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2RISØ National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark. 3University of Odense, Campusvej 55, DK-5230 Odense M, Denmark. (Received: 18 April ...



Journal of Atmospheric Chemistry 28: 195–207, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

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Carboxylic Acids: Seasonal Variation and Relation to Chemical and Meteorological Parameters K. GRANBY1 , A. H. EGELØV1 , T. NIELSEN2 and C. LOHSE3 1

National Environmental Research Institute, PO Box 358, DK-4000 Roskilde, Denmark RISØ National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark 3 University of Odense, Campusvej 55, DK-5230 Odense M, Denmark 2

(Received: 18 April 1996; in final form: 6 March 1997) Abstract. Formic and acetic acid measured as daily averages in 1993–1994 show equal and highly correlated concentrations up to 3 ppb in the summer (May–August). In the winter (October–March) the formic acid/acetic acid ratio was 0.6 and the formic acid concentrations were usually below 1 ppb. In winter the carboxylic acids correlate with O x , NOy , SO2 and particulate sulphur. The main sources are suggested to be ozonolysis of anthropogenic alkenes and reactions between peroxyacetyl radicals and RO2 radicals. In spring–summer the carboxylic acids correlate with O3 , Ox , HNO3 , PAN, NOy , SO2 , particulate sulphur and temperature. In addition to the sources of the winter a contribution from ozonolysis of biogenic alkenes is likely. Quite similar formic acid/acetic acid ratios for all wind directions suggest that the source(s) are atmospheric oxidation processes distributed over large areas. The highest concentrations occurring for winds from east to south and the correlation with e.g., particulate sulphur indicate chemical production in polluted air masses during long range transport. Key words: carboxylic acids, formic acid, acetic acid, ambient air.

1. Introduction Although formic and acetic acid are ubiquitous constituents of the troposphere their sources are still not adequately explained. Many different sources have been proposed or demonstrated. Direct emissions from the sea (Graedel and Weschler, 1981), the soil (Sanhueza and Andreae, 1991; Bingemer et al., 1991), the vegetation (Kesselmeier et al., 1994), ants (Graedel and Eisner, 1988), biomass burning (Lefer et al., 1994) or anthropogenic combustion sources like vehicles and stationary sources (Kawamura et al., 1985; Talbot et al., 1988) may occur. Formic and acetic acid can also be produced from various oxidation processes in the troposphere. OH radical reactions with aromatic hydrocarbons from carboxylic acids (Bierbach et al., 1994). In clouds hydrated formaldehyde can react with OH to produce formic acid, which then equilibrates with the gas phase (Chameides and Davies, 1983). Oxidation of anthropogenic or biogenic alkenes with ozone is an important source of the carboxylic acids (Grosjean, 1992; Hatakeyama and Akimoto, 1994). The formation proceeds through ozonolysis of an alkene forming a Criegee intermediate which quickly rearranges to a carboxylic acid. Ethene, e.g., in the presence of water vapour, has been demonstrated to produce formic acid

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(Horie et al., 1994). The reaction with ozone, which is rate determining, is slowest for ethene (k = 1:7  10,18 cm3 molecule,1 s,1 ) and relatively fast for 2-methylbutene (k = 493  10,18 cm3 molecule,1 s,1 ) (Atkinson, 1990). The formation time ranges from hours to days. An additional mechanism of acetic acid production is the reactions of peroxyacetyl radicals with RO2 radicals (Moortgat et al., 1989; Madronich et al., 1990). The formation time is in the order of hours. The main sinks of formic and acetic acid in the planetary boundary layer are wet and dry deposition. Hartmann et al. (1991) estimated lifetimes of >five days for the tropical dry season. The present investigation includes the seasonal variation of formic acid and acetic acid based on daily mean measurements of 444 days in parts of 1993 and in 1994. The measurements are compared to O3 , Ox , NOy , SO2 , particulate sulphur and meteorological parameters and for the spring 1993 to additional photochemical products. Results of 12-hour measurements from the summer of 1995 are also presented. 2. Methodology The measurements were made at the Danish Tropospheric Ozone Research (TOR) station Lille Valby, 30 km west of Copenhagen in an agricultural area at 12070 3400 E, 55 410 1400 N, 15 m a.s.l. A north–south highway with a traffic density of a few thousand cars per day passes 1.5 km west of the station. The influence from industrial sources is limited. The carboxylic acids were collected according to Grosjean and Parmar (1990). A filter pack with a teflon filter, 1 m, 47 mm diameter in front of 2 Na2 CO3 impregnated cellulose filters (Whatman 41) was used. Na 2 CO3 was used for impregnation because it only retains a minor fraction of PAN (Grosjean and Parmar, 1990). To lower the blank values, the cellulose filters were washed 2  15 minutes by gently shaking with water before they were impregnated by shaking with a 2% Na2 CO3 in ethanol-water (3:1) and drying. The sampling (24 hours in 1993–1994: 800– 800 h local winter time and 12 hours in 1995: 700–1900 h and 1900–700 h) was performed by a sampler for 8 filter packs including one blank. The samples were changed twice a week. Recovery tests have shown that the formate and acetate do not degrade during that period. The sampling flow was 40  0:5 l min.,1 . Mostly 90–100% of the formic and acetic acid were trapped on the first of the two serial Na2 CO3 filters but both filters were analyzed. After sampling the filters were kept frozen at –18  C until chemical analyses were performed using an ion chromatograph with a Dionex AS-9 column and guard column, a micromembrane suppressor and a conductivity detector. The eluent was 1 mM Na2 B4 O7 at 1 mL min.,1 and the regenerent 0.0025 M sulphuric acid at 5 mL min.,1 . The precision of the sampling method was 0.07 ppb for formic acid and 0.06 ppb for acetic acid (n = 11). The detection limits based on three standard deviations of the blanks depended on the season with 0.05 ppb during the winter

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and 0.1 ppb during the summer. However in April–June 1993, the detection limit rose to 0.2–0.3 ppb. Measurements of nitric acid and nitrate were made by 24 hours collection on NaCl coated denuders (0.4 mm i.d.  50 cm, flow 1 l min.,1 ) followed by NaF coated filters and analyzed by ion chromatography with UV-detection. PAN was measured by gas chromatography with electron capture detection. O3 was measured by a UV-absorption monitor, NO and gas NOy by a two chamber chemiluminescence monitor, SO2 by UV-fluorescence and temperature, global radiation, precipitation and wind direction with conventional micro-meteorological instruments. Particulate sulphur (mostly as SO24, ) was sampled 30 km NNE of Lille Valby and analyzed by PIXE (proton induced X-ray emission spectroscopy). Ox was estimated as ‘O3+ gas (NOy –NO)’ where gas (NOy –NO) is an approximation of NO2 (Nielsen et al., 1995). 3. Results and Discussion 3.1. SEASONAL VARIATION The seasonal variations of formic acid, acetic acid, ozone and temperature 1993– 1994 (Figure 1) show that the highest carboxylic acid concentrations (2–3 ppb) coincide with relatively high ozone concentrations and temperatures. The carboxylic acids show the highest monthly means in spring and summer (Table I). On an annual basis the carboxylic acids correlate with O3 , Ox , temperature, global radiation, SO2 and particulate sulphur (Table II). In the summer (May–August) the formic and acetic acid concentrations are similar and highly correlated (r = 0:95 (the stars symbolize the 95%(), 99%( ) and 99.9%( ) confidence levels.)) which indicate a common source (Figure 2a, orthogonal regression plot). Even if the results are divided in wind sectors, they show similar formic acid/acetic acid ratios for each sector (Figure 3). The mean concentrations of formic acid is 0:8  0:5 ppb and of acetic acid 0:8  0:6 ppb. The carboxylic acids correlate with ozone, Ox , temperature and also with gas NOy , SO2 and particulate sulphur (Table II) which generally show elevated concentrations in polluted air masses i.e., air arriving from eastern-Central Europe. During the period April–June 1993 the carboxylic acids also correlate with the photochemical products HNO3 and PAN but not with H2 O2 . 12 hour sampling measurements during May–July 1995 show that the daytime (700–1900 h) concentrations are roughly the double of the nighttime (1900–700 h) concentrations. The nighttime loss may be due to deposition of the carboxylic acids in the nocturnal boundary layer. The dry depositions were estimated by Hartmann et al. (1991) to be 0:64  0:20 cm s,1 for formic acid and 0:50  0:22 cm s,1 for acetic acid. Although not significantly different our nighttime formic to acetic acid ratio was 14% lower than the daytime ratio indicating a higher nighttime deposition of formic acid.

1 2 3 4 5 6 7 8 9 10 11 12

Month

0.11 (0.00) n

0.37 (0.2) 0.82 (0.6) 1.21 (0.7) 0.62 (0.4)

Formic acid 93

= 20

0.45 (0.3) 0.24 (0.1) 0.63 (0.5) 0.64 (0.4) 0.44 (0.3) 1.14 (0.6) 0.61 (0.4) 0.30 (0.2) 0.26 (0.2) 0.17 (0.1) 0.13 (0.1)

94

0.52 (0.4) n 0.37 (0.2) n 0.81 (0.5) n

0.18 (0.1)

95

= 17 = 25 = 12 0.17 (0.1) n

0.65 (0.4) 0.94 (0.6) 1.24 (0.6) 0.54 (0.3)

Acetic acid 93

= 20

0.73 (0.4) 0.37 (0.2) 0.82 (0.5) 0.73 (0.5) 0.43 (0.3) 1.06 (0.5) 0.66 (0.4) 0.39 (0.2) 0.42 (0.2) 0.27 (0.2) 0.23 (0.1)

94

0.47 (0.4) n 0.37 (0.2) n 0.93 (0.5) n

0.42 (0.3)

95

= 17 = 25 = 12

Table I. Monthly mean formic acid and acetic acid concentrations (ppb), standard deviations (ppb) (in brackets) and number of days (n) if only a part of a month is measured

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O3 Ox NOy S SO2 Temperature Radiation HNO3 Total NO, 3 PAN H2 O2 MSA

=

A 0.42 0.63 0.21 0.54 0.26 0.41 0.46 – – – – –

All year F 0.55 0.71 0.07 0.47 0.17 0.59 0.54 – – – – –

=

0.59 0.72 0.30 0.53 0.46 0.60 0.25 – – – – –

Summer F 0.54 0.68 0.36 0.50 0.49 0.52 0.22 – – – – –

A

=

–0.01 0.26 0.26 0.68 0.53 –0.43 0.27 – – – – –

Winter F –0.11 0.19 0.40 0.63 0.44 –0.49 0.35 – – – – –

A

0.58 0.70 0.35 – 0.57 0.60 – 0.70 0.33 0.62 0.00 –0.40

0.50 0.68 0.40 – 0.61 0.48 – 0.68 0.42 0.61 0.02 –0.40

Spring 1993 F A

=

Table II. Correlations between formic acid (F), acetic acid (A) and chemical and meteorological parameters 1993–1994 (n 444), summer (May–Aug., n 162), winter (Oct.–Mar., n 201) and spring 1993 (Apr.–Jun., n 90)

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Figure 1. Seasonal variations of daily means 1993–1994 of ozone, formic acid, acetic acid (ppb) and temperature.

Figure 2. Formic acid versus acetic acid (ppb) (a) May–August 1993–1994 and (b) October– March 1993–1994.

In winter (October–March) the formic acid concentration is 0:2  0:2 ppb and acetic acid 0:4  0:3 ppb. Thus, more acetic acid than formic acid is present

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Figure 3. Dependency of formic acid and acetic acid on wind direction for summer and winter 1993–1994.

during the winter, especially for northerly wind directions. The formic acid/acetic acid ratio is 0.6 with a fair correlation (r = 0:87 ) (Figure 2b). The carboxylic acids correlate with Ox , gas NOy , SO2 and particulate sulphur. In addition total nitrate (HNO3 + NO, 3 ) measured in March correlates with e.g., formic acid (r = 0:62 ). HNO3 does not correlate with the carboxylic acids in March although the correlations for April–June are 0.7 . This is perhaps due to locally high ammonia concentrations in March (Sørensen et al., 1994) depleting the nitric acid. 3.2. SOURCES OF THE CARBOXYLIC ACIDS The high correlations between formic acid and acetic acid in summer for all wind sectors suggest a common source, sources with the same formic acid/acetic acid ratios or sources that are not local but distributed over large geographic areas. Local sources of carboxylic acids are not significant. Granby et al. (1996) found that winter time concentrations in Copenhagen and at the Danish TOR station were similar (r = 0:90–0.92), which means that the carboxylic acid source was not of local origin and that direct emissions from car exhausts was not an important source. The importance of biomass burning as a carboxylic acid source was interpreted. Emissions of carboxylic acids from domestic and industrial wood burning

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were estimated from their emission factors relative to CO2 (HCOOH: 6:9  10,6 , CH3 COOH: 6:9  10,5 (Talbot et al., 1988)) and from Danish CO2 emissions (Fenhann and Kilde, 1994). The emissions correspond to formation of 0.0004 ppb day,1 formic acid and 0.003 ppb day,1 acetic acid. These emissions are low compared to the concentration levels. As burning of agricultural or forest areas only takes place by accident, the contribution from biomass burning is generally low. During the growing season biogenic precursor emissions (isoprene, terpenes) may be an important source of carboxylic acids. However, e.g., the reaction between O3 and isoprene is slow (halflife  1 day (Jacob and Wofsy, 1988)) and competes with the faster reaction between isoprene and HO. The isoprene may therefore be distributed over large areas before the carboxylic acids are formed. Biogenic emissions of hydrocarbons often depend on the temperature (Isidorov et al., 1985). If carboxylic acid emissions depend on temperature (they are related to transpiration (Kesselmeier et al., 1994) and therefore probably also to temperature), the following example indicates that local biogenic emissions are not a dominant carboxylic acid source: during 22 July to 4 August 1994 the daily temperature levels were the same but the concentrations of carboxylic acids varied by a factor of four and were at their maximum (up to 2.6 ppb formic acid) when the air masses arrived from east-southerly directions (Figure 4). O3 showed the same pattern with a maximum of 100 ppb. The further discussion is based on wind direction dependencies (local wind directions (Figure 3). For the summer the formic and acetic acid concentrations are highly correlated for all wind sectors. However, the concentration levels vary and can be divided into three main sectors. The 90–180 sector (SE) shows the highest concentrations, the 180–315 sector (W) shows concentrations more than a factor 2 lower and the 315–90 sector (N) concentrations in between. The W sector is most influenced by marine areas (the North Sea and the Atlantic Ocean) which generally show lower carboxylic acid concentrations than continental areas (Arlander et al., 1990). The carboxyic acids are negatively correlated with the marine product methane sulphonic acid (MSA) (Granby et al., 1994) (Table II). If the carboxylic acid source is supposed to be governed by photochemical processes taking place over regional or larger scales, the lowest carboxylic acid concentrations are expected from the W sector. The amount of precipitation is about double in the W sector compared to the other sectors (Table III) and a larger part of the carboxylic acids may therefore be scavenged before arrival at the measuring site. The higher amount of precipitation in the W sector, and generally large scavenging ratios for carboxylic acids and particles explain a good correlation with particulate sulphur (r = 0:69–0.70 ). Dry deposition of carboxylic acids to the sea (pH 8.2) will also remove them from the planetary boundary layer, whereas nocturnal dry deposition over continental areas is supposed to result in some revolatilization the following day due to the acidity of the water layer at the surfaces (pKa for formic and acetic acid are 3.62 and 4.75 respectively).

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Figure 4. Long-range transport to Lille Valby, 22 July–4 August 1994. While the temperature reaches about the same daily level during the period the carboxylic acids and ozone vary showing the highest concentrations when the wind arrives from the east–south.

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Table III. Formic acid/acetic acid ratios and daily mean precipitation (mm) for different wind sectors

Ratio Summer: Winter: Precipitation Summer: Winter:

N

SE

W

0.95 0.52

0.92 0.67

0.93 0.59

0.5 0.7

0.9 1.2

2.1 2.1

The SE sector shows the highest carboxylic acid concentrations. It also shows the best correlations with O3 (r = 0:57–0.64 ) and Ox (r = 0:65–0.71 ) which indicates that the carboxylic acids are produced by oxidation processes during long range transport of polluted air masses. Both ozonolysis of alkenes and reactions between peroxyacetyl radicals and RO2 radicals may be important sources. Satsumabayashi et al. (1995) used field measurements as well as a model to show that the acetic acid was produced by oxidation of anthropogenic hydrocarbons during long-range transport. Talbot et al. (1995) found a fair relationship between ozone and the carboxylic acids (r 2  0:5). However, the known hydrocarbon oxidation pathways could not account for the concentrations found in their photochemical experiment. Talbot et al. (1995) suggest a natural source, probably soil emission to be of importance. It is difficult to estimate how much biogenic VOC emissions are precursors for the observed amounts of carboxylic acids but considerable parts of Eastern Europe are forest areas. Jacob and Wofsy (1988) constructed a model showing that isoprene mainly produces formic acid. However, Martin et al. (1991) found in a deciduous forest that acetic acid concentrations were about 80–90% of formic acid concentrations. Martin et al. (1991) also found that the morning increase in carboxylic acids significantly lacked that of known biogenic emitted hydrocarbons and suggest that the source may not be terrestrial. The summer concentrations of carboxylic acids from the N sector are relatively high, which is difficult to explain if the dominant source has reactions involving anthropogenic hydrocarbons. The sector represents Scandinavia which is sparsely populated and with major nature reserves. For this sector either biogenic emissions or a large scale carboxylic acid production by oxidation processes may represent the most likely sources. In the winter the average formic acid and acetic acid concentrations are about a quarter and the half compared to the summer. The W sector still shows the lowest concentrations. Biogenic emissions are not a likely source during the winter. Formic acid shows the highest concentrations from the SE sector. Oxidation of anthropogenic hydrocarbons may be an important source for the SE sector.

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The acetic acid concentrations show the same level for the N and SE sector in winter but anthropogenic precursor emissions from the N sector are supposed to be much smaller than from the SE sector. However, the ozone levels are less depleted in the N sector compared to the SE sector, which to some extent may counterbalance the difference in precursor emissions. The seasonal variation in the formic acid/acetic acid ratio (0.6 in winter compared to 1.0 in summer) may be due to seasonal variation in the relative composition of hydrocarbons and the seasonal variation in the product yields from the ozonolysis and ‘acetylperoxy + HO2 ’ reactions. The variation in the product yields result from changes in the relative composition of the reactants (e.g. O3 , OH, HO2 ) and in the rate constants with temperature. For ozonolysis of alkenes, it competes with the OH oxidation mechanisms. In winter the OH concentrations may be one order lower and O3 not more than two times lower compared to the summer. Although the temperature dependencies of the rate constants reduce the seasonal differences, the result is that in winter the relative product yields for ozonolysis reactions (forming formic and acetic acid) are larger and the relative importance of the different ‘O3 + alkene’ reactions changes (Altschuller, 1991). For ethene the reaction with OH is important both summer and winter while for higher alkenes the ozonolysis reactions are important, especially in winter (Altschuller, 1991). For the reaction between acetylperoxy and HO2 or alkylperoxy radicals the rate constant has a negative temperature dependence but the reactant HO 2 is estimated to be one order lower in winter compared to summer which may result in lower importance during the winter. The seasonal variation in the formic/acetic acid ratio may also be affected by the assumed faster deposition rate for formic acid which will deplete it more from the longer enduring nocturnal boundary layer during the winter. 4. Conclusions The main known carboxylic acid sources in the winter are suggested to be ozonolysis of anthropogenic alkenes and for acetic acid also reactions between peroxyacetyl radicals and RO2 radicals. In the summer the sources are suggested to be as for the winter and in addition a contribution from ozonolysis of biogenic alkenes. As the wind direction distributions of formic acid and acetic acid for the summer are quite similar, it appears the concentrations of carboxylic acids are governed by large scale processes acting over several types of large-scale source regions. The seasonal variation in the formic/acetic acid ratio may be explained by the variation in the relative composition of the reactants (O3 , OH, HO2 , hydrocarbons), the changes in the rate constants with temperature and perhaps by differences in deposition velocities between the carboxylic acids.

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Acknowledgements The work has been supported by the Danish Research Academy and the Centre of Air Pollution Processes and Models under the frame of the Danish Environmental Research Programme 1992–1996. Thanks to Risø National Laboratory for providing results of the micro-meteorological parameters.

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