Introduction
Organic acids comprise a large fraction of the non-methane hydrocarbons in
the atmosphere (Khare et al., 1999). Volatile organic compounds (VOAs) are
known to have both anthropogenic and biogenic sources (Mellouki et al.,
2015). Globally biomass combustion and vegetation are known to be major
sources of formic and acetic acids (Kesselmeier and Staudt, 1999), but also
C3–C7 VOAs have been detected in biomass burning emissions
(Ciccioli and Mannozzi, 2007). Locally traffic and food cooking may be
important sources of C2–C7 VOAs (Ciccioli and Mannozzi, 2007). In
addition, VOAs are produced in ambient atmospheric air from the oxidation of
other volatile organic compounds (VOCs) (Orzechowska et al., 2005). These
reactions include the reaction of ozone with olefinic hydrocarbons, carbonyl
oxidation by hydroxyl radicals and radical recombination reactions between
acetyl peroxy radicals and peroxy radicals (Rosado-Reyes and Francisco,
2006). In addition, anaerobic processes such as composting are well-known
sources of VOAs (Brinton, 1998). Acids are usually metabolic byproducts of
anaerobic respiration and are breakdown products of more complex organic
compounds such as oils and fats present in raw waste. Several VOAs have been
found to have high odour potentials at concentrations as low as the ppb level
(Brinton, 1998).
The VOAs react with hydroxyl (OH) and nitrate radicals in the air or undergo
dry or wet deposition (Khare et al., 1999; Rosado-Reyes and Francisco, 2006).
Atmospheric fate of smaller VOAs is dominated by wet deposition through
scavenging by rain, cloud and fog water (Mellouki et al., 2015) and they can
strongly influence the acidity of the deposition and cloud water especially
in remote areas (Kesselmeier and Staudt, 1999). However, solubility decreases
with increasing carbon number and larger VOAs partition more strongly into
the gas phase and their main loss mechanism is gas-phase reaction with OH
radicals (Mellouki et al., 2015). Through the reactions with OH radicals VOAs
contribute to the oxidative capacity of the atmosphere and formation of ozone
(Mellouki et al., 2015).
The VOAs potentially play also a role in the production of secondary organic
aerosols by undergoing heterogeneous reactions on particles (Shen et al.,
2013; Tong et al., 2010) and by acting as organic coatings of aerosol
particles (Russell et al., 2002). Effective irreversible uptake of acetic,
propanoic and butanoic acids on mineral dust and ammonium nitrate films for
instance has been observed (Shen et al., 2013). Ammonium nitrate is a
well-known component of aerosol particles. Heterogeneous reactions of other
organic compounds on particles can also produce VOAs, which can be evaporated
back to the gas phase (Ervens et al., 2013). However, VOAs are expected to
occur mainly in the gas phase (Yatavelli et al., 2014). Kawamura et
al. (2000) found that C1–C10 monocarboxylic acids exist mainly in
free volatile forms and the particulate-phase fraction represented less than
10 % of the total organic acids in the air of Southern California in
October 1984.
There are several studies on the concentrations of gas-phase VOAs in ambient
air, but these investigations had predominantly focused on formic and acetic
acids (Chebbi and Carlier, 1996, and references therein). Kawamura et
al. (2000) and Nolte et al. (1999) studied gas-phase C1–C10 VOAs
in Los Angeles in 1984–1985 and 1993 by collecting on impregnated quartz
filters and Veres et al. (2011) used negative-ion proton-transfer chemical-ionisation mass spectrometry (PTR-MS) with 1 min time resolution to
study formic, acrylic, methacrylic, propanoic and pyruvic/butanoic acids in
the urban air masses in Pasadena, CA, in 2010. In addition, there are studies
on other gas-phase and particulate-phase organic acids. Terpenoic acids in
particles have been studied using liquid chromatographs (LC) with mass
spectrometers (Vestenius et al., 2014; Kristensen and Glasius, 2011), higher
carboxylic acids in gas phase simultaneously with ultrafine (≤ 50 nm)
particles using gas chromatograph (GC) and LC (Parshintsev et al., 2011) and dicarboxylic acids
in particles and gas phase using ion chromatographs with mass spectrometers
(e.g. Fisseha et al., 2006). However, these methods are labour intensive and
their time resolution is low. In addition a novel online system, filter inlet
for gas and aerosols (FIGAERO), has been used with a high-resolution
time-of-flight chemical-ionisation mass spectrometer for measurements of
formic and monoterpenoic acids in boreal forest (Lopez-Hilfiger et al.,
2014).
Even though ambient air studies of C2–C7 VOAs are scarce, there
are studies on anthropogenic sources of them: two such studies, Zahn et
al. (1997) and McGinn et al. (2003), measured emissions from pig and beef
production facilities. In their studies acids were collected on sorption
tubes and analysed later by GCs. However, the detection limits (DLs) for these
methods were too high for ambient air studies.
There is a paucity of knowledge of VOAs, other than formic and acetic acids
in gas phase, and this dearth of information is at least partly due to the
lack of sensitive enough measurement methods for detecting concentrations in
ambient air. In the present study we developed an in situ gas chromatograph–mass spectrometer (GC-MS) measurement
method for measuring C2–C7 monocarboxylic VOAs in gas phase with
2 h time resolution at ambient air concentration levels, which we used
to measure ambient air concentrations in a boreal forest site. Earlier these
types of in situ GC-MS methods have been used e.g. for measurements of
aromatic hydrocarbons and monoterpenes (Hakola et al., 2012; Hellén et
al., 2012a).
Retention times (RT), ions used for the quantitation (Tgt ion) and
qualitation (Q1 ion), blank values (BL), detection limits (DL), precision
(UPrec) and total expanded uncertainties (Utot) for
studied compounds at mean ambient air mixing ratios during the measurement
campaign at SMEAR II in June 2015.
RT (min)
Tgt ion (Th)
Q1 ion (Th)
BL (pptv)
DL (pptv)
Uprec ( %)
Utot ( %)
Acetic acid
31.3
59.9
–
156
130
7
16*
Propanoic acid
34.4
73.9
72.9
5
23
15
32
Isobutyric acid
35.4
72.9
87.9
–
16
–
–
Butanoic acid
37.3
59.9
72.9
3
7
19
39
Isopentanoic acid
38.3
59.9
86.9
–
1
–
–
Pentanoic acid
40.0
59.9
72.9
–
5
38
76
Isohexanoic acid
41.6
57.0
73.0
–
13
–
–
Hexanoic acid
42.5
59.9
72.9
–
7
20
40
Heptanoic acid
44.7
59.9
72.9
–
19
–
–
Benzene
8.4
78.0
77.0
6
20
53
108
Toluene
12.4
91.0
92.0
8
9
35
72
* Acetic acid has an additional error source which was not taken into account
in these calculations (see main text).
Experimental
GC-MS sampling and analysis
A method for measurements of gas-phase VOAs in air was developed for an in
situ thermal desorption unit (TD; Unity 2 + Air Server 2, Markes
International Ltd, Llantrisant, UK) with a gas chromatograph (Agilent 7890A,
Agilent Technologies, Santa Clara, CA, USA) and a mass spectrometer (Agilent
5975C, Agilent Technologies, Santa Clara, CA, USA). Samples were taken every
other hour. In the 3 m long fluorinated ethylene propylene (FEP) inlet
(0.32 cm I.D.) an extra flow of 2.2 L min-1 was used to avoid losses
of the compounds on the walls of the inlet tube. Samples were collected
directly from this ambient air flow into the cold trap (U-T17O3P-2S,
Markes International Ltd, Llantrisant, UK) of the TD. The sampling time was
60 min and the sampling flow through the cold trap 30 mL min-1. A
schematic diagram of the sampling system can be found as supplement Fig. S1.
All the lines and valves in the TD were kept at 200 ∘C. Excess water
can block the cold trap and therefore it was removed by keeping the
hydrophobic cold trap at 25 ∘C during sampling and using a
post-sampling line purge (10 min, 30 mL min-1), post-sampling trap
purge (10 min, 20 mL min-1) and pre-trap fire purge (10 min,
10–11 mL min-1). During desorption the cold trap was heated to
300 ∘C for 3 min and flushed with a helium flow of
10–11 mL min-1. The polyethylene glycol column used for separation
was the 30 m DB-WAXetr (J&W 122-7332, Agilent Technologies, Santa Clara,
CA, USA) with an inner diameter of 0.25 mm and a film thickness of
0.25 mum. Helium (> 99.9996 %, Linde AG, Pullach,
Germany) was used as a carrier gas. During the analysis the GC oven was first
kept at 50 ∘C for 10 min, heated to 150 ∘C with the rate
of 4 ∘C min-1 and then to 250 ∘C with the rate of
8 ∘C min-1, where it was kept for 5 min. The total run time
was 52.5 min.
The system was calibrated using liquid standards in Milli-Q water injected
into adsorbent tubes filled with Tenax TA (60/80 mesh, Supelco, Bellefonte,
USA) and Carbopack B (60/80 mesh, Supelco, Bellefonte, USA). After
injection the tubes were flushed with nitrogen (HiQ N2
6.0 > 99.9999 %, Linde AG, Pullach, Germany) flow of
80 mL min-1 for 10 min to remove the water. Standard tubes were
heated to 300 ∘C by the TD and desorbed samples
were directed to the cold trap in helium flow and analysed using the same
method as for the samples, which were collected directly to the cold trap
from ambient air. Fresh standards were prepared from a volatile free acid
mixture (CRM46975, Supelco, Bellefonte, USA) 1 day before the analysis.
Used quantitation and qualification ions are listed in the Table 1. The
stability of the mass spectrometer was followed by running gaseous field
standards containing aldehydes and aromatic hydrocarbons after every 50th
sample taken and using tetrachloromethane as an “internal standard”. The
concentration of tetrachloromethane in ambient air is stable, and thus it was
possible to detect sampling errors or shifts in calibration levels by
following its concentration.
Test site and ambient air measurements
An ambient air sampling campaign was conducted at SMEAR II forest research
station in Hyytiälä (61∘51′ N, 24∘17′ E;
181 m a.s.l.), Finland, between 11 and 27 June 2015. The SMEAR II station
is a dedicated facility for studies of forest ecosystem–atmosphere
associations (Hari and Kulmala, 2005). The measurement station is located in
a Scots pine stand that is approximately 50 years old. The continuous
measurements at that location include leaf, stand and ecosystem-scale
measurements of greenhouse gases, VOCs, pollutants (e.g. O3, SO2,
NOx) and many different aerosol properties. In addition, a full suite of
meteorological measurements of the site is continuously recording.
Proton-mass-transfer time-of-flight mass spectrometer
(PTR-TOFMS) measurements
During the measurement campaign at SMEAR II a PTR-TOFMS (Ionicon Analytik
GmbH, Innsbruck, Austria; Graus et al., 2010; Jordan et al., 2009) was run in
parallel with in situ GC-MS. The PTR-TOFMS instrument was operated at a drift
tube pressure of 2.3 mbar and a drift tube voltage of 600 V. These settings
resulted in an E/N of 130 Td, where E is the electrical field strength
and N the gas number density. The air was sampled at a flow of
20 L min-1 through a 3.5 m PTFE inlet, which had an inner diameter of
4 mm. A total flow at the rate of 500 mL min-1 went to the instrument
via a three way valve (type: 6606 with ETFE, Bürkert GmbH & Co.,
Ingelfingen, Germany), 10 cm of 1.6 mm (I.D.) PTFE and 10 cm of 1 mm
(I.D.) PEEK tubing. There, 30 mL min-1 of the flow was sampled and the
remainder served only as a by-pass flow in order to decrease the response
time and wall losses. A 20 min background measurement was performed three
times a day, during which the air from the 3.5 m inlet was let through a
custom build catalytic converter. The instrument was calibrated every
2–3 weeks, as described in Schallhart et al. (2016). The calibration gas did
not contain acetic acid or propanoic acid, and therefore sensitivity was
estimated. As both molecules fragment, when measured with PTR-MS (von
Hartungen et al., 2004), the sensitivities were estimated to be 50 % of
the acetone sensitivity. The instrumental background for acetic acid was
clearly correlated with ambient measurements. This can be explained by a
memory effect (of the inlet and/or instrument) of those compounds. This has
already been observed by de Gouw et al. (2003). Therefore, the reported
concentrations of acetic acid are underestimated, as an excessively high
background signal had been subtracted. The mean DLs for acetic
and propanoic acids during the campaign were 34 and 8 pptv respectively.
Calculation of the uncertainties
Total uncertainty of the measurements (Utot) was calculated from
precision (Uprec) and systematic errors (Usys):
Utot2=Uprec2+Usys2.
The precision was calculated by
Uprec=13DL+RSD×χ,
where DL is the detection limit, RSD is the relative standard deviation between the
samples, when known amounts of acids were injected into the N2 flow, and
χ is the mean mixing ratio of the acid in ambient air during the
measurement campaign at the SMEAR II site. The DL is the
dominant factor for low mixing ratios whereas the secondary term used
describing reproducibility of the instrument and this becomes more important
for higher mixing ratios.
The systematic error includes uncertainty of the standard solution
(Ustdmix) given by the producer, uncertainty of the standard
preparation (Ustdprep) estimated for the equipment that was used,
uncertainty of the sample volume (Uvol) that was obtained for the
uncertainty of the mass flow controller, errors due to blank corrections
(Ublank) and further instrument problems (e.g. error due to
correction of the drift of the calibrations using tetrachloromethane,
Udrift):
Usys2=Ustdmix2+Ustdprep2+Uvol2+Ublank2+Udrift2.
Results
Method validation
Peaks of the different acids were separated very well in the chromatograms
(Table 1 and Fig. S2 in the Supplement). Background values of VOAs in the
system were estimated by sampling clean nitrogen (HiQ N2
6.0 > 99.9999 %, Linde AG, Pullach, Germany) using the same
method as used for the samples. Blank values were obtained for acetic,
propanoic and butanoic acids (Table 1). The DLs were defined as
3 times the standard deviation of the blank values or alternatively as
signal-to-noise ratio (3:1). Detection limits varied between 1 and
130 pptv and were highest for acetic acid due to the high blank values.
Some memory effect was found for all studied acids after running calibration
tubes and standard gases. The calibration standards contained amounts that
corresponded to ambient mixing ratios up to 10 000 pptv and the field gas
standard up to 40 000 pptv, whereas the mean ambient mixing ratios varied
between 1 and 1160 pptv. Blank samples of clean nitrogen (HiQ N2
6.0 > 99.9999 %, Linde AG, Pullach, Germany) run immediately
after field gas standard showed mixing ratios < 3 % of the ones
from gas standard (Fig. S4 in the Supplement). In the ambient data increased
mixing ratios were detected in five samples after running field gas standard
(Fig. S5 in the Supplement). Therefore, the first five samples after
calibration checks were always disregarded. Using lower concentration for the
calibrations would be expected to solve this issue. In ambient samples
concentration variations are not as high and therefore these memory effects
are not expected to be as significant.
The desorption efficiency (DE) of the cold trap was determined by
redesorption at a higher temperature (320 ∘C) after running a
sample. The amount of the sample found in the first desorption was compared
to the total amount of the sample. DEs from the cold trap were
> 98 % for all compounds.
The precision (Uprec) was checked by injecting known amounts of
acids into the N2 (HiQ N2 6.0 > 99.9999 %,
Linde AG, Pullach, Germany) flushed through the inlet lines. Mixing ratios
varied between 0.1 and 1994 ppbv. The precision calculated for the ambient
mixing ratios found at SMEAR II using the Eq. (2) was found to vary from 7 to
38 % for the acids of interest. The total expanded uncertainties of the
studied acids varied between 16 and 76 % (Table 1). The highest relative
uncertainties were found for the compounds with mixing ratios closest to the
DLs. The uncertainties for benzene and toluene were as high as
108 and 72 % respectively. Earlier studies that used the same instrument
(Kajos et al., 2015) found the relative analytical uncertainties of benzene
and toluene to be much lower (4 and 5 % respectively). However, the
present study found the mean mixing ratio of benzene was at the DL (20 pptv) and the mean mixing ratio of toluene was very close to it.
The relative uncertainties for these low values are expected to be high due
to high influence of DL in Eq. (2).
The real uncertainty of the acetic acid in these measurements is expected to
be higher than that reported due to calibration issues mentioned above. The
precision for the acetic acid was good (7 %), but acetic acid has an
additional systematic error, which was not found for the other compounds
studied. There was a high background level of acetic acid in the
calibrations, which was probably due to the preparation of the calibration
solutions and adsorbent tube standards that caused non-linearity of the
calibration curve. This high background concentration was estimated by
analysing blank adsorbent tubes, i.e. tubes that had been prepared with only
the solvents but without any acetic acid. A better calibration method such as
one that uses the permeation device could remove this source of uncertainty.
Mixing ratios (pptv) of volatile organic acids (C2–C6) and
trace gases together with meteorological parameters at SMEAR II station in
Hyytiälä, Finland.
It is expected that a proportion of acids will be lost in the inlet tubes;
therefore inlet loss estimation tests were conducted using a permeation oven
(FlexStream Base, Kin-Tek laboratories, Inc., Laurel La Marque, USA) with a
nitrogen flow of 0.50 or 0.75 L min-1. The permeation vials were
filled with the studied acids and placed into the oven at 40 ∘C.
These tests were performed both with dry and humid nitrogen flow and the
concentrations of acids varied between 0.2 and 1994 ppb (Table 2). Four
different configurations were tested: (1) one with humidified N2 flow of
0.75 L min-1 and 4 m long FEP tube (0.32 cm I.D.) at room
temperature, (2) one with humidified N2 flow of 0.75 L min-1 and
1 m long stainless steel tube (0.175 cm I.D.) heated to 120 ∘C and
used for ozone removal in terpenoid sampling (Hellén et al., 2012b),
(3) one with humidified N2 flow of 0.75 L min-1 and 3 m long FEP
tube heated to 120 ∘C and (4) one with dry N2 flow of
0.50 L min-1 and 3 m long FEP tube heated to 120 ∘C. Samples
were taken before and after the inlets. The comparison results for toluene
are included in Table 2. The results for all configurations were acceptable
(within ± 20 %). The first configuration was chosen for further
tests and for ambient air sampling. The ozone removal tube was not selected
because the studied acids are not reactive towards ozone, but the test was
conducted for the situations where ozone reactive compounds (e.g.
sesquiterpenes) can be measured using the same system.
Recoveries (%) from the inlets together with amounts and mixing
ratios (vmr) used in the tests.
Amount
vmr
1
2
3
4
ng sample-1
ppbv
%
%
%
%
Acetic acid
8.6
4.0
101
104
98
97
Propanoic acid
1.7
0.6
105
107
109
-
Isobutyric acid
6470
1992
99
100
112
90
Butanoic acid
109
16
96
101
108
95
Pentanoic acid
0.8
0.2
87
98
123
94
Hexanoic acid
16
3.6
104
107
93
98
Toluene
15
4.6
100
101
105
97
1: 4 m FEP tube (0.32 cm I.D.) at room
temp, humidified N2 flow 0.75 L min-1 2: 1 m stainless
steel tube (0.175 cm I.D.) at 120 ∘C, humidified N2 flow
0.75 L min-1. 3: 3 m FEP tube (0.16 cm I.D.) at
120 ∘C, humidified N2 flow 0.75 L min-1. 4:
3 m FEP tube (0.16 cm I.D.) at 120 ∘C, dry N2 flow
0.75 L min-1.
Mixing ratios (pptv) of volatile organic acids at SMEAR II station
in Hyytiälä, Finland, between 11 and 27 June 2015 and in earlier
studies.
Present study
Boreal forest
Nolte et al. (1999)
Kawamura et al. (2000)
Veres et al. (2011)
pptv
Mean
Min
Max
Background
Urban
Urban
Rural
Acetic acid
1160
910
1520
720
6560
290–2640
–
Propanoic acid
81
< DL
130
30
550
29–211
0–6100
Isobutyric acid
< DL
< DL
20
6
80
5–18
–
Butanoic acid
40
20
100
3
160
9–50
0–240
Isopentanoic acid
1
<DL
4
–
–
–
–
Pentanoic acid
10
< DL
20
0
60
3–20
–
Isohexanoic acid
< DL
< DL
< DL
–
–
–
–
Hexanoic acid
20
< DL
80
4
90
4–32
–
Heptanoic acid
< DL
< DL
< DL
0
30
2–30
–
Benzene
20
< DL
90
–
–
Toluene
20
<DL
70
–
–
Results from ambient air measurements
Mixing ratios in a boreal forest
The highest mixing ratios were measured for acetic acid (Table 3). The mixing
ratios of isobutyric, isohexanoic and heptanoic acids stayed below their
DLs during the whole campaign. The mixing ratios generally
decreased with increasing carbon number except for hexanoic acid. Hexanoic
acid was more abundant than pentanoic acid. Such a VOA profile was also seen
in the measurements of Kawamura et al. (2000) but in the urban air of
Southern California in 1984.
Hexanoic acid had the highest relative variations in mixing ratios (Fig. 1).
The variation in sources and source strengths together with higher reactivity
of hexanoic acid may explain this. Reaction rates of VOAs with hydroxyl
radicals increased with increasing carbon number (Mellouki et al., 2015) and
trees and other vegetation are known to produce stress-induced emissions of
green leave volatile organic compounds which are aldehydes, esters and
alcohols with 6-carbon atoms (Hakola et al., 2001; Scala et al., 2013).
Oxidation of these compounds could be a source of hexanoic acid. However,
based on the current knowledge even direct emissions of hexanoic acid cannot
be ruled out.
Butanoic acid emissions peaked (100 pptv) on 14 June (Fig. 1). This peak
occurred at the same time as the peak of 1-butanol (2500 pptv). 1-Butanol was
being used at the same site in other instruments including particle counters.
During malfunctions of these instruments 1-butanol may have been released
into the ambient air. Butanoic acid was expected to be produced in the
oxidation reactions of 1-butanol in the atmosphere. Maximum mixing ratio
occurred in the middle of the night (01:30–02:30 local time, LT), which gave
an indication that butanoic acid has been produced from nitrate radical
reactions. Lower boundary layer present during the nigh may also explain
higher night-time mixing ratios, but since this clear peak was not seen for
the any other compounds, we believe that there was an additional butanoic
acid source.
Acetic acid was measured at the same site in August 2001 using an annular
denuder system and IC analysis (Boy et al., 2004). The diurnal means of
concentrations of acetic acid varied between 166 and 1666 pptv, which is
close to values measured in this present study in June 2015. Information on
mixing ratios of VOAs higher than C2 is scarce. Kawamura et al. (2000)
measured C1–C10 VOAs in Southern California in October 1984 and
their mixing ratios were at similar levels as found in our measurements in
the present study (Table 3). However, Veres et al. (2011) found clearly
higher mixing ratios using PTR-MS in June 2010 in Pasadena, California. The mean mixing ratio
of propionic acid was 1740 pptv whereas it was only 81 pptv in our study
and 29–211 pptv in the study of Kawamura et al. (2000). Veres et al. (2011)
found evidence that organic acids were photochemically and rapidly produced
from urban emissions transported from Los Angeles. Nolte et al. (1999) also
detected much higher mixing ratios of C2–C10 acids at the four
urban sites in Southern California in September 1993, but mixing ratios found
at San Nicolas Island (background) were lower than in our measurements. The
vegetation in Southern California is very different compared to our boreal
site and differences in primary and secondary sources may explain the
differences.
Mean diurnal variation of the mixing ratios with standard deviations
(error bars) at SMEAR II between 11 and 27 June 2015.
Comparison of mixing ratios (pptv) measured by GC-MS and PTR-TOFMS
at SMEAR II in June 2015.
Diurnal variation of mixing ratios
Acetic and propanoic acids had the highest mixing ratios during the day and
lowest during the night (Fig. 2). Hexanoic acid had the opposite diurnal
variation with the maximum concentration occurring during the night. Butanoic
and pentanoic acids did not show any clear diurnal cycle. Direct emissions
from vegetation and production in photochemical reactions are expected to be
highest during the day when there is more light and higher temperature
(Gabriel et al., 1999; Finlayson-Pitts and Pitts, 2000). However, reactions
of VOAs and mixing are also faster during the day and this phenomenon, in
addition to the lower boundary layer present during the night, may explain
the high night-time concentrations of faster reacting VOAs. High night-time
concentrations have also been measured at the site for monoterpenes even
though their emissions are clearly highest during the day (Hakola et al.,
2012). During the night VOAs may also be produced from ozone and nitrate
radical reactions (Monks, 2005; Khare et al.. 1999).
Similar diurnal pattern of propionic acid with daytime maxima was also found
in the study of Veres et al. (2011) in California in June 2010. Those authors
found daytime maxima for pyruvic/butanoic acid, but in the present study we
found that butanoic acid did not have any clear diurnal variation.
Comparison with other trace gases and meteorological
parameters
Data for the other trace gases and meteorological parameters (Fig. 1) were
obtained from the SmartSmear AVAA portal (Junninen et al., 2009; Williams et
al., 2011). All data are for the height of 4.2 m except wind speed, which is
for 8.4 m. Acetic acid had a weak correlation with temperature
(R2 = 0.35) and propanoic acid with ozone (R2 = 0.25).
Correlation plots are presented as Fig. S6 in the Supplement. Hexanoic acid
concentration correlated with toluene (R2 = 0.42), α-pinene
(R2 = 0.42) and CO (R2 = 0.52). The highest hexanoic acid
concentrations were measured during nights with low wind speed. This
indicates that mixing ratios of shorter chain VOAs were more dependent on
photochemical production or temperature and light-dependent emissions,
whereas the diurnal cycle of longer chain VOAs were more strongly affected by
reactivity and mixing of air.
Comparison with PTR-TOFMS data
The PTR-TOFMS measured acetic and propanoic acids, whereas the other VOAs
remained below their respective DLs. The variations of the
mixing ratios were quite similar for both instruments (Fig. 3). The
correlation was relatively good when the mixing ratios of acetic acid
(GC > 1300 ppt, R2 = 0.78) and propanoic acid
(GC > 80 ppt, R2 = 0.52) were highest. Low correlations
with lower values were expected due to the high uncertainties for both
instruments when the levels of the VOAs being analysed were close to their
respective DLs.
The mean mixing ratios of acetic and propanoic acids measured by GC-MS were
5.7 and 2.3 higher than those measured by the PTR-TOFMS method. The main
reason for the large discrepancy for acetic acid is the overestimation of the
background due to memory effects in the PTR-TOFMS as discussed in Sect. 2.3.
The measurements were conducted in separate containers but were close to
each other (5 m). Therefore, some differences were expected, but not large
differences. The overall variations of the signal of the two instruments are
comparable, and thus the main difference between them seems to be due to the
background problem or problems in calibrations of the instruments. The
calibration curve of acetic acid for the GC-MS measurements suffered from
high background at low levels. More accurate measurements of these compounds
require that better calibration methods be developed. In addition to this,
using different inlet line and valve materials could help to reduce the
memory effect and lower the background.
Conclusions
A novel in situ GC-MS method for the quantification of volatile organic
acids was evaluated. Despite the relatively high uncertainty, the method is
uniquely capable of detecting VOAs at low concentrations with only a
2 h time resolution. Experimentally determined recoveries of VOAs from
FEP and heated stainless steel inlets were acceptable and different VOAs
were fully desorbed from the cold trap and were well separated in the
chromatograms. Detection limits varied between 1 and 130 pptv between
individual VOAs.
The mixing ratios of acetic and propanoic acids measured with the novel
GC-MS method were compared to PTR-TOFMS data. Similar variations of mixing
ratios were captured by both analytical setups, but absolute levels
deviated significantly. High background concentration was a problem for both
instruments and especially for the measurement of acetic acid by the
PTR-TOFMS method. Replacing the inlet line and valve materials could improve
the situation. A better calibration method, especially for acetic acid in
GC-MS measurements, would also improve the quality of the data for acetic
acid.
The system performed well for ambient air measurements at a boreal forest
site. We found that acetic acid had the highest mixing ratios, but hexanoic
acid concentrations varied the most. The lightest VOAs (acetic and propanoic
acids) had their maxima in the afternoon, whereas hexanoic acid had opposite
diurnal variation.
This novel in situ TD-GC-MS method will allow us to study diurnal and
seasonal variations of VOAs in ambient air and produce new data on, which
will benefit atmospheric chemistry and new particle formation studies.