Gas- and aqueous-phase formic and acetic acids at a tropical cloud forest site

Atmospheric Environment Vol. 26A, No. 8, pp. 1421. 1426, 1992. Printed in Great Britain.

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0004&981/92 $5.00+0.00 1992 Pergamon Press plc

GAS- A N D AQUEOUS-PHASE FORMIC A N D ACETIC ACIDS AT A TROPICAL C L O U D FOREST SITE EUGEN10 SANHUEZA, MAGALY SANTANA a n d MARIELA HERMOSO IVIC Atmospheric Chemistry Laboratory, Apartado 21827, Caracas 1020-A, Venezuela (First received 1 June 1991 and in final fl~rm 25 October 1991)

Abstract Atmospheric gas-phase and aqueous-phase (dew and fog) formic and acetic acids were measured over a cloud forest in Venezuela. The gaseous acids showed diurnal cycles, with higher mixing ratios during daytime. Higher concentrations were observed during the dry season (HCOOH 1.7+/-0.5ppb, CH3COOH 1.4+,: 0.6ppb) in comparison with the rainy season (HCOOH 0.79+/-0.24ppb; CH3COOH 0.54+/-0.20 ppb). Liquid-phase concentrations in dew and fog are of the same order and range from 8.1 to 69.5 ItM for HCOOH and 4.3 to 15.3 ,uM for CH3COOH. The field-observed Henry's Law coefficients, calculated from the simultaneous measurements of gas- and liquid-phase acids, do not show a significant trend with the pH of the solution, in contrast to theoretical considerations. Dry deposition velocities to the nighttime dew are 1.1 + / - 0 . 6 and 0.68 + / - 0 . 4 2 cm s-1 for formic and acetic acids, respectively. A loss of 0.054 ppb HCOOH and 0.022 ppb CH3COOH from the atmospheric boundary layer to the dew is produced nightly. Key word index: Formic and acetic acids, dew, fog, field Henry's Law coefficients, dry deposition velocities.

1. INTRODUCTION

The gas-phase oxidation of these organic acids is rather slow (i.e. the lifetime for the reaction with the O H radical is > 1 0 days) (Lelieveld and Crutzen, 1991) and removal from the atmosphere is largely due

F ° r m i c ( H C O O H ) and acetic ( C H 3 C O O H ) acid gases are important constituents of the "natural" atmosphere; they contribute a large fraction (likely to be over 25%) to the nonmethane hydrocarbon atmospheric mixture. Measurements have been carried out in rural areas of North America (Dawson et al., 1980; Dawson and Farmer, 1988; Talbot et al., 1988) and E u r o p e ( P u x b a u m et al., 1988; Winiwarteret al., 1988; H a r t m a n n et al., 1989), a t " n a t u r a l " l o c a t i o n s in tropical America, the Amazon forest (Andreae et al., 1988; Talbot et al., 1990) and in a Venezuelan savanna site (Hartmann et al., 1991); also measurements were performed over the Equatorial forest in Africa during a period of vegetation burning (Helas et al., 1988). These acids also play a major role in the rainwater acidity observed in the tropics (Galloway et al., 1982; Andreae et al., 1988, 1990; Sanhueza et al., 1989, 1991; Gillet et al., 1990). The atmospheric sources of these acids are still very uncertain; likely sources include direct emissions from biomass burning (Talbot et al., 1988), ants (Graedel and Eisner, 1988), plants (Talbot et al., 1990) and soils (Sanhueza and Andreae, 1991), and also oxidation of precursor compounds in the gas phase (Madronich and Calvert, 1990). The contribution of the in-cloud production of H C O O H from H C H O oxidation (Chameides and Davis, 1983; Jacob, 1986) to the gasphase atmospheric composition is doubtful, even though formic acid is produced by oxidation of H C H O in aqueous phase, in the overall process clouds seem to be a net sink for gaseous formic acid (Sanhueza et al., 1991; Lelieveld and Crutzen, 1991).

to wet and dry depositions. Considering the watersolubility of H C O O H and C H a C O O H , dry deposition over wet surfaces may play a significant role in controlling the atmospheric concentration of these acids, however, until now, no evaluation of this deposition process has been published. In this paper we report measurements, made in a cloud forest in Venezuela, of gas-phase levels of H C O O H and C H a C O O H and their concentrations in nighttime dew and fog; experimental Henry's Law coefficients and deposition velocities to the dew for both acids are derived. 2. EXPERIMENTAL 2.1. Sampling site The study was performed at an altitude of 1750 m above sea level, in the campus of the Instituto Venezolano de Investigaciones Cientificas (IVIC), located in Altos de Pipe (lff' 30'N, 66° 56'W), in the Coastal Mountain Range in the north part of Venezuela. Samples were collected on the roof of the Atmospheric Chemistry Laboratory (~ 10 m above the ground). The sampling site is surrounded by a relatively well-preserved tropical cloud forest, formed by a large number of species of trees (Cuenca and Herrera, 1987). The humidity, maintained by the fog formation from the wet and warm air coming from the ocean, is the prime environmental factor for the development of the evergreen flora. Trade winds from the Caribbean Sea prevail in this site. Climatic average over 10 years shows that the annual precipitation is 994mm, well distributed throughout the year, with only two months of relatively dry climate (FebruaryMarch). The annual mean temperature is 18~C.

1421

1422

E. SANHUEZAet al.

2.2. Gas-phase measurements Gas-phase organic acids were sampled using the aqueous scrubber technique (Cofer et al., 1985); the air was first passed through a 2,urn pore diameter Zefluor filter to remove aerosol particles, and then exposed to a fine deionized water mist. The average sampling flow rate was 12 f rain-1. The samples were collected over 30 rain and the absorption solution (about 5 ml) analysed for organic acids immediately after collection. Due to the low concentrations of formate and acetate in aerosol, no significant bias results from gas-aerosol exchange on the prefilter (Andreae et al., 1988). In the same way, blanks were collected for each sample, passing 4 ~ of air through the scrubber. Formic and acetic acids were analysed by ion exchange chromatography using a Dionex (Model QIC) chromatograph equipped with a HPIC-AS4 separator column, an anion micromembrane suppressor column, and a conductivity detector. Tetraborate eluent (1.25mM Na2B,~Ov) and sulfuric acid regenerant (5 raM) were used. Atmospheric concentrations are normalized at standard temperature and pressure (0°C and 1 atm) and reported as parts per billion by volume (ppbv). Detection limits for formic and acetic acids were of the order of 3 pptv. 2.3. Dew and fog measurements Nighttime dew was collected using the technique described by Pierson et al. (1986). The collector of 1 m 2, consists of a 50-/~m thick FEP Teflon film mounted on a slab of styrofoam (4.8-cm thick) hinged in the middle. At the onset of dew formation the collector (previously rinsed with deionized water and then dried with cellulose filter paper) was placed on a bench of 60-era height; samples were collected for periods of 2 or more hours, until the dew formation stopped. Fog samples were collected using a passive collector constructed as an array of 370 Teflon strings (0.53-mm dia., 45cm long), spaced 3 mm apart in two concentric circles. The fog droplets accumulating on the strings slid down and were collected by a funnel which led into a polyethylene bottle (Mohnen and Kadlecek, 1989). The collector was cleaned with deionized water until the conductivity was 1 #S cm- 1, and then covered. Thecover was removed at the onset of fog. The aqueous (dew and fog) samples were preserved with chloroform to prevent microbial consumption of the organic acids. The analysis was made by ion chromatography in a similar way as the one described for the gas-phase samples. The detection limit was 0.02/~M for both formate and acetate.

3. RESULTS AND DISCUSSION 3.1. Gas-phase atmospheric concentrations Twenty-five-hour samplings were performed twice a month during March, June, October and November-December 1989. The results are presented in Fig. 1. N o rain fell during the sampling periods. Fog events are indicated in the figure. Dew formation was observed every night (see Section 3.2). 3.1.1. Diurnal variations. Both acids show a diurnal cycle, with lower concentrations during nighttime and early afternoon maximum levels. The variations are more pronounced during the dry season (March), when relatively higher daytime concentrations are observed. The lower concentrations of H C O O H and C H 3 C O O H during the night are likely due to a reduction of the convective circulation and also to an increase of their removal by dry deposition to the dew

produced at this time of the day (see Section 3.2). It is worth mentioning that relatively high levels were recorded during the nighttime, indicating that o u r " h i g h altitude" sampling site remains connected to the boundary layer during the night. N o significant changes were observed in the gas-phase concentrations during nighttime fog events. The H C O O H / C H 3 C O O H ratios obtained during all sampling periods are plotted in Fig. 2, against the local time. The results show that the ratio is fairly constant during the whole day, with an average value of 1.49_+ 0.53 (n = 192). The results obtained in Altos de Pipe contrast with the variations observed at a Venzuelan savanna site (Guri, Bolivar State) (Hartmann et al., 1991). In the savanna, the consumption due to dry deposition of H C O O H and C H 3 C O O H inside the shallow nocturnal mixed layer produced very low (close to zero) concentrations of these acids during nighttime; also, due to a faster removal of formic acid, compared with acetic acid, nighttime H C O O H / C H 3 C O O H ratios less than one were ohserved most of the time. 3.1.2. Seasonal variations. Vertical mixing is stronger around mid-day and atmospheric concentrations recorded during this time of the day should be more representative of boundary layer conditions than the daily averages. Table 1 summarizes the concentrations of both acids obtained between 1000 and 1700 h; since 30 N o v e m b e r was a foggy day (see Fig. 1) and convective circulation is diminished, the values recorded during that day were not included in the average. The results show a significant variation between the concentrations recorded during March (dry season H C O O H 1.6 ppb, C H 3 C O O H 1.4 ppb) and the period June December (rainy season, H C O O H 0.79 ppb, C H 3 C O O H 0.54 ppb). It is likely that the higher levels observed in March are due to a lower rate of removal by wet deposition, and to a higher atmospheric production from the oxidation of organic compounds emitted by vegetation burning; i.e. permutation reaction of organic peroxy radicals that could produce both formic and acetic acids (Madronich and Calvert, 1990) should increase during dryburning periods. Considering that vegetation burning emits 10 times more acetic than formic acid (Talbot et al., 1988), and that no significant variation in the H C O O H / C H 3 C O O H ratio was recorded between seasons, it seems that direct emission of H C O O H and C H 3 C O O H from biomass burning does not contribute in a significant way to the atmospheric budget of these acids. Similarly, Sanhueza et al. (1991) conclude that the higher concentrations of H C O O H in rainfall during the burning period should be mainly related to a larger concentration of H C H O during this period; formic acid is produced by oxidation of H C H O in the aqueous phase and the potential amount of H C O O H formed depends on the concentration of H C H O in the gas phase. As is shown in Table 2, the boundary-layer concentrations of H C O O H and C H 3 C O O H found in our

Acids in a tropical cloud forest (a)

1423

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Fig. 1. (a) Diurnal variation of gaseous formic and acetic acids during March to June. "f" denotes periods of fog. (b) Diurnal variation of gaseous formic and acetic acids during October to December. "f" denotes periods of fog.

cloud-forest monitoring site, are of the same order as the levels reported in other parts of the world. The higher mixing ratios reported for Central Africa are due to the impact of emissions from biomass burning (Helas e t al., 1988); at the Amazon forest, the large removal of both acids by wet deposition explain the relatively lower levels observed during the rainy season (Talbot et al., 1990).

3.2. F o r m i c a n d a c e t i c a c i d s in d e w a n d f o g Dew formation is observed practically every night at the monitoring site. The dew formed per night ranges between 0.18 and 0.40 ( m - 2 with an average of 0.24_+ 0.08 ( m - 2 (n = 6); due to the collection procedure, it is likely that these values are underestimated by 10-20%. This dewfall rate compares relatively well

1424

E. SANHUEZAet al. 5

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Table 1. Atmospheric

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concentration

for

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period

HCOOH (ppb)

n

CH3COOH (ppb)

n

1.7_+0.5 0.71-+0.23 0.96+0.23 0.61-+0.22 0.79 -+ 0.24

15 15 16 9 40

1.4+0.6 0.57+0.15 0.67+0.18 0.35-+0.10 0.54 -+ 0.20

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aqueous phase is a function of the Henry's Law con-

Fig. 2. HCOOH/CHaCOOH ratios in the gas phase,

stant, Kr~: Kn(M a t m - t) = Aaq (M)/As~s (atm).

with the scarce literature data, which fluctuates between 0.1 and 0.4 Em -2 per night (Brimblecombe, 1978; Mulawa et al., 1986; Pierson et al., 1986, 1988). Table 3 summarizes the concentrations of formic and acetic acids in dew and fog collected at our monitoring site; pH values are also given. Large nightto-night variations in pH are observed in both dew and fog-water solution, however, these variations do not correlate with the changes observed in H C O O H and C H a C O • H , indicating that other compounds that contribute to the acid-basic equilibrium must control the free acidity of the solutions (i.e. HNO3, NH3). Liquid-phase concentrations range between 8.1 and 69.5 #M for H C O O H and from 4.3 to 15.3 #M for CH3COOH. These concentrations in dew and fog are higher than the values recorded in the rain collected at the same site, during n o n burning periods (Hermoso et al., 1990; Sanhueza et al., 1991). 3.2.1. Partition between the gas and the liquid phase, The partitioning of soluble gases between gas and

For acids that form ions upon dissolution, it is necessary to express the gas-liquid equilibrium in terms of the pseudo Henry's Law constant, which takes dissociation and p H into account (Schwartz, 1984; Winiwarter et al., 1988): K* (M atm - 1) = KH (1 + K a / [ H ÷ ] ). The field-observed K~ values for H C O O H and C H 3 C O O H , obtained from the simultaneous measurements of the acids in dew or fog (Table 3) and the corresponding gas-phase concentrations (Fig. 1) are shown in Fig. 3. The theoretical pseudo-Henry's Law coefficients for both acids are also given (dashed line); the KH and Ka values were taken from Winiwarter et al. (1988). In contrast with the prediction from the "theoretical" considerations, the results show that the experimental K* values do not present a significant trend with pH, and very large deviations between theoretical and experimental values are observed at higher

Table 2. Atmospheric concentration of formic and acetic acids at different locations Location A. Pipe (Venezuela)* A. Pipe (Venezuela)* Guri (Venezuela)* Amazonia (Brazil)*t Amazonia (Brazil):[: Central Afric~::~ E. Virginia (U.S.A.)* S.W.U.S.A. (remote region) Germany:~

Season

HCOOH (ppb)

dry rainy dry/rainy dry rainy dry growing non growing September

Austria (rural) * Ground level measurements during daytime. t In a clearing in the forest. :~Boundary layer airplane measurements. §Continental anticyclone. IIMarine influence.

1.7 0.79 1.3 ~ 2.0 0.43 3.7 1.89 0.69 0.9 1.04§ 0.1711 0.9

CHaCOOH (ppb)

Reference

1.4 0.54 0.70 ~ 2.0 0.34 2.7 1.31 0.70 0.6

this work this work Hartmann et al. (1991) Andreae et al. (1988) Talbot et al. (1990) Helas et al. (1988) Talbot et al. (1988)

1.24§ 0.7211 ~0.45

Hartmann et al. (1989)

Dawson and Farmer (1988)

Puxbaum et al. (1988)

Acids in a tropical cloud forest

1425

Table 3. HCOOH and CHaCOOH concentrations in dew and fog Dew Water dep. (ml m- 2 h - ~)

pH

HCOOH (~M)

26-27 June 1900-2100 2100-2300 23004) 100 01004)300 03004)500 06004)800

31.5 37.7 31.5 31.5 66.5

5.44 5.36 5.34 5.38 5.40

10.6 12.8 12.1 8.1 9.8

5-6 October 22304)030 00304)240 02454)430 04004)600

22.5 22.0 53.7

Period

10-11 October 1830-2100 21004)000 00004)300 0300-0400

21.7 86.7 19.0 114.0

30 November-1 December 1930-2200 80.0 22004)100 102.0 01004)500 65.0 05004)700 25.0

5.30 4.85

6.80 6.65 5.44

5.55 5.75 5.15

38.8 33.6 26.4

16.1 49.8 13.3

23.4 9.42 11.8 44.9

Fog CH3COOH (,uM)

pH

HCOOH (#M)

CH3COOH (/~M)

5.65

57.5

12.7

4.87 4.75

67.3 69.5

4.3 9.6

4.57

69.0

6.9

6.56

42.9

10.4

6.34

27.9

10.2

7.5 7.1 6.5 5.0 4.9

7.5 6.1 5.5

11.7 15.3 6.7

5.2 5.1 7.3 11.5

pH. At present we do not know the cause of these deviations. Winiwarter et al. (1988) found similar deviations in concurrent gas- and liquid-phase organic acid measurements during radiation fog episodes in the Po Valley (Italy); these authors hypothesized that inhomogeneities occurring within the fog system may explain the discrepancies. Very recently, Pandis and Seinfeld (1991) proposed that mixing of droplets with different pH that are individually in Henry's Law equilibrium with the surrounding atmosphere always results in a bulk mixture that is supersaturated with acids (i.e. H C O O H ) with respect to the original atmosphere; this situation will result in outgassing of weak acids in an effort to equilibrate with the atmosphere. Deviations from Henry's Law are not exclusive of organic acid atmospheric measurements, but significant deviations have also been observed for SO 2 and N H 3 (Pandis and Seinfeld, 1991); these deviations should be considered when calculating the partition of species between gas and aqueous phases in the atmosphere. 3.2.2. Deposition to the nighttime dew. The results indicate that significant fluxes of formic and acetic acids from the gas phase into the dew occur at our sampling site. With an average deposition rate of 0.24 f of dew water per m - 2 per night, and the volume weighted averages of H C O O H (15.9/~M) and C H 3 C O O H (6.4/~M), the nightly fluxes are 3.8 and 1.5 ,umol m - 2 , respectively. Assuming a tropospheric boundary layer of 2000 m, an atmospheric loss of

acids, daily sources of at least similar strength must exist in the region. Dry deposition velocities (va) of H C O O H and C H 3 C O O H to the dew water were calculated from the deposition fluxes and the corresponding (simultaneous) atmospheric concentrations. The data shows a large dispersion with average values of 1.1 +0.6 cm s- 1 (n = 16) for H C O O H and 0.68 +0.42 cm s - 1

0.054 ppb of H C O O H and 0.022 ppb of C H 3 C O O H occurs every night to the dew. Therefore, in order to maintain the atmospheric concentrations of both

Fig. 3. Theoretical (dashed line) and fieldobserved Henry's Law coefficients as a function of pH.

HCOOH o Dew 6 - . Fog

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1426

E. SANHUEZAet al.

(n = 17) for C H 3 C O O H . As expected, H C O O H is i n c o r p o r a t e d to the liquid phase faster t h a n C H 3 C O O H (i.e. VatHcoom~ 1.6 Valcn3coom) a n d the H C O O H / C H 3 C O O H ratios in dew a n d fog water are larger t h a n the ratios recorded in the gas phase. As far as we know, these are the first values reported for the dry deposition velocities of formic a n d acetic acids to dew. These dry deposition velocities c o m p a r e relatively well with the values estimated at a Venezuelan sava n n a site: H C O O H 0.64-1.0 cm s - 1 a n d C H a C O O H 0.5-1.0 cm s - 1 ( H a r t m a n n et al., 1991). At the s a v a n n a the vd were o b t a i n e d from the decrease of the acid c o n c e n t r a t i o n s inside the n o c t u r n a l mixed layer; they are also in good agreement with deposition rates derived from theoretical considerations (Wesely, 1989). In conclusion, the results clearly indicate t h a t removal of H C O O H a n d C H 3 C O O H to wet surface should play a key role in controlling the a t m o s p h e r i c concentrations of these gases, especially during periods without rainfall. Acknowledgement--We are very grateful to Prof. M . O . Andreae for lending us the ion chromatograph.

REFERENCES Andreae M. O., Talbot R. W., Andreae T. W. and Harris R. C. (1988) Formic and acetic acids over the Central Amazon region, Brazil. J. geophys. Res. 93, 1616-1624. Andreae M. O., Talbot R. W., Berresheim H. and Beecher K. M. (1990) Precipitation chemistry in Central Amazonia. J. geophys. Res. 95, 16 987-16 999. Brimblecombe P. (1978) "Dew" as a sink for sulphur dioxide, Tellus 30, 151-157. Chameides W. L. and Davis D. D. (1983) Aqueous phase source of formic acid in clouds. Nature 304, 427-429. Cofer III W. R., Collings V. G. and Talbot R. W. (1985) Improved aqueous scrubber for collection of soluble atmospheric trace gases. Envir. Sci. Technol. 19, 557-560. Cuenca G. and Herrera R. (1987) Ecophysiology of aluminium in terrestrial plants growing in acids and aluminium rich tropical soils. Ann. Soc. R. Zool. Belgique 117 (suppl.1), 57-74. Dawson G. A. and Farmer C. J. (1988) Soluble atmospheric trace gases in the southwestern United States 2. Organic species HCHO, HCOOH, CH3COOH. J. geophys. Res. 93, 5200-5206. Dawson G. A., Farmer J. C. and Moyers J. L. (1980) Formic and acetic acids in the atmosphere of the southwest, U.S.A. Geophys. Res. Lett. 7, 725-728. Galloway J. N. and Likens G. E. (1976) Calibration of collection procedures for the determination of precipitation chemistry. Water Air Soil Pollut. 6, 24t-258. Galloway J. N., Likens G. E., Keene W. C. and Miler J.M. (1982) The composition of precipitation in remote areas of the world. J. geophys. Res. 87, 8771-8786. Gillett R. W., Ayers G. P. and Noller B. N. (1990) Rainwater acidity at Jabiru, Australia, in the wet season of 1983/84. Sci. total Envir. 92, 129-144. Graedel T. E. and Eisner T. (1988) Atmospheric formic acid from formicine ants: a preliminary assessment. Tellus 40B, 335-339. Hartmann W. R., Andreae M. O. and Helas G. (1989) Measurements of organic acids over Central Germany. Atmospheric Environment 23, 1531-1533.

Hartmann W. R., Santana M., Hermoso M., Andreae M. O. and Sanhueza E.(1991) Diurnalcycles offormic and acetic acids in the northern part of the Guayana Shield, Venezuela. d. atmos. Chem. 13, 63-72. Helas G., Bingemer H. and Andreae M. O. (1988) Measurements of organic acids in equatorial Africa during Decafe 88. Eos 69, 1066. Hermoso M., Santana M. and Sanhueza E. (1990) Carcteristicas ~.cido-b~sicas de las lluvias de un sitio urbano y otro suburbano de Caracas. Acta Cient. Venez. 41, 191-198. Jacob D. J. (1986) Chemistry of OH in remote clouds and its role in the production of formic acid and peroxymonosulphate. J. geophys. Res. 91, 9807-9826. Lelieveld J. and Crutzen P. J. (1991) The role of clouds in tropospheric photochemistry. J. atmos. Chem. 12, 229-267. Madronich S. and Calvert J. G.(1990) Permutation reactions of organic peroxy radicals in the troposphere. J. geophys. Res. 95, 5697-5715. Mohnen V. A. and Kadlecek J. A. (1989) Cloud chemistry research at Whiteface Mountain. Tellus 41B, 79-91. Mulawa P. A., Cadle S. H., Lipari F., Ang C. C. and Vandervennet R. T. (1986) Urban dew: its composition and influence on dry deposition rates. Atmospheric Environment20, 1389-1396. Pandis S. N. and Seinfeld J. H. (1991) Should bulk cloudwater or fogwater samples obey Henry's Law. J. geophys. Res. 96, 10791-10798. Pierson W. R., Brachaczek W. W., Gorse R. A. Jr, Japar S. M. and Norbeck J. M. (1986) On the acidity of dew. J. geophys. Res. 91, 4083-4096. Pierson W. R., Brachaczek W., Japar S., Cass G. R. and Solomon P. A. (1988) Dry deposition and dew chemistry in Claremont, California, during the 1985 nitrogen species methods comparison study. Atmospheric Environment 22, 1657-1663. Puxbaum H., Rosenberg C., Gregori M., Lanzerstorfer C., Ober E. and Winiwarter W. (1988) Atmospheric concentrations of formic and acetic acids and related compounds in eastern and northern Austria. Atmospheric Environment 22, 2841-2850. Sanhueza E. and Andreae M. O. (1991) Emission of formic and acetic acids from tropical savanna soils. Geophys. Res. Lett. 18, 1707-1710. Sanhueza E., Elbert W., Rond6n A., Arias M. C. and Hermoso M. (1989) Organic and inorganic acids in rain from a remote site of the Venezuelan savanna. Tellus 41B, 170-176. Sanhueza E., Ferrer Z., Romero J. and Santana M. (1991) HCHO and HCOOH in tropical rains. Ambio 20, 115-118. Schwartz S. E. (1984) Gas-aqueous reactions of sulphur and nitrogen oxides in liquid water clouds. In SO2, NO and NO 2 Oxidation Mechanism: Atmospheric Considerations (edited by Calvert J. G.), pp. 173-208. Butterworths, London. Talbot R. W., Beecher K. M., Harriss R. C. and Corer III W. R. (1988)Atmospheric geochemistry of formic and acetic acids at a mid-latitude temperate site. J. geophys. Res. 93, 1638-1652. Talbot R. W., Andreae M. O., Berresheim H., Jacob D. J. and Beecher K. M. (1990) Sources and sinks of formic, acetic, and pyruvic acids over Central Amazonia: 2. Wet season. J. geophys. Res. 95, 16 799-16 811. Wesely M. L. (1989) Parameterization of surface resistance to gaseous dry deposition in regional-scale numerical models. Atmospheric Environment 23, 1293-1304. Winiwarter W., Puxbaum H., Fuzzi S., Facchini M. C., Orsi G., Beltz N., Enderle K. and Jaeschke W. (1988) Organic acid gas and liquid-phase measurements in Po Valley fall-winter conditions in the presence of fog. Tellus 40B, 348-357.