaction of OH with the known concentration of the
reactive molecule and the reaction of OH with sinks in ambient air, thus
enabling measurement of the ambient OH reactivity. Absolute measurement of
the physical loss rate of OH is not required for the technique, and the
limit of detection of comparative reactivity instruments is determined by
the sensitivity to changes in the signal corresponding to the concentration
of the reactive species. However, OH radicals are typically produced in
comparative reactivity instruments through Reactions (R1–R2), in a similar
manner to that used in flow tube instruments and thus also producing high
concentrations of HO2. Interferences resulting from OH production from
HO2+ NO are thus also potentially problematic for comparative
reactivity instruments and knowledge of NO concentrations are required to
correct for any interferences. In addition, the amount of OH produced is
dependent on humidity, and it is essential to ensure constant humidity
between measurements made in “zero” air and those made in ambient air, with
significant corrections often necessary to account for any differences
(Michoud et al., 2015).
The laser flash photolysis technique (Sadanaga et al., 2004a) produces OH in
isolation (i.e. with no simultaneous production of HO2) via laser
photolysis of O3, typically at a wavelength of 266 nm, followed by
reaction of O(1D) with ambient H2O (Reactions R4–R5):
O3+hν(λ=266nm)→O2+O(1D)O(1D)+H2O→2OH.
The production of OH without initial co-production of HO2 minimises
potential interferences from HO2+ NO and renders the flash
photolysis technique more suitable to high NOx (NOx= NO + NO2) environments. The laser flash photolysis method also has the
advantage that the production of OH radicals is uniform throughout the
reaction cell, minimising the risk of poor mixing which is potentially
problematic for the flow tube and comparative reactivity techniques. Flow
rates of sampled air are typically lower for the laser flash photolysis
instruments (∼ 12–20 standard L min-1) (Sadanaga et al., 2004a) than for
those using flow tubes (∼ 300–900 standard L min-1) (Kovacs and Brune,
2001; Kovacs et al., 2003; Ingham et al., 2009; Hansen et al., 2014), and
the resulting laminar flow of gas reduces contact of the gas with the walls
of the instrument, thus reducing the physical loss rate of OH and associated
uncertainties. Although averaging of data is often required to improve
signal-to-noise, a significant advantage of the laser flash photolysis
technique is the ability to measure ambient OH reactivity in “real-time”
through time-resolved measurements of the OH decay following photolysis. The
technique has the potential for significantly enhanced time resolution, both
in terms of the number of time points obtained during the decay of OH, and
the averaging time over which the data are reported, compared to the flow
tube or comparative reactivity methods.
The first atmospheric measurements of total OH reactivity were made at an
urban background site in Nashville, TN, USA, in summer 1999 using the flow
tube technique (Kovacs and Brune, 2001; Kovacs et al., 2003). Calculations
of OH reactivity, using VOC measurements co-located with the reactivity
measurements, underestimated the total observed reactivity by
∼ 30 % on average owing to unmeasured or unknown VOCs and
VOC oxidation products (Kovacs et al., 2003). Subsequent measurements at an
urban background site in New York, NY, USA, were, on average, within 10 %
of the calculated reactivity in summer 2001 (Ren et al., 2003), but were
underestimated by 30–40 % during morning and evening rush hours in winter
(Ren et al., 2006a). Significant underestimation of the measured OH
reactivity in the morning rush hour was also reported for observations in
the Mexico City Metropolitan Area (MCMA), with the observed reactivity
reaching 120 s-1 (Shirley et al., 2006). High OH reactivity has also
been observed in Paris during the MEGAPOLI campaign in 2010, with kOH
reaching 130 s-1 for continental air masses and calculations based on
measured VOC concentrations underestimating the reactivity by up to 75 %
(Dolgorouky et al., 2012). Reactivity measurements in Tokyo were
underestimated in summer, spring and autumn, but reproduced to within 5 %
in winter, with the reactivity correlating well with NOx throughout the
year (Sadanaga et al., 2004b; Yoshino et al., 2006; Chatani et al., 2009;
Yoshino et al., 2012). Aircraft measurements of OH reactivity have also
shown that reactivity tends to decrease with altitude, with discrepancies
between observed and calculated reactivity most pronounced at altitudes up
to 2 km and tending towards agreement at altitudes above 4 km (Mao et al.,
2009).
Flow tube measurements at an urban site in Houston, US, during the
TEXAQS2000 and TRAMP2006 campaigns (Mao et al., 2010) and at a forested site
at Whiteface Mountain, NY, USA (Ren et al., 2006b), were well reproduced by
model calculations. However, measurements made at a coastal site in Norfolk,
UK, typically characterised by relatively “clean” air were significantly
underestimated and attributed to the presence of numerous high molecular
mass VOCs at low concentrations which were not included in the VOC
measurement suite (Lee et al., 2009; Ingham et al., 2009). The presence of
unmeasured VOCs was also indicated for the PROPHET 2000 campaign at a
forested site in Michigan, USA, during which the measured OH reactivity was
underestimated by ∼ 50 % on average, with the “missing” OH
reactivity exhibiting a strong temperature dependence potentially resulting
from temperature-dependent emissions of unmeasured biogenic VOCs (Di Carlo
et al., 2004). Uncertainties in emissions and chemistry of biogenic VOCs,
particularly in the oxidation chemistry of isoprene and its oxidation
products, have also been responsible for underpredictions of observed OH
reactivity in forested regions in Suriname (Sinha et al., 2008) and Borneo
(Whalley et al., 2011; Edwards et al., 2013). Model calculations of OH
reactivity in Borneo underestimated the observed diurnal mean reactivity by
30 %, and indicated that uncertainties in the chemistry and deposition
rates of secondary oxidation products could potentially explain the observed
reactivity without the need for additional primary VOC emissions, and that
at least 50 % of the carbon-containing compounds which react with OH were
not measured (Edwards et al., 2013). Biogenic VOCs also dominated the
daytime OH reactivity in the Pearl River Delta region, China, with isoprene
and its oxidation products comprising ∼ 40 % of the total
OH reactivity in the afternoon and observed reactivity underestimated by
∼ 50 % when calculated from measured OH sinks but
reproduced by model calculations considering the contributions from
secondary oxidation products (Lou et al., 2010). However, observations of OH
reactivity in a forested region in Colorado, USA, during the BEACHON-SRM08
campaign were underestimated by model calculations (∼ 30 %), with the dominant VOCs found to be 2-methyl-3-buten-2-ol (MBO) and
monoterpenes (Nakashima et al., 2014).
Using a branch enclosure technique, Kim et al. (2011) demonstrated that
underestimations of observed OH reactivity at the PROPHET field site,
Michigan, USA, during the 2009 CABINEX campaign were related to oxidation
products of known and measured biogenic VOCs, rather than to unknown or
unmeasured primary VOC emissions. Model calculations were able to reproduce
the CABINEX OH reactivity observations below the forest canopy, but
discrepancies were apparent above the canopy, indicating the presence of
unmeasured trace gases above the forest canopy (Hansen et al., 2014). Model
calculations and experiments using the comparative reactivity method at a
forested site in Finland also observed differences between OH reactivity
measured in the forest canopy and above the canopy (Mogensen et al., 2011;
Nölscher et al., 2012). While the in-canopy reactivity was typically higher
than the above-canopy reactivity, transport of wildfire plumes to the site
significantly increased the above-canopy reactivity, increasing it above the
in-canopy level and increasing the “missing” reactivity above the canopy
from 58 % for “normal” conditions to 73 % for periods impacted by
transported pollution (Nölscher et al., 2012).
OH reactivity measurements have also been used to determine ozone production
rates in southwestern Spain during the DOMINO campaign (Sinha et al., 2012)
and in London during the ClearfLo campaign (Whalley et al., 2016), and have
been used in laboratory studies to assess our understanding of combustion
systems (Nakashima et al., 2010) and atmospheric isoprene oxidation
mechanisms (Nakashima et al., 2012; Nölscher et al., 2014).
Measurements of OH reactivity thus have a number of applications, and can be
used to improve our understanding of atmospheric composition and chemistry.
In this work we present the design and characterisation of an instrument
using laser flash photolysis coupled with laser-induced fluorescence
(LFP-LIF) to measure OH reactivity in the field and in chamber experiments.
Schematic of the laser flash photolysis laser-induced fluorescence
OH reactivity instrument for experiments and field measurements in which the
FAGE detection cell was situated at the end of the reaction cell and sampled
from the centre of the photolysed volume, leading to OH decays described by
a single exponential. All laboratory and field measurements shown in this
work were obtained with the instrument configuration as shown here. See text
for further details.
Experimental
A schematic of the OH reactivity instrument is given in Fig. 1. The
instrument comprises a reaction cell (described in Sect. 2.1) and a
detection cell (described in Sect. 2.2), with the two cells typically
situated approximately 5 m above ground level on the roof of a shipping
container housing the FAGE (fluorescence assay by gas expansion) mobile laboratory during ambient measurements.
During laboratory and chamber measurements, the instrument is configured
within the laboratory. Thus, for ambient measurements, and laboratory and
chamber measurements made at room temperature, the temperature in the OH
reactivity instrument is the same as the source of the air being sampled.
Reaction cell
The reaction cell consists of a cylindrical stainless steel tube of 50 mm
internal diameter and 85 cm in length. Ambient air is drawn through a
stainless steel sampling line (50 mm internal diameter and 20 cm in length),
enters the reaction cell at 90∘ to the air flow in the tube, and is
drawn along the tube by an extractor fan (612F, DC Axial Fan, EBM-Papst)
mounted on the exit arm situated immediately prior to the OH detection cell
(Sect. 2.2), as shown in Fig. 1. More recently, the connection between
the reaction cell and the exhaust has been replaced with an exhaust that
draws air out of the cell over the full circumference of the cell. Although
the data recorded with the new exhaust are not reported in this paper, we do
not see any significant change in results between the different exhaust
designs, but this will be discussed in future publications.
The fan speed determines the flow rate of gas in the reaction cell, and is
set to ensure a laminar flow of air through the cell with a Reynold's number
less than 2300. The flow rate of gas, determined by measurement of the flow
velocity using a hot-wire anemometer (TSI Air velocity transducer 8455-150)
or set by calibrated mass flow controllers during laboratory experiments
(Sects. 4, 6, and 7) and measurements of instrument zeroes (Sect. 3.1),
is in the range 12 to 14 standard L min-1, giving a residence time of 7 to 8 s in the
reaction cell and a Reynold's number of ∼ 360, which is below
that required for laminar flow.
Production of OH radicals within the reaction cell is achieved by the 266 nm
laser photolysis of O3 in the presence of water vapour (Reactions R4–R5). A flashlamp pumped Nd : YAG laser (Big Sky Laser CFR 200, Quantel USA)
is used to generate laser light at 1064 nm, which is frequency doubled to
532 nm (lithium triborate, LiB3O5, doubling crystal) and then
frequency doubled to generate the fourth harmonic 266 nm radiation (caesium
lithium triborate, CsLiB6O10, doubling crystal) with pulse
energies of ∼ 50 mJ, pulse length 8 ns, and beam diameter of
6.35 mm. The pulse repetition frequency is typically 1 Hz, and has been
varied in experiments between 0.1 and 1 Hz with no significant impact
observed. The laser is operated with a Q-switch to modulate the intracavity
losses and maximise the pulse energy.
The 266 nm laser head is situated adjacent to the reaction cell in order to
minimise the footprint of the instrument when used in the field. The laser
head is powered, controlled and water cooled by an Integrated Cooler and
Electronics unit (Big Sky Laser ICE450, Quantel USA) which is housed within
the FAGE shipping container and powered via an uninterruptible power supply
(APC 1000VA, American Power Conversion by Schneider Electric).
Laser light exiting the laser head is directed into the reaction cell using
two dielectrically coated 266 nm turning mirrors of 1′′ diameter (Thorlabs,
NB1-K04). Immediately prior to the reaction cell, the 266 nm beam is
expanded to a diameter of ∼ 10 mm by a telescope incorporating
a plano-concave lens (Thorlabs LC4252, focal length =-30 mm) and a
plano-convex lens (Thorlabs LA4148, focal length = 50 mm) housed in a lens
tube (SM1M20, Thorlabs) to increase the photolysis volume within the
reaction cell. The photolysis laser enters the reaction cell through a fused
silica window, initiating OH radical production.
Typically, there is sufficient production of OH in the instrument from
Reactions (R4–R5) at ambient concentrations of O3 and water vapour in
order to measure a temporal decay of OH. At low ambient concentrations of
O3 (< 10 ppb) or during laboratory tests (Sects. 4, 6 and 7)
and measurement of instrument zeroes (Sect. 3), the OH radical
concentration in the reaction cell is increased by passing a small flow
(0.5 standard L min-1) of humidified ultra-high purity air (BTCA 178, BOC Special Gases)
across a low pressure Hg vapour lamp and mixed with the main sampled air
flow (12–14 standard L min-1) in the inlet to the reaction cell. The mixing ratio of
ozone in the reaction cell is increased by ∼ 50 ppb by this
method (measured by an ozone analyser (Thermo Environmental Instruments
Inc., 49C O3 Analyser) situated at the end of the reaction cell during
laboratory tests). Given knowledge of the rate coefficient for reaction of
OH with O3 (kOH+O3=7.3×10-14 cm3 s-1,
Atkinson et al., 2004), the chemical loss of OH resulting from the addition
of 50 ppb O3 is < 0.1 s-1 at 298 K, and any loss of VOCs
in the reaction cell through reaction with ozone is extremely small given
the reaction times involved.
OH detection cell
OH radicals in the reaction cell are monitored by laser-induced fluorescence
(LIF) using the FAGE technique. The
LIF-FAGE detection cell has been described previously in detail (Ingham et
al., 2009), thus only a brief description will be given here.
Initial experiments were conducted with the detection cell situated midway
along the reaction cell, and sampling at 90∘ to the direction of air
flow along the reaction cell, in a similar design to that described by
Sadanaga et al. (2004a) and Lou et al. (2010). However, the observed OH
decays in such a configuration displayed biexponential behaviour, comprising
a fast initial decay followed by a slower decay representative of the
expected OH reactivity, as observed in previously described instruments
(Sadanaga et al., 2004a; Lou et al., 2010). In this configuration, the
photolysis laser is aligned such that the beam passes across the inlet to
the detection cell without hitting the inlet. The air sampled in to the
detection cell thus likely contains air that has experienced the photolysis
laser (containing elevated OH concentrations) and air that has not
experienced the photolysis laser (which will have significantly lower or
zero OH concentrations), with this mixing of air potentially leading to an
apparent increase in the initial OH decay rate owing to dilution of the air
containing elevated OH concentrations with air containing lower (or zero)
concentrations. Once mixing of the air having experienced the photolysis
laser with that outside the beam diameter has occurred sufficiently to give
uniform OH concentrations in the reaction cell the observed OH decay will
result from the chemical losses in the instrument, leading to biexponential
decays. Such biexponential behaviour has been attributed to similar effects
of non-homogeneous spatial distributions of OH near the inlet to the
detection cell (Lou et al., 2010) and to local heating and turbulence of the
gas flow caused by the photolysis laser (Sadanaga et al., 2004a). A
comparison between sampling at 90∘ to the direction of air flow along
the reaction cell and sampling along the axis of the direction of air flow
from the centre of the reaction cell has been reported previously (Amedro et
al., 2012), with different fitting procedures required to extract the OH
reactivity for the different instrument configurations attributed to
differences in physical effects such as diffusion which were more
significant when sampling at 90∘ (Amedro et al., 2012).
Subsequent experiments in this work (including all those described below)
were performed with the detection cell situated at the end of the reaction
cell along the same axis as the direction of air flow to sample air directly
from the centre of the reaction cell. This configuration reduces the chance
of sampling air into the detection cell that has not experienced the
photolysis laser beam, and thus reduces the impact of physical effects such
as diffusion. The observed OH signals in this instrument configuration are
described by a single exponential decay, although biexponential decays can
still be obtained if the photolysis laser is not correctly aligned along the
axis of the reaction cell.
Air is sampled from the centre of the reaction cell through a pinhole of 0.8 mm diameter and 0.5 mm thickness into the aluminium detection cell, which
consists of three orthogonal axes and is black anodised to minimise light
scattering within the cell. The pressure in the cell is measured by a
capacitance manometer (Sensotec Z/606-01ZA) and is maintained at
∼ 1.5 Torr by a roots blower backed by a rotary pump (Leybold
Vacuum SV200/WAU1001), resulting in an air flow of approximately 4 standard L min-1 and a
supersonic expansion of the air as it is drawn through the pinhole.
The probe laser consists of a Nd : YAG pumped Ti : sapphire laser (Photonics
Industries) which generates broadband radiation in the range 690–1000 nm. A
diffraction grating is used to select radiation with λ= 924 nm,
which is frequency tripled through generation of the second harmonic at 462 nm followed by sum-frequency mixing of the 462 nm radiation
with that at 924 nm to produce the 308 nm light with a pulse repetition frequency (PRF) of
5 kHz, pulse length (full width half maximum (FWHM)) of 35 ns, laser line
width (FWHM) of 0.065 cm-1 and beam diameter of ∼ 3 mm
(Bloss et al., 2003).
A reference fluorescence cell, containing a heated nichrome wire filament
and humidified air at ∼ 2 Torr to produce a constant stable
source of OH radicals from dissociation of water vapour, is used to
facilitate tuning the probe laser to the precise wavelength required for the
desired OH transition. Approximately 1 mW of the 308 nm laser light is used
for this purpose, with ∼ 9 mW used to make measurements of OH
reactivity and a further ∼ 13 mW remaining to make
measurements of ambient OH, HO2 and RO2 concentrations in a
separate instrument (see, for example, Whalley et al., 2016).
The probe laser, reference cell and pumps are all situated inside the
shipping container. The ∼ 9 mW of the 308 nm laser light used
to measure OH reactivity is passed to the detection cell on the roof of the
shipping container via an anti-reflective coated optical fibre with an
angled and polished end (Oz Optics, QMMJ-55-UVVIS-200/240-3-30-AR2-SP,
length = 30 m) through a baffled side-arm at 90∘ to the air flow. The
probe laser light exits the detection cell through a baffled side-arm and is
directed onto a photodiode (New Focus Large Area Photoreceiver 2032) to
measure the laser power to enable normalisation of fluorescence signals for
fluctuations in laser power. For a recent intercomparison at the SAPHIR
chamber, the OH reactivity instrument, comprising the reactor flowtube and
OH fluorescence and associated equipment was placed in the shipping
container itself.
Fluorescence from electronically excited OH radicals resulting from
excitation of the Q1(1) A2Σ+ (v′=0) – X2Π3/2 (v′′=0) transition at 308 nm is collimated by a symmetrical
biconvex collimating lens (Melles-Griot, focal length = 50 mm at
λ= 546.1 nm, diameter = 50 mm) and focused onto the
photocathode of a channeltron photomultiplier tube (PMT) (Perkin Elmer C
943P) by two plano-convex focusing lenses (UQG Optics Ltd., focal
length = 75 mm at λ=250 nm, diameter = 50 mm). A narrow band UV
interference filter (Barr Associates Inc., FWHM bandwidth of 8 ± 1.6 nm centred at 309 ± 1 nm with a peak transmission
of > 50 % at 308 nm and a blocking factor of 106 at other wavelengths) is
situated between the excitation region in the detection cell and the PMT to
minimise detection of scattered solar photons. The solid angle from which
fluorescence is collected is effectively doubled through the use of a
spherical concave mirror coated for high UV reflectance which is mounted in
the detection cell opposite the side-arm bearing the PMT. Discrete photon
signals on the PMT are processed using a multi-channel scaler photon
counting card (Becker and Hickl, PMS 400, minimum bin width of 250 ns) in
the computer used to control the instrument.
Instrument control
A digital delay pulse generator (Stanford Research Systems DG535) produces a
5 kHz TTL (transistor-transistor logic) pulse to trigger the Ti : sapphire
laser and a second delay generator (Stanford Research Systems DG535) which
subsequently triggers the gating of the PMT detector for the reactivity
instrument and a third digital delay pulse generator (Berkeley Nucleonics
Corporation 555) to trigger the 266 nm photolysis laser and the photon
counting card at the specified pulse repetition frequency in synchronisation
with the 308 nm probe laser. A personal computer is used to automate data
collection, with analogue signals from measurements of the pressure in the
detection cell and the power of the 308 nm probe laser at the photodiode
attached to the detection cell digitised by an A/D card (Measurement
Computing, PCI-DAS 1200). Electrical power to all parts of the instrument is
supplied via an uninterruptible power supply (APC 1000VA).
Data acquisition
Data acquisition is initiated by triggering of the photon counting card,
with a background signal measured for 100 ms before triggering of the 266 nm
photolysis laser and production of OH in the reaction cell. To avoid
saturation of the PMT resulting from detection of the 308 nm laser pulse
itself, the PMT is gated off at the onset of the 308 nm laser pulse (35 ns
FWHM) until ∼ 100 ns after the laser pulse, thereby preventing
detection of any reflected or scattered laser light. The fluorescence signal
is typically collected for 1 µs following each 308 nm probe laser
pulse. Repeated measurements of the OH fluorescence signal are taken for 900 ms following each 266 nm photolysis laser pulse, during which time the OH
concentration and hence the fluorescence signal will decay to the background
level. Under normal conditions this occurs within ∼ 300 ms of
the photolysis laser pulse, although this is of course dependent upon the
magnitude of the OH reactivity, and may be longer. The pulse repetition
frequency of the 308 nm probe laser (5 kHz) results in measurement of the OH
fluorescence signal every 200 µs throughout the measurement period. The
data collection cycle, as illustrated in Fig. 2 is typically repeated
every 1 s (i.e. with the photolysis laser having a pulse repetition
frequency of 1 Hz). Experiments, both in the laboratory and in the field, in
which the PRF of the photolysis laser was varied between 0.1 and 1 Hz showed
no effect on the observed OH reactivity (Sect. 8).
(a) Schematic to illustrate the Stanford Research Systems delay
generator controlled gate timing of the PMT detector and photon counting
card in the OH reactivity instrument. The blue hatched region indicates the
overlap between the OH fluorescence signal and the photon counting gates;
(b) Schematic to illustrate the photon counting bin structure used to collect OH
fluorescence photons after each 308 nm probe laser pulse (5 kHz pulse
repetition frequency). Four 50 µs wide photon counting bins cover the
time period between each 308 nm laser pulse, but only the bins immediately
after the laser pulse collect any fluorescence photons (shaded bins), and
only the photon counts from these bins are used to construct the OH decay.
When measurements of OH reactivity are made alongside those of ambient OH
concentrations, the acquisition of OH reactivity data is linked to
measurements of ambient OH concentrations owing to the dual use of the 308 nm excitation laser. Under such circumstances, measurements are taken on an
approximate 7 min cycle, with a 5 min “online” period during which the 308 nm laser is at the precise wavelength to excite the OH transition, followed
by a 1 min “offline” period during which the wavelength of the laser is
moved to a nearby wavelength at which the OH transition is not excited in
order to enable measurement of a background signal for determination of
ambient OH concentrations (see, for example, Whalley et al. 2010).
Approximately 1 min is then required to scan the laser wavelength over the
OH transition to find the maximum OH fluorescence signal in the reference
cell (Sect. 2.2). OH reactivity measurements are thus taken during the 5 min online period, and data from successive measurement cycles during each
online period are co-added to improve the signal-to-noise ratio. Figure 3
shows typical OH decays derived from the co-addition of data recorded
throughout 5 min online periods during the Clean Air for London (ClearfLo)
campaign in summer 2012.
Typical OH time profiles following photolysis of ambient air
(mixed with a small flow of N2/ O2/ O3/ H2O) observed
during the Clean Air for London (ClearfLo) campaign (black points) with fits
of Eq. (5) (red lines) to the LIF data to determine kOH′ for data
recorded (a) during a polluted period on 25 July 2012 (kOH′= (46.6 ± 3.2) s-1) and (b) during
a cleaner period on 7 August 2012 (kOH′= (13.9 ± 0.9) s-1). Time zero is defined
as the time at which photolysis occurs. Decays represent data co-added
throughout 5 min periods.
Measurements of OH reactivity may also be made independently of any other
use of the 308 nm probe laser, in which case the timescale over which
successive measurement cycles are co-added may be selected as desired, with
the laser periodically scanned over the OH transition to ensure that the
maximum OH signal is obtained.
Calibration of the FAGE detection cell
Calibration of the detection cell, although not strictly necessary for
measurements of OH reactivity, is required to ensure that pseudo-first-order
conditions are met in the reaction cell (i.e. combined concentrations of OH
sinks are in excess over the OH concentration) and provides a means to
determine any potential interferences from production of OH via ambient
HO2+ NO in the reaction cell (from ambient HO2 which may
survive the sampling inlet, and any HO2 generated following oxidation
of OH sinks in the instrument) and to monitor potential changes in
instrument sensitivity with time.
The calibration procedure has been described in detail by Commane et al. (2010). Production of OH (and HO2) is achieved through Reaction (R1) (and
HO2 through Reaction R2) by passing a turbulent flow of humidified
ultra-high purity air (BTCA 178, BOC Special Gases) across a low pressure
mercury vapour lamp to photolyse water vapour at λ= 184.9 nm.
H2O+hν(λ=184.9nm)→H+OH
The concentration of OH is given by Eq. (4):
[OH]=[H2O]σH2OφOHFδt,
where σH2O is the absorption cross-section of H2O at 184.9 nm (7.1 ± 0.2) × 10-20 cm2 (Cantrell et al., 1997;
Creasey et al., 2000), φOH is the quantum yield for OH
production (φOH=1), F is the photon flux of the mercury
lamp at 184.9 nm and δt is the residence time in the photolysis
region. The product Fδt is determined by N2O actinometry
(Commane et al., 2010), with F varied by changing the current supplied to the
lamp, and δt controlled by the flow rate of the gas used in the
calibration. The concentration of water vapour in the flow is determined by
diverting a small known flow of the air to a dew point hygrometer (CR4, Buck
Research Instruments), and was varied between 300 and 10 000 ppm during
calibration experiments.
The calibration for OH was conducted over a range of mercury lamp fluxes and
water vapour mixing ratios (between 300 and 10 000 ppm), giving a
calibration factor (COH) of (2.13 ± 0.27) × 10-8 counts s-1 molecule-1 cm3 mW-1. The
1 σ instrumental limit of detection for OH radicals was determined to be
∼ 107 cm-3 for a 5 min integration period, enabling
observation of sufficient changes in OH radical concentrations in the
reaction cell to allow measurements of ambient OH reactivity.
For a minimum ambient O3 mixing ratio of 10 ppb (below which
∼ 50 ppb is added to the instrument (Sect. 2.1), an
absorption cross-section for O3 of 9.65 × 10-18 cm2
at 266 nm (Atkinson et al., 2004), typical laser fluence of ∼ 50 mJ cm-2 and a quantum yield of 0.9 for production of O(1D)
(Matsumi et al., 2002), the initial O(1D) number density following
photolysis is ∼ 1.6 × 1011 cm-3. For a
water vapour concentration of 5 × 1017 cm-3
(∼ 2 %), competition between reaction of O(1D) with
water (kO1D+H2O= 2.1 × 10-10 cm3 s-1, Atkinson et al., 2004), leading to 2 OH, and quenching of O(1D) by
N2 (kO1D+N2= 3.1 × 10-11 cm3 s-1, Atkinson et al., 2004) or O2 (kO1D+O2= 4.0 × 10-11 cm3 s-1, Atkinson et al., 2004), to produce
O(3P), typically leads to an initial OH concentration in the reactivity
instrument of > 3 × 1010 cm-3, in agreement
with the calibration results. At higher ambient mixing ratios of O3,
the initial OH concentration in the reactivity is increased, whilst
maintaining pseudo-first-order conditions for OH. For example, an ambient
O3 mixing ratio of 100 ppb, as observed in some polluted environments,
would be expected to generate > 3 × 1011 cm-3
OH (at 2 % humidity), leading to improvements in the signal-to-noise
ratio and minimisation of any potential interferences in measurements of OH
reactivity (Sect. 6).
Determination of OH reactivity
The observed pseudo-first-order rate coefficient for OH loss (kloss) is
determined by least-squares fitting Eq. (5) to the time-resolved OH
decay:
SOH,t=SOH,0exp(-klosst)+b,
where SOH,t is the fluorescence signal at time t after firing of the
266 nm photolysis laser, SOH,0 is the fluorescence signal at time zero
(i.e. immediately following firing of the 266 nm laser and production of OH
in the reaction cell), kloss is the observed rate coefficient for loss
of the fluorescence signal, t is the time since firing of the 266 nm
photolysis laser and b is the background fluorescence signal measured by the
PMT averaged for the 100 ms prior to firing of the photolysis laser
(typically zero). Fits are typically started within 5 ms of the peak in the
OH signal, and data are fitted until the OH signal is essentially back to
the background level by the end of the fit. Values for SOH,0 and
kloss are permitted to vary in the fitting process. Since the OH decays
are well described by first-order kinetics, the fitted values for
SOH,0 and kloss do not depend in any way on the time period over
which the decays are fitted. Figure 3 shows typical fits of Eq. (5) to
measurements of OH reactivity made in ambient air.
The value for kloss determined from the fit contains a contribution from
kOH,obs′, the rate coefficient for OH loss owing to chemical losses of
OH in the reaction cell (the OH reactivity), and kphys, the instrument
“zero” corresponding to the rate coefficient for physical losses of OH owing
to diffusion out of the sampling volume and heterogeneous losses on the
walls on the reaction cell. The chemical loss of OH in the reaction cell is
thus given by Eq. (6), and in order to determine the OH reactivity from
measurements of kloss it is therefore essential to characterise
kphys (Sect. 3.1).
kOH,obs′=kloss-kphys
At low ambient concentrations of ozone (< 10 ppb) and in laboratory
experiments (Sects. 4, 6 and 7) and measurements of kphys, it was
necessary to add a small flow of humidified air containing a constant mixing
ratio of ozone (∼ 50 ppb) to the main air flow sampled in
order to produce sufficient OH radicals in the reaction cell. This
“non-ambient” ozone added to the reaction cell results in a small loss of OH
owing to the reaction of O3 with OH, but is expected to be < 0.1 s-1 at 298 K (Sect. 2.1). However, addition of the small
ozone-containing air flow (0.5 standard L min-1) to the sampled flow of ambient air (12 standard L min-1) does require a correction for the dilution of the ambient air flow,
such that the OH reactivity (kOH′) is given by Eq. (7):
kOH′=kOH,obs′(1+f),
where f is the dilution factor of the ambient air flow, given by the ratio of
the small ozone-containing flow rate to the total flow rate of the air in
the reaction cell (∼ 0.04 for the conditions used in this
work). Potential errors arising from errors in measurements of kphys and
f have been included in overall reported errors for kOH′, and contribute,
on average, 70 and 25 %, respectively, to the total uncertainty in
kOH′.
Determination of kphys
Determination of kphys is critical to the evaluation of the true OH
reactivity from observations of the total OH loss rate in the instrument
(Eqs. 7 and 8), and requires the measurement of the OH loss rate in
the absence of any chemical removal processes such that kloss is equal
to kphys. A single value for kphys is determined (from several
experiments) independently of any measurements of kobs, and is fitted
over as much of the observed OH decay as possible using the procedure
described above in order to obtain the best possible determination of
kphys since fitting kphys over as much of the decay as possible
reduces the uncertainty in the fit. As described above, determinations of
kphys (and kobs) are independent of the time period over which the
data are fitted since the observed OH decays follow first-order kinetics and
can be described with a single exponential function.
To minimise the chemical losses of OH in the reaction cell (and thus to
minimise kOH,obs′) the loss of OH in the instrument is measured in
ultra-high purity air (BTCA 178, BOC Special Gases) passed through scrubbers
(Gatekeeper Gas Purifiers) to remove H2, CO and CO2 to sub-ppb
levels. Despite the use of scrubbed ultra-high purity air, low levels of
residual VOCs can remain in the air, leading to chemical losses. Such
residual VOCs in the scrubbed ultra-high purity air have been quantified by
gas-chromatography and their contributions (< 1 s-1) to the
observed OH loss subtracted.
Furthermore, humidification and addition of a small amount of O3 to the
ultra-high purity air are necessary for the production of OH in the
instrument during experiments to determine kphys. Approximately 50 ppb
of O3 is added to ensure production of sufficient OH, leading to a
chemical loss of < 0.1 s-1 at 298 K through the reaction of OH
with O3. Moreover, despite the use of purified water for
humidification, obtained using a water purification system (PURELAB flex
PRIPLB0163, Elga LabWater, Veolia Water Solutions & Technologies),
impurities in the water can lead to significant chemical losses for OH and
the components in the purification system must remain uncontaminated in
order to ensure accurate determinations of kphys.
Determination of kphys in the laboratory and in the field for the
ClearfLo campaign in London in 2012 gave an average value (1.1 ± 1.0) s-1 (precision of 0.4 s-1) and (1.25 ± 0.42) s-1
(precision of 0.2 s-1) for the campaign in York in 2014 (Sect. 7).
Instrumental validation via measurements of kOH+CO and
kOH+CH4
As a real-time technique, the accuracy of the time axis during which the OH
decay is obtained is determined by the accuracy of the delay generators used
to trigger the lasers and other delays (as described in Sect. 2.4 and
shown in Fig. 2), which should be absolute within 1 ps. Hence the method
should be absolute in terms of the time separation between points in the
decay. However, owing to various reasons, for example the appropriateness of
the function used to fit the decay, or any recycling of OH from oxidation
products (for example the reaction of HO2 with NO), it is prudent to
characterise the instrument through the use of known concentrations of
reactants for which the rate coefficient with OH is also well known. In
order to validate measurements of ambient OH reactivity, the well-known rate
coefficients for reactions of OH with CO and CH4 were both measured
under pseudo-first-order conditions using the instrumental setup described
above. Ultra-high purity air (BTCA 178, BOC Special Gases) was mixed with an
excess of either CO (5 % in N2, BOC Special Gases) or CH4 (BOC,
CP grade, 99.5 %), producing a main flow of 11.5 standard L min-1 with known
concentrations of CO or CH4, prior to mixing with a small flow of
humidified air (0.5 standard L min-1) containing ∼ 50 ppb O3 generated
by passing the air flow across a mercury vapour lamp.
Bimolecular plots of pseudo-first-order rate coefficients
describing OH loss (kOH′) against known concentrations of reactive gases
during laboratory tests (black points) with best fit lines (blue) and
literature values (red) for (a) CO at 298 K (intercept = 1.1 s-1);
(b) CH4 at 298 K (intercept = 1.3 s-1); (c)n-butanol
(n-C4H9OH, sampling from the chamber) at 298 K (intercept = 1.1 s-1). Literature values are taken from Atkinson et al. (2004).
Corrections for dilution have been applied (Eq. 7). Errors are 1 σ from the fits to the observed OH decays. Note the change in vertical scale
between the three panels.
Figure 4 shows the OH reactivity, determined by fitting Eq. (5) to the OH
decay and subtracting kphys (Sect. 3), for a series of CO and CH4
concentrations. The bimolecular rate coefficients for OH + CO
(kOH+CO) and OH + CH4 (kOH+CH4), determined at 298 K from the
relationships kOH′=kOH+CO[CO] and kOH′=kOH+CH4[CH4],
were found to be (2.4 ± 0.2) × 10-13 and (6.4 ± 0.6) × 10-15 cm3 s-1, respectively (errors are 1 σ). The values for
kOH+CO and kOH+CH4 determined here are in agreement with the
literature values of (2.3+0.6-0.5)× 10-13 and (6.4+1.3-1.1)× 10-15 cm3 s-1
at 298 K (Atkinson et al., 2004), respectively, providing
confidence in measurements of ambient OH reactivity.
Limit of detection, precision and total uncertainty
The instrumental limit of detection for measurements of kOH′ is
determined by the fit error in kloss (Eq. 5), the determination of
kphys, and its associated uncertainty, and the number of measurements
used to determined kphys. For the ClearfLo campaign, kphys was
determined to be (1.1 ± 1.0) s-1, with a precision of 0.4 s-1 and a 1 σ limit of detection for kOH′ of 1.0 s-1.
During the York campaign (Sect. 7), kphys was determined to be (1.25 ± 0.42) s-1, with a precision of 0.2 s-1 and a 1 σ
limit of detection for kOH′ of 0.4 s-1.
Experiments described in Sect. 4 indicate that OH reactivities of up to
∼ 150 s-1 can be measured reliably with the instrument
described in this work, with the possibility for measurements of higher
reactivities described in Sect. 9. Replicates of kOH′ measurements at
fixed concentrations of CO and CH4 (Sect. 4 and Fig. 4) also enable
determination of the instrument precision. The majority of ambient
kOH′ measurements are < 35 s-1, for which the measurements under
controlled conditions using known concentrations of CO and CH4 indicate
a precision of ∼ 1 s-1. For reactivities of
∼ 110 s-1, which were observed in London during the
ClearfLo campaign (Sect. 7, Whalley et al., 2016), measurements using known
concentrations of CO and CH4 indicated a precision of ∼ 5 s-1. In the field, reactivities of up to ∼ 140 s-1
have also been measured, with fit errors of ∼ 6 s-1,
although the precision at higher reactivities is worse compared to lower
reactivities.
On average, the total uncertainty in measurements of kOH′ is 6 %,
with the uncertainty in kphys comprising ∼ 70 % of the
total uncertainty. When addition of O3 to the instrument is necessary
to improve the OH signal (at ambient mixing ratios of O3 of less than
10 ppb and during laboratory experiments and measurements of kphys), the
uncertainty associated with the dilution of the main flow contributes
∼ 25 % to the total uncertainty in kOH′.
Potential interferences
Potential interferences in measurements of kOH′ were investigated
through model simulations of OH decay traces under various scenarios to
investigate the possible effects of OH recycling, and subsequent impacts on
measurements of kOH′, through the reaction of ambient NO with any
HO2 or RO2 radicals that may be generated within the instrument.
The initial OH concentration in the instrument, following photolysis of
O3 by the 266 nm laser, is estimated at > 3 × 1010 cm-3 (Sect. 2.5), with no co-production of HO2
following photolysis of O3. However, there is production of HO2
from the reaction of OH with CO in the instrument, and of RO2 radicals
from the reactions of OH with VOCs. The HO2 radicals produced have the
potential to recycle OH through reactions with NO, which would lead to an
apparent reduction in the observed OH reactivity. In addition, there is
potential for production of HO2 from the photolysis of oxygenated
volatile organic compounds (oVOCs) in the instrument, with previous studies
indicating significant radical production following photolysis of oVOCs in
urban environments (Volkamer et al., 2010; Sheehy et al., 2010). During the
ClearfLo campaign in London (Sect. 7), the mean observed HCHO mixing ratio
was 9.3 ppb, potentially leading to production of ∼ 1 × 108 cm-3 HO2 following the photolysis of HCHO at
266 nm within the reactivity instrument. Other oVOCs, notably CH3CHO,
C3H7CHO, IPRCHO ((CH3)2CHO), C4H9CHO,
methacrolein (MACR) and methyl vinyl ketone (MVK) during the ClearfLo
campaign, are also potentially photolysed by the 266 nm laser in the OH
reactivity instrument, leading to production of HO2. For the mean oVOC
mixing ratios observed during ClearfLo, and literature absorption
cross-sections at 266 nm (Atkinson et al., 2004), there is potential for
production of ∼ 2.5 × 108 cm-3 HO2 in
the reactivity instrument, including that produced by photolysis of HCHO,
with CH3CHO representing the dominant oVOC photolysed. The effects of
HO2 and RO2 radicals produced by OH reactions within the
instrument, and of HO2 production following oVOC photolysis, were
therefore investigated over a range of NO mixing ratios.
Model simulations, using the numerical integration package Kintecus (Ianni,
2002), were initiated with a conservative estimate of the initial OH radical
concentration of 1 × 1010 cm-3 and a total OH reactivity
of 25 s-1 comprised of losses to CO (producing HO2), VOCs
(producing RO2) and NO2 (leading to loss of radicals from the
system). The relative contributions of CO and VOCs to the total OH
reactivity (7 and 68 %, respectively) were set to the average values
determined by Whalley et al. (2016) for field measurements in London
(Sect. 7). The contribution from NO2 was set to increase with increasing NO in
the model with the NO2: NO ratio set to the average ratio observed
during measurements in London (Bohnenstengel et al., 2015; Whalley et al.,
2016), and the model includes the reaction between HO2 and NO2 to
form HO2NO2, and the reverse reaction (which is slow and
negligible on the timescale of the reactivity measurement). Initial
concentrations of HO2 were, in separate model simulations, set to 0, 1 × 108 and 2.5 × 108 cm-3 to
investigate the potential effects of oVOC photolysis. Initial mixing ratios
of NO were varied from zero to 75 ppb, the maximum observed during field
measurements of kOH′ in London in 2012 (Whalley et al., 2016). The model
was run forwards in time and output for OH was analysed in an identical
manner to that applied to the OH decays measured by the reactivity
instrument to determine kOH′. Comparison of the values for kOH′
determined by fitting to the simulated OH decays to the total reactivity
input to the model thus enables assessment of any potential interferences.
Figure 5 shows the impact of NO on the OH reactivity determined by fitting
to the model output for OH. For model simulations with the initial
concentration of HO2 set to zero (i.e. no photolytic sources of
HO2), but with subsequent generation of HO2 following reaction of
OH with CO, it can be seen that the OH reactivity determined by fitting to
the model output for OH shows only a small deviation of from the true OH
reactivity in the model, through the recycling of OH through HO2+ NO. At 75 ppb of NO, the deviation from the true OH reactivity is
∼ 4 %, which is less than the overall uncertainty in
measurements of kOH′. For model runs which simulate photolytic
production of HO2 from oVOCs, with initial HO2 concentrations of
1.0 × 108 or 2.5 × 108 cm-3, it
can be seen that the impact of OH recycling through HO2+ NO is
dependent on the initial OH concentration. For initial OH concentrations of
109 cm-3, the impact of photolytic HO2 production is
potentially large, with a deviation of 18 % from the true reactivity for
an initial HO2 concentration of 1.0 × 108 cm-3 and a
deviation of 10 % for an initial HO2 concentration of 2.5 × 108 cm-3 at 75 ppb of NO. However, as discussed in Sect. 2.5,
the initial OH concentration produced in the instrument is expected to be
significantly higher than 109 cm-3. For an initial OH
concentration of 1010 cm-3, which is still a conservative estimate
of the initial OH concentration, the impact of any photolytically generated
HO2 is minimal, with the deviation from the expected OH reactivity at
75 ppb of NO similar to that determined in the absence of any photolytically
produced HO2 at ∼ 5 %. However, we note that it is not
possible to fully assess the impact of unmeasured or unknown oVOCs, and
recommend maintaining high initial OH concentrations in the reactivity
instrument in order to maximise the OH : HO2 ratio and thereby minimise
any potential interferences arising from production of HO2 in the
instrument.
Impact of NO on the ratio of the kOH′ fitted to model
simulations for OH to the kOH′ used as input for the model (i.e. the
equivalent of kobserved′/ktrue′). Black points show results for
model simulations with the initial HO2 concentration set to zero; blue
data points show results for model simulations with the initial HO2
concentration set to 1 × 108 cm-3; red data points show
results for model simulations with the initial HO2 concentration set to
2.5 × 108 cm-3; circles represent data points with an
initial OH concentration of 109 cm-3; diamonds represent data
points with an initial OH concentration of 1010 cm-3; filled data
points represent simulations with the initial RO2 concentration set to
zero; open data points represent simulations with an initial RO2
concentration of 1 × 1010 cm-3.
Model simulations were also performed with an initial RO2 concentration
of 1 × 1010 cm-3 to investigate the possible effects of
incompletely refreshing the gas sample in the reactivity instrument between
photolysis pulses, which could potentially lead to increased RO2
concentrations in the reaction cell through OH + VOC reactions. A reaction
between RO2 and OH was also added to the model, with a rate coefficient
of 1 × 10-10 cm3 s-1, as observed for
CH3O2+ OH (Bossolasco et al., 2014) and C2H5O2+ OH (Farago et al., 2015). Figure 5 shows that the impact of RO2
chemistry within the instrument is not expected to have a significant impact
on the observed OH reactivity, with the potential increase in observed OH
reactivity of 1 s-1 from an initial RO2 concentration of 1 × 1010 cm-3 and kRO2+OH= 1 × 10-10 cm3 s-1,
being less than the total uncertainty in ambient
measurements of kOH′. In addition, experiments in which the pulse
repetition frequency of the photolysis laser was varied between 0.1 and 1 Hz, thus varying the extent to which the gas sample was replaced between
laser pulses, did not show any significant change in the observed OH
reactivity. Despite the expectation that the impact of OH + RO2
chemistry on observations of kOH′ will be minimal, we include an
additional 1 s-1 uncertainty in ambient measurements of kOH′ to
reflect the potential for interferences owing to RO2 production in the
reactivity instrument. We note that the contribution to the uncertainty in
the total OH reactivity from RO2+ OH chemistry is itself subject to
significant uncertainty and the additional 1 s-1 uncertainty applied to
the total OH reactivity represents an upper limit which is included in order
to be thorough and complete.
We thus conclude that the reactivity instrument described in this work does
not suffer from significant interferences associated with potential
production of HO2 or RO2 within the instrument. We do note,
however, that measurements of OH reactivity using this instrument in
environments which may contain significant concentrations of oVOCs would
benefit from high initial concentrations of OH. We also note that any OH
decays observed during field or laboratory experiments that cannot be
reliably fitted by a single exponential function describing a first-order
loss for OH (Eq. 5) would be treated with caution and, where
appropriate, other fitting functions would be applied.
Field measurements
The laser flash photolysis OH reactivity instrument was deployed at the
North Kensington measurement site (51∘ 31′ N, 0∘ 12′ W) during the
Clean Air for London (ClearfLo) summer campaign in July and August 2012
(Bohnenstengel et al., 2014), with near-continuous measurements made from
the 21 July to 18 August 2012, alongside FAGE measurements of
OH, HO2 and RO2 radical concentrations. Measurements of O3,
CO, NO, NO2, HONO, VOCs and aerosol mass and composition were also made
at the site during the campaign.
Figure 6 shows the full time series of measured OH reactivity for the
campaign. The observed reactivity was highest for air masses that had
previously passed over central London (24–27 July (Julian days
206–209) and 8–10 August (Julian days 221–223)), with a
maximum reactivity of 116 s-1 recorded during rush hour on 24 July 2012. Measurements taken on the 25 July 2012 (Julian day 207)
are shown in Fig. 7 to highlight the capability of the instrument, and the
average diurnal profile for the campaign is shown in Fig. 8. A peak
reactivity, on average, of ∼ 27 s-1 was observed during
morning rush hour, with a minimum of ∼ 15 s-1 during the
afternoon and a second peak during evening rush hour. Detailed analysis of
these data, including model calculations using the Master Chemical Mechanism
constrained to observed concentrations of long-lived species, is described
by Whalley et al. (2016). The modelling study shows that the observed OH
reactivity can be reproduced by the model (to within 6 %) when larger
VOCs than those typically measured are included in the model, and
demonstrates the importance of oxidation intermediates and the role of heavy
VOCs, particularly biogenics, in controlling the total OH reactivity and the
oxidation budget in a megacity such as London.
Time series of OH reactivity observed during the Clean Air for
London (ClearfLo) campaign (21 July to 18 August 2012).
Uncertainties (represented by the shaded area) represent 1 σ combined
uncertainties from the fits to observed OH decays (Eq. 5),
determinations of kphys (Eq. 6) and uncertainties in the dilution
factor, f (Eq. 7).
Time series of OH reactivity observed during the Clean Air for
London (ClearfLo) campaign on the 25 July 2012 (Julian day 207).
Error bars represent 1 σ combined uncertainties from the fits to
observed OH decays (Eq. 5), determinations of kphys (Eq. 6)
and uncertainties in the dilution factor, f (Eq. 7).
Diurnal average for observed OH reactivity (15 min averages)
during the Clean Air for London (ClearfLo) campaign (21 July to
18 August). Shaded areas represent the measurement variability from
day to day during the campaign.
Field measurements of OH reactivity have also been made at a site at the
University of York (53∘ 56′ N, 1∘ 02′ W) from the
19 May to 16 June 2014, approximately 3 km south-east of the
centre of York and 2 km west of a major road, with a small wooded area
immediately to the east, and thus subject to anthropogenic emissions and
local biogenic emissions. Figure 9 shows the average diurnal during this
period. The observed OH reactivity was typically lower than that observed
during the ClearfLo campaign, with a maximum in the diurnal average of
∼ 6 s-1. Measurements of O3, CO, NO, NO2, VOCs
and were also made at the site during this period, alongside measurements by
a new instrument coupling an OH reactor to measurements of VOCs by gas
chromatography with time of flight mass spectrometry (GC-ToFMS) to aid
identification of any “missing” OH reactivity. Detailed analysis of the
results will be given in future publications.
Diurnal average for observed OH reactivity (15 min averages)
during in York (19 May to 16 June 2014). Shaded areas
represent the measurement variability from day to day.
Chamber measurements
The field instrument described above has also been modified in order to
interface to the Highly Instrumented Reactor for Atmospheric Chemistry
(HIRAC) to enable measurements of OH reactivity during VOC oxidation under
controlled conditions. For complex reaction mechanisms, the oxidation
pathway followed will have a characteristic time-evolution of the reactivity
as secondary products are generated, and measurement of OH reactivity and
comparison with a model prediction provides greater constraint for
experimental determination of the mechanism.
Observed OH reactivities for a fixed gas composition (for which
the expected OH reactivity is shown in red) (a) as a function of the total
flow rate through the reaction cell (slm = standard L min-1) and
(b) as a function of the pulse repetition frequency of the photolysis laser.
HIRAC is a 2.25 m3 stainless steel chamber equipped with UV photolysis
lamps to initiate photochemistry and a comprehensive suite of analytical
instrumentation, including gas chromatography (GC), Fourier transform
infrared (FT-IR) spectroscopy, cavity ringdown spectroscopy (CRDS) and
LIF-FAGE for radical measurements. Photolysis lamps within the chamber
enable initiation of photochemistry, and experiments can be conducted at
temperatures between 203 and 343 K and pressures up to 760 Torr (Glowacki et
al., 2007; Malkin et al., 2010; Winiberg et al., 2015).
Gas is sampled from HIRAC through 1/2′′ PTFE tubing at a flow rate
of 1 standard L min-1 and diluted with 5 standard L min-1 of ultra-high purity air immediately on
exiting the chamber, then diluted further with 9 standard L min-1 of humidified
ultra-high purity air and 1 standard L min-1 of ultra-high purity air passed over a low
pressure Hg lamp in order to generate O3, giving a total flow of 16 standard L min-1
and hence a dilution factor of 1:16. The diluted gas flow, containing
∼ 45 ppb O3, is then directed into the reaction cell of
the OH reactivity instrument, with instrument operation and analysis as
described in Sects. 2 and 3 (including the correction of observed
reactivity for dilution of sampled gas from the chamber using Eq. (7),
which is significant for these experiments in order to avoid measurement of
high reactivities (Sect. 9) and to reduce the volume of gas removed from
the chamber for the reactivity measurements). The uncertainty associated
with the dilution contributes ∼ 25 % to the total
uncertainty in kOH′, which is approximately 6–8 %.
Experiments were conducted to verify the sampling procedure by filling HIRAC
with air containing known concentrations of a reactive gas, with a
well-characterised rate coefficient for reaction with OH, followed by
measurement of the OH reactivity in the chamber. Determination of the
pseudo-first-order rate coefficients describing the OH loss for each of the
given reactive gas concentrations in the chamber enabled determination of
the bimolecular rate coefficient for reaction of the reactive gas with OH
for comparison with literature values, as for experiments described in Sect. 4. Figure 4c shows the bimolecular plots for experiments in which
the chamber was filled with n-butanol (n-C4H9OH) in air at total
pressures of 760 Torr and temperatures of 298 K. A bimolecular rate
coefficient of (8.5 ± 0.1) × 10-12 cm3 s-1 was
obtained, in comparison to the literature value of (8.5+3.5-2.5)× 10-12 cm3 s-1 (Atkinson et al.,
2004), thus indicating the validity of the sampling procedure. No dependence
of the observed reactivity was observed on the total flow rate through the
instrument, which was varied between 10 and 22 standard L min-1, or on the pulse
repetition frequency of the photolysis laser, which was varied between 0.1
and 1 Hz, as shown in Fig. 10.
The coupling of OH reactivity measurements to chamber studies will enable
detailed assessment of our understanding of the chemistry of secondary
products in complex oxidation mechanisms by providing increased constraint
on oxidation budgets during chamber experiments, and will be explored
further in future work.
Effects of averaging time and future improvements to sampling
Measurements have also been made in HIRAC to investigate the effect of the
averaging time on the measured OH reactivity. Figure 11 shows the observed
OH reactivities, for a given set of conditions, as a function of the
averaging time, showing successful measurements with an averaging time of 10 s and indicating the potential for further improvements for future
integration of ambient OH reactivity observations with flux measurements.
Observed OH reactivities, for a fixed gas composition (for which
the expected OH reactivity is shown in red), as a function of averaging
time, obtained using 15 mW of 308 nm probe laser power.
Experiments described in this work using known concentrations of reactive
gases have been able to reproduce recommended literature values for known
rate coefficients, indicating the validity of the technique described here
over the dynamic ranges investigated. However, recent work in Leeds has
shown that, at higher reactivities, observed kinetics can be influenced by
sampling issues related to the effects of velocity distributions on the
transport time of sampled gas from the pinhole nozzle to the point at which
fluorescence is excited and detected in the FAGE cell. The effects of the
velocity distributions on the time taken for sampled gas to travel from the
pinhole to the point of detection coupled with the true kinetics of the OH
decay can lead to underestimations of very high reactivities; this is
described in detail by Stone et al. (2016). Successful measurements with
known concentrations of CH4 at reactivities of ∼ 150 s-1 (Sect. 4, Fig. 4b), although scattered owing to the small
number of time points over which fast decays can be measured and poorer
precision compared to lower reactivities, indicate that such effects should
be minimal for the instrument described in this work, even for the highest
reactivities observed during the ClearfLo campaign (> 100 s-1). However, experiments incorporating OH reactivity measurements in
chamber studies, such as those described in Sect. 8, must also ensure that
the gas sampled from the chamber has been sufficiently diluted so as to
avoid the measurement of high reactivities directly.
Future work will incorporate a new inlet designed to minimise the distance
between the pinhole nozzle and the point of excitation fluorescence and
detection, ideally such that detection occurs within the supersonic jet
formed on expansion of the gas as it flows through the pinhole. The new
inlet will not only increase the dynamic range over which reactivity
measurements can be made, but sampling within the supersonic jet will also
lead to increased signal-to-noise and enable further reductions in the
averaging time required to achieve adequate signal-to-noise for measurements
with high time resolution.
Conclusions and outlook
In this work we present the design and characterisation of an instrument to
make field and chamber measurements of OH reactivity by laser flash
photolysis (LFP) coupled with laser-induced fluorescence (LIF) using the
FAGE technique. The LFP-LIF reactivity instrument, its operation and data
analysis have been described in detail. Ambient reactivity measurements
obtained during field campaigns in London, UK, and York, UK, have been
presented, and will be discussed further in future work. The instrument has
also been coupled to an atmospheric chamber, and preliminary results have
been shown to demonstrate the potential for reactivity measurements during
future chamber experiments.
Reactivity measurements have been made using an averaging time of 10 s,
indicating potential for integration of ambient OH reactivity observations
with flux measurements. Future development of the instrument will increase
the dynamic range over which measurements can be made and will enable
reduced averaging times owing to improvements in the signal-to-noise ratio.
Acknowledgements
This work was supported by the National Environment Research Council (NERC)
under grants NE/H003193/1, NE/J008990/1 and NE/L010798/1. D. Stone is grateful to
NERC for the award of an Independent Research Fellowship (NE/L010798/1). C. A. Brumby
is grateful to the Engineering and Physical Sciences Research Council
(EPSRC) for support. We are also thankful to the National Centre for
Atmospheric Science (NCAS), which is funded by NERC, for ongoing support.
Edited by: R. Volkamer
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