Atmospheric formic and acetic acids in Venezuela

Pergamon Atmospheric Environment Vol. 30, Nos. 10/11, pp. 1861-1873, 1996 Copyright © 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 1352-2310/96 $15.00 + 0.00

1352-2310(95)00383-5

ATMOSPHERIC FORMIC AND ACETIC ACIDS IN VENEZUELA

E U G E N I O S A N H U E Z A , * t LUIS F I G U E R O A * and M A G A L Y S A N T A N A * *IVIC, Atmospheric Chemistry Lab., Apartado 21827, Caracas 1020-A, Venezuela; and i'MPI fiir Chemic,

Luftchemie, Postfach 3060, 55020 Mainz, Germany

(First received 13 December 1994 and in final form 1 September 1995)

Abstract--Gas phase and rain concentrations of HCOOH and CHaCOOH have been measured at various sites in the sawtnnah climatic region, a cloud forest site and a coastal site in Venezuela. Gas phase and rain water were sampled using the aqueous scrubber technique and a wet only collector, respectively. Analyses were made b) ion chromatography. The results indicate that formic and acetic acids are important components of the Venezuelan atmosphere. They are homogeneously distributed, suggesting a widespread source. Boundary layer concentrations during the dry season (HCOOH, 1.8 ppbv; CH3COOH, 1.25 ppbv) are higher than in the wet season (HCOOH, 1.0 ppbv; CHsCOOH, 0.7 ppbv), mainly due to a longer lifetime of the acid during the dry season ( ~ 6 days) compared with the wet season ( ~ 2 days). The overall concentrations in rain are 7.0 and 4.0 #M for formic and acetic acids, respectively. The estimated annual total depositions are: HCOOH, 17 mmol m- 2 yr- 1 and CH3COOH, 10 mmol m - 2 yr- 1; around half of the acids are removed by dry deposition. It is established that a larger source ( ~ 1.8 times) of both acids is present during the wet season. We speculate that atmospheric oxidation of hydrocarbons should be the main source of HCOOH and CH3COOH in the Venezuelan atmosphere; soil emissions could make a significant contribution during the dry season.

Key word index: Formic acid, acetic acid, tropical atmosphere, dry deposition, wet deposition.

1. INTRODUCTION

Formic (HCOOH) and acetic (CH3COOH) acids are important constituents of the "natural" atmosphere, both in gas phase (e.g. Talbot et al., 1990; Hartmann et al., 1991) and in hydrometeors (e.g. Sanhueza et al., 1992a, b). They contribute to a large fraction (likely to 5-10%) to the nonmethane hydrocarbon atmospheric mixture and they play a major role in the rain water acidity observed in the tropics (Andreae et al., 1988; Sanhueza et al., 1989, 1992b; Gillett et al., 1990; Lacaux et al., 1992; Sanhueza and Santana, 1994). The atmospheric sources of these acids are still very uncertain; likely continental sources include direct emissions from biomass burning (Talbot et al., 1988; Hartmann, 1990), motor vehicles (Kawamura et al., 1985), ants (Graedel and Eisner, 1988), plants (Talbot et al., 1990; Sch~ifer et al., 1992), and secondary formation in the atmosphere by oxidation of reactive hydrocarbons (Jacob and Wofsy, 1988; Madronich et al., 1990; Horie et al., 1994; Sanhueza et al., 1995). The contribution of the in-cloud production of HCOOH from HCHO oxidation (Chameicles and Davis, 1983; Jacob, 1986) to the gas-phase atmospheric composition is doubtful. Even though formic acid is produced

by oxidation of HCHO in aqueous phase, in the overall process clouds seem to be a net sink for gaseous formic acid (Sanhueza et al., 1992b; Lelieveld and Crutzen, 1990). The gas-phase oxidation of these organic acids is rather slow (i.e. the lifetime for the reaction with the OH radical is > 10 days) and removal from the atmosphere is largely due to wet and dry depositions. In this paper we report gas phase and rain concentrations of formic and acetic acids observed at various sites in Venezuela (coastal, cloud forest and savannah) during both dry and wet seasons. Diurnal and seasonal variations of acid concentrations are discussed. An evaluation of the sinks and an attempt to identify major sources of these acids in the region is made.

2. FIELD MEASUREMENTS

Gas phase and rain concentrations of HCOOH and CHsCOOH were measured at two sites in the Venezuelan savannah climatic region, at a cloud forest site and at a coastal site; locations are shown on the map in Fig. 1. In the map are also indicated the locations where organic acid measurements have been reported previously (Sanhueza et al., 1989, 1991,

1861

1862 E. SANHUEZA et al.

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Fig. 1. Location of the sampling sites.

1992; Hartmann et al., 1991; Morales and Bifano, 1992).

2.1. Characteristics of the sampling sites

2.1.1. Coastal site (Los Caracas). This site is located between the Caribbean sea and the Coastal Mountain Range (10°37'N; 66°34'W). Due to a local microclimate there is an average of ~ 1000 mm of rain fall in the area; the driest months are March to May, with an average of ~ 50 mm per month. The annual mean temperature is 25.6°C. Measurements were made at a few meters from the sea shore and when the wind was blowing from the ocean. With the exception of the traffic in the coastal route (very light during weekdays), this site is relatively far from pollution sources (the nearest urban center, La Guaira, is ~ 30 km downwind of the sampling site).

2.1.2. Cloud forest (Altos de Pipe). Measurements were performed at an altitude of 1750 m above sea level, on the campus of the Instituto Venezolano de Investigaciones Cientificas (IVIC), located in Altos de Pipe (10°30'N, 66°56'W), in the Coastal Mountain Range in northern Venezuela. 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 envir-

onmental factor for the development of the evergreen flora. Trade winds from the Caribbean Sea prevail in this site. Climate average over 10 yr shows that the annual precipitation is 994mm, well distributed throughout the year, with only two months of relatively dry climate (February-March). The annual mean temperature is 18°C. The site is located nearby urban activities; however, the air quality is surpris- ingly good (e.g. Sanhueza et al., 1979; Escalona and Sanhueza, 1981), showing the good dispersion capa- city of the tropical atmosphere.

2.1.3. Savannah region (Calabozo and Canaima). At Calabozo the measurements were made in the Estaci6n Biol6gica de los Llanos, Calabozo, Gu~rico state (8°53'N; 67°19'W). The site is located in the central part of Venezuela and is minimally influenced by urban or industrial pollution sources. The scrub-grass savannah is covered with graminea grasses (Trachypogon sp and Axonopus canescens) interrupted by trees and scrubs (Curatella american, Boudichia virgilioides, and Byrsonima crassifolia). Two well-defined climatic periods occur in the area: a dry season from December to April, and a rainy season from May to November. The rainfall at the site (MARNR station) shows a 22 yr average of 1300 mm. Around 98% of the rainfall occurs between April and November. The annual mean temperature is 27.6°C.

Atmospheric formic and acetic acids 1863

Canaima is the most remote (pristine) site included in this study. It is located in the southern part of the savannah region close to the border of the wet tropical forest. The sampling site was located ~ 2 km from the Canaima village (6°15'N; 62°52'W) at an altitude of ~400 m above sea level. Dry and wet season periods are similar to those observed in Calabozo. The annual rainfall in the region is 2300 mm and the average temperature 25.6°C.

2.2. Sample collection and analysis

Rain samples were collected by event, using an Aerochem Metric Inc., Model 301 wet and dry auto- matic collector. The material was prepared in the laboratory following the directions given by Gallo- way and Likens (19'76). The pH was measured within 2 h of the end of the rain event. A fraction of the sample was preserved with chloroform (3 ml per 0.5 E of rain) for the analysis of the organic acids.

Gas-phase organ:ic acids were sampled using the aqueous scrubber technique (Cofer et al., 1985); other details of our sampling procedure are given in San- hueza et al. (1992). The sampled air volume was deter- mined with a dry gas meter and the sampling period ranges from I0 to 30 min. At Altos de Pipe, Calabozo and Canaima sites the sample solutions were analyzed immediately after collection. At Los Caracas, samples were preserved with CHC13 and were analyzed within 24 h after their collection.

Formic and acetic acids were analyzed by ion- exchange chromatography using a Dionex (Model QIC) chromatograph equipped with a HPIC-AS4 separator column, an anion micromembrane sup- pressor column, and a conductivity detector. Tet- raborate eluent (1.25 mM Na2B407) and sulfuric acid regenerant (5 mM) were used. Detection limits for gaseous formic and acetic acids were of the order of 20 pptv.

2.3. Samplin9 periods

2.3.1. Rainfall. Rain samples were collected "occa- sionally" during 1990 and 1993 at Calabozo: 8 events in 1990, 32 in 1991, 17 in 1992 and 18 in 1993. At Altos

Table 1. Measurement periods of gaseous acids in Calabozo a~d Altos de Pipe

Calabozo A. de Pipe

1990 14 January-1 February 14-17 March a 20-27 June ~ 5-11 October a

15-24 October 3-5 November 1-13 December a

1993 30 March-2 April 21-23 March 23-24 April

8-11 June 24-25 May 9-10 July

10-23 September 10-11 August

a Results published by Sanheuza et al. (1992a).

de Pipe 18 rain samples were collected during 1988, 13 in 1989 and 35 in 1993. In Canaima, 7 events were collected during April 1994. The analysis of 8 events collected (April 1994) at the top of the Auyantepuy at an altitude of 2100 m (see Fig. 1) is reported in this paper.

2.3.2. Gaseous acids. At Los Caracas coastal site, samples were collected once a month between 1100 and 1400 h for the period March-August 1993. At A. de Pipe and Calabozo sites the acids were measured over a 24 h period (the dates are given in Table 1). These periods cover dry and wet seasons. At Canaima the diurnal variation of the acids levels was only measured during the dry season (April 1994).

3. RESULTS

3.1. Diurnal variations of gas phase HCOOH and CHaCOOH

Twenty-four hour measurements were made at a A. Pipe, Calabozo and Canaima sites. The concentrations of both acids as a function of local time are shown in Fig. 2.

3.1.1. Wet season. At Calabozo, formic and acetic acids show a diurnal cycle, with lower concentrations during nighttime and early afternoon maximum levels. Similar behaviour was observed at Guri, also located in the Venezuelan savannah region (Hart- mann et al., 1991). The lower concentrations during the night are likely due to a reduction of the vertical circulation and also to an increase of their removal by dry deposition to the dew produced at this time. The rapid increase observed in the morning hours should be mainly due to downward mixing of upper air, which in comparison with the depleted surface layer is enriched in HCOOH and CH3COOH.

At the cloud forest, formic acid follows the same pattern as in the savannah, with higher concentrations during the day; however, acetic acid does not show a clear diurnal variation and quite similar levels were observed during day and nighttime. In a previous study, at the same site, both acids showed lower concentrations at night; simultaneous measurement in the gas phase and dew indicate that an atmospheric loss of 0.054 ppbv of HCOOH and 0.022 ppbv of CH3COOH occurs every night to the dew at this cloud forest site (Sanhueza et al., 1992).

3.1.2. Dry season. In Calabozo and Canaima sites, an important increase of concentration of both acids was observed during late afternoon, reaching levels up to ~ 6 and ~7 ppbv for HCOOH and CH3COOH, respectively. These high concentrations were maintained during the evening and decreased slowly during the night, producing nighttime averages higher than daytime averages. This pattern is very different from the one observed during the wet season, which had maximun levels between 1100 and 1700, and a rapid decrease during late afternoon. It is likely that the afternoon increase observed in the dry season is


DRY SEASON

5

CANAIMA 4

5

4

2

t

0

CALABOZO

, i, tlttt!t ,

• ~ #Clt.,jCOOH

WET SEASON

00:00

A. DE PIPE

06:00 12:00 LOCAL TIME

I • I I i

18:00 00:00 00:00 08:110 12:00 18:00 00:00 LOCAL TIME

Fig. 2. Diurnal variation of gaseous formic and acetic acids during dry and wet seasons at Altos de Pipe, Calabozo and Canaima sites. Error bars are standard deviations.

produced by the reaction of reactive hydrocarbons (mainly emitted by biomass burning) with ozone (see Section 4.2.2); much higher ozone concentrations ( ~ 3 times) are observed in the savannah region during the dry season (unpublished results); relatively high levels of ozone are maintained during the evening and night and the chemical production of the acids inside the shallow nocturnal mixing layer should compen- sate the deposition to the surface. Due to dryness of the savannah surface, deposition of acids inside the nocturnal mixing layer is slower during the dry season.

In the cloud forest highest formic acid concentrations were observed during daytime, whereas the highest concentrations of acetic acid were recorded at night. In this case the dry deposition of HCOOH to the wet cloud forest surface exceeds

its chemical production; direct emission of CH3COOH from vegetation burning may also contribute to maintain the high levels of acetic acid during the night.

3.1.3. HCOOH/CH3COOH ratios. The HCOOH/ CH3COOH ratios obtained at the various sites are plotted in Fig. 3, against the local time. The ratios show strong diurnal variations, with values above 1 during daytime, and below 1 at night. This is likely due to a more rapid nighttime removal of more soluble HCOOH than of CHaCOOH inside the nocturnal mixing layer. The partition between the gas and liquid phase, Henry's law constants, has been given by Sanhueza et al. (1992). Daytime ratios, more represen- tatives of boundary layer concentrations, indicate that there is always more HCOOH than CH3COOH in the Venezuelan atmosphere.

"1- 0 0 0 2 - r

"1" 0 1 0 0

CANAIMA

Atmospheric formic and acetic acids

DRY SEASON

WET SEASON

CALABOZO

,ttt tltttt * t I I I I I

tt

1865

0 04):00

A. DE PIPE

I 06:00

t÷. t i 12 00 18:00

LOCAL TIME

i I

0 0 : 0 0 0 0 : 0 0 0 6 : 0 0 1 2 : 0 0 L O C A T I M E

I

1 8 : 0 0 0 0 : 0 0

Fig. 3. Diurnal variation of the HCOOH/CH3COOH ratios during dry and wet seasons at Altos de Pipe, Calabozo and Canaima sites. Error bars are standard deviations.

3.2. Seasonal variation of boundary layer concentrations

In the tropics, vertical mixing is very strong during the hours of high solar irradiation and surface concentrations measured at this time should be representa- tive of boundary layer concentrations. Figures 4 and 5, and Table 2 summarized the concentrations of HCOOH and CHaCOOH recorded in Venezuela.

At the coastal site (Los Caracas) sampling was only performed when air was coming from the ocean. Simultaneous measurements of carbon monoxide indicate that the sampled air was practically not affected by anthropogenic sources of air pollution. Com- pared with the levels measured at the continental sites (see Table 2), lower concentrations of both acids were recorded at Los Caracas. No significant differences

1.5

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Fig. 4. Seasonal variation at the coastal site (Los Caracas). Averages of measurements made between 1100 and 1600

local time. Error bars are standard deviations.

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Fig. 5. Seasonal variation of H C O O H and C H 3 C O O H at the continental sites. Measurement periods are given in Table 1. Averages of measurements made between 1100 and 1600 local time. Error bars are

s tandard deviations. The Gury data are from Har tmann et al. (1991).

Table 2. Atmospheric concentration of formic acetic acids in Venezuela for the period 1100-1600 h

HCOOH= C H a C O O H a Site Season (ppbv) (ppbv) C H a C O O H / H C O O H

Guri b Dry/1989 1.44 + 0.5 (13) 0.72 + 0.55 (13) 0.50 (Savannah) Wet/1988 1.28 + 0.34 (40) 0.64 _ 0.19 (40) 0.50 A. de Pipe c Dry/1989 1.7 + 0.31 (15) 1.4 ___ 0.6 (15) 0.82 (Cloud forest) Wet/1989 0.79 ___ 0.24 (40) 0.54 +__ 0.2 (39) 0.68 Calabozo d Dry/1990 1.32 + 0.68 (40) 0.82 _ 0.57 (40) 0.62 (Savannah) Wet/1990 1.84 _ 0.91 (24) 1.51 + 0.92 (24) 0.82 Calabozo d Dry/1993 3.28 + 0.93 (10) 1.64 _ 0.39 (10) 0.50 (Savannah) Wet/1993 0.82 + 0.28 (57) 0.67 + 0.39 (57) 0.82 Los Caracas d Dry/1993 0.66 _ 0.31 (21) 0.78 + 0.51 (19) 1.18 (Coast) Wet/1993 0.62 _ 0.34 (24) 0.53 -t- 0.34 (23) 0.85 A de Pipe d Dry/1993 1.53 + 0.35 (13) 1.27 + 0.13 (15) 0.83 (Cloud forest) Wet/1993 0.97 + 0.37 (15) 0.93 + 0.61 (14) 0.95 Canaima d Dry/1994 2.17 ___ 0.37 (26) 1.33 _ 0.91 (26) 0.61 (Savannah)

a Average +_ Standard deviation (number of data). b Har tmann e t al. (1992). c Sanhueza et al. (1992a). d This work.


were observed between the dry and the wet seasons (Fig. 4), suggesting constant sources and sinks during the entire year. The levels of HCOOH and CH3COOH of ~0.65 ppbv reported here are consistent with the concentrations found by Norton (1992) in Mauna Loa (HCOOH, 0.45 ppbv; CH3COOH, 0.37 ppbv) and Ar- lander et al. (1990) in the Pacific Ocean boundary layer (HCOOH, 0.80 ppbv, CH3COOH, 0.78 ppbv).

The results in Fig. 5 show that at A. de Pipe (cloud forest), Calabozo and Guri (savannah) higher concentrations were observed during the dry season compared with the wet season. Relatively high levels were also observed in Canaima during the dry season. It is interesting to note that quite high levels of acids were also observed in Calabozo during a prolongate dry period in October 1990. A very significant seasonal variation of boundary layer concentrations has been reported over Central Amazon, with concentrations

5 times lower during the rainy season (Talbot et al., 1990). Relatively high levels were found (HCOOH, 3.7 ppbv; CH3COOH, 2.7 ppbv) above the rain forest of northern Congo during the dry season (Helas et al., 1992a). Lower rate of removal by dry and wet depositions seems to be 1:he main cause of the higher levels during the dry sea:~on in Venezuela (see Section 4.2). As we discuss later~, it is likely that the acids originate from different sou:~ces during dry and wet seasons; during the wet season oxidation of hydrocarbons produced by the vegetation is probably the main source, whereas in the dry season acids are produced by the oxidation of hydrocarbons emitted by biomass burning.

3.3. Relationship between gaseous HCOOH and CH3COOH, and ~ith other short-lived gases

As is shown in Table 2, with the exception of Los Caracas during the dry season, a higher concentration

of formic acid (CH3COOH/HCOOH < 1) is observed in the Venezuelan atmosphere. The results of the linear regressions between the boundary layer concentrations (measurements between 1100 and 1600) of formic and acetic acids at the various sites, during both seasons are summarized in Table 3. The table also contains the results obtained when the sites are bundled together. As an example, in Fig. 6 the relationship between HCOOH and CH3COOH observed at the continental sites (Calabozo, Altos de Pipe and Canaima) during the dry and wet seasons is shown. With only one exception, Canaima during the dry season, significant correlations are obtained, strongly suggesting a common source of both acids (Keene and Galloway, 1988). The lack of correlation at the Canaima site is likely due to a significant incidence of "local" direct emissions of acetic acid from biomass burning during part of the measurement period. The closest site to large urban activities is Altos de Pipe; however, the ratios CH3COOH/ HCOOH at this site are quite similar to those obtained at the other sites, suggesting that direct emissions of acids from anthropogenic combustion sources, which mainly emit acetic acid (Kawamura et al., 1985), do not contribute, in a significant way, to the atmospheric concentrations recorded at the rural sites. At present, we have no explanation for the difference (around a factor of 2) between ACH3COOH/AHCOOH in dry and wet seasons. In general, the large variation observed for HCOOH (ACH3COOH/AHCOOH < 1) may indicate a larger "turnover" (i.e. larger sources and shorter lifetime) of this acid in the atmosphere, compared with acetic acid.

During September 1993 (rainy season) as part of ASTROS (Atmospheric Studies in TROpical

Table 3. Regression analysis between the concentration of formic and acetic acids in gas phase and rain water

Site Season Regression equation r ~ n

Gas phase Calabozo Dry A Calabozo Wet A A. d:e Pipe Dry A A. d:e Pipe Wet A Los Caracas Dry & Wet A Canaima Dry A Cot~tinental Dry A sites ~ Wet A

Dry & Wet A All :Sites Dry & Wet A

Rain Calbozo Wet A A. de Pipe Wet A Canaima Wet A Auyantepuy Wet A All sites Wet A

= 0.44F = 0.84F = 0.62F = 1.10F = 0.72F = 0.19F = 0.40F = 0.84F = 0.53F = 0.49F

= 0.35F = 0.61F = 0.56F = 0.40F = 0.47F

+ 0.17 0.91 52 + 0.07 O.92 83 + O.28 0.86 15 - 0.18 0.68 14 + 0.16 0.58 39 + 0.68 0.17 b 26 + 0.30 0.70 92 - 0.05 0.81 97 + 0.18 0.76 189 + 0.27 0.73 232

+ 0.83 0.82 71 + 0.43 0.86 68 + 0.31 0.98 7 + 0.59 0.99 7 + 1.17 0.69 153

a The correlations have 99% confidence. b DO not correlate. Calabozo, Altos de Pipe and Canaima.


Savannah), at the Calabozo site, isoprene, H C H O , CHaCHO, H202 and total peroxides were measured simultaneously with formic and acetic acids (San- hueza et al., 1995). The results indicate that H C O O H and C H 3 C O O H correlate with isoprene (the more

abundant olefin in the savannah atmosphere), H202, and total peroxides. H C O O H also correlates with H C H O and CH3COOH. As we discuss later (Sec- tion 4.3.2), this supports the idea of a significant chemical product ion of both acids in the atmosphere.

3.4. HCOOH and CHaCOOH in rainfall

In Fig. 7 are the concentrations of HCOOH and CH3COOH measured in rain events in Altos de Pipe, Calabozo, Canaima and Auyantepuy. As expected a large variation is observed. The volume-weighted average (VWA) concentrations given in Table 4 are in the range of the values reported previously at other Venezuelan sites (Sanhueza et al., 1992b).

As in the gas phase, also a good correlation between formic and acetic acids is observed in rain samples (see

3.5

3

~ . 2 . 5

"I" 2 0 0 l.s O F> 1 "1-

O 05

0

3.5

3

2.5 e~ "1" 2 0 0 1.5 o -r 0 0.5

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WET SEASON

~ ~ 0 '

~ , El, , , J ,

[ ] CALABOZO X CANAIMA + A. DE PIPE

DRY SEASON D

x

~DX ~ L /~ f~F

~ ~ ~x×

, , , , , , , ,

05. 1 1.5 2 25. 3 3.5 4 HCOOH (ppbv)

4.5

Fig. 6. Relationship between formic and acetic acids in the gas phase during dry and wet seasons. The results of the

regression analysis are given in Table 3.

100

~" 10

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Aug-87 Dec-88 May-90 Sep-91 Jan-93 Jun-94

• CALABOZO o A. DE PIPE o AUYANTEPUY FJ CANAIMA

Fig. 7. Formic and acetic acids in Venezuelan rainfall.

Table 4. Volume-weighted average of formic and acetic acids in Venezuelan rains

HCOOH CHaCOOH Site Period (n) pH (~tM) 0 a M ) CH3COOH/HCOOH

Calabozo a 90-93 (75) 5.2 6.5 3.5 0.54 Canaima a 1994 (7) 4.8 2.9 1.7 0.59 Auyantepuy ~ 1994 (8) 5.3 4.2 1.9 0.45 A. de Pipe a 88 93 (66) 5.6 6.4 5.6 0.87

(58) b 5.8 3.0 3.1 1.0 La Paragua c 1985 (10) 4.7 9.3 9.5 1.0 J. Tigre c 1985 (11) 5.1 8.2 8.5 1.0 Chaguaramas ~ 1987 (11) 5.4 8.5 7.8 0.92 Guri c 1988 (9) 4.6 6.6 4.3 0.65 La Esperanza a 88-89 (42) 5.7 14 San Carlos de Rio Negro e 1980 (14) 8-24 2-8

a This work. bDoes not include rains occurred during the dry season or at the beginning of the rainy season.

Sanhueza et al. (1992). d Morales and Bifano (1992); located downwind pollution sources.

Galloway et al. (1982).


Table 3 and Fig. 8). The ACHaCOOH/AHCOOH (0.47) is lower than the values observed in the gas phase during the rainy season at the continental sites (0.84). The larger variation of HCOOH with respect to CH3COOH in rains is likely due to a more efficient below-cloud scaw~nging of the more soluble formic acid, which also ihas higher atmospheric gas-phase levels.

During September 1993, HCHO was also measured in the Calabozo rain samples (C. de Serves, Stockholm University, unpublished results). A good correlation is observed between HCOOH and HCHO; these data have been plotted (Fig. 9) together with

100

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I

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ee • e mm urns .~m~UU~

I n * ~ x , Up- ~ • •

,~ ~'~ •

l*I~ •

o.1 , i ~ 1 i i i i I I i I I I i I l l i , I t l l l t , v

0.1 1 10 100 HCOOH ¢p.M)

ICALABOZO eA. DE PIPE

AAUYANTEPUY DCANAIMA

Fig. 8. Relationship of formic and acetic acids in Venezuelan rains. The results of the regression analysis are

given in Table 3.

lOOO

, 10o

8 tO • t- lO

t

~ ~ ~[]

i , i i i i i I I I I ' ' ' '

lO lOO HeRO (/~4)

Fig. 9. Relationship between HCHO and HCOOH: ( . ) Calabozo and ([3) other Venezuelan sites reported by San-

hueza et al. (1991); R = 0.88, n = 49.

previous measurements made in Venezuela by San- hueza et al. (1991). Since HCHO is more soluble than HCOOH and (HCHO/HCOOH)g,sphase > (HCHO/ HCOOH)rain it is likely that in cloud, oxidation of formaldehyde plays a major role in controlling the concentration of formic acid in tropical rains (San- hueza et aL, 1991).

4. DISCUSSION

4.1. Distr ibut ion of HCOOH and CH3COOH m Venezue la

The results in Table 2 and Fig. 5 show that the continental sites have very similar levels of acids, with higher concentrations during the dry season. These results suggest that the sources of these acids are well distributed all over the country, during both wet and dry seasons. In the Amazon forest also higher concentrations were recorded during the dry season (Talbot et al., 1990). A longer atmospheric lifetime of acids seems to be the main reason that explains the higher concentrations during the dry season (see below). It is interesting to note that this behaviour observed in the tropics contrasts with the variations recorded at temperate latitudes (e.g. Talbot et al., 1988), where higher levels of acids are produced during the growing season. From the available data (Table 2) we estimate for the continental region of Venezuela average concentrations of 1.8 and 1.25 ppbv for HCOOH and CH3COOH during the dry season¢ and 1.0 and 0.7 ppbv in the wet season. Concentration ranges are given in Table 5.

At the coastal site lower concentrations were recorded in "marine" air (regional trade winds blow from the ocean to the continent), without any significant seasonal variation, suggesting constant sources and sinks. As far as we know there are no measurements of gaseous organic acids at a marine site cover- ing the different seasons of the year. A seasonal cycle has been observed in rainfall collected in a remote marine location in the southern Indian Ocean, Am- sterdam Island (Moody et al., 1991). Higher levels of both acids were recorded during the spring, likely associated with phytoplankton blooms, which would produce a high concentration of precursor hydrocarbons.

Venezuelan rains also show a quite homogeneous distribution of HCOOH and CH3COOH all over the country. The VWA concentrations range between 2 and 9 #M, with higher concentrations of HCOOH in most of the cases (see Table 4). Similar results have been reported at other tropical American sites (for a review, see Sanhueza and Santana, 1994), confirm- ing the ubiquitous presence of formic and acetic acids in the tropical atmosphere. Average rain concentrations of HCOOH of 7/~M and CH3COOH of 4 #M were established to estimate the regional wet deposition of these acids in the continental region of Venezuela (see Table 5).


Table 5. Input data for deposition calculations

Dry season Wet season

HCOOH CH3COOH HCOOH CH3COOH

Gas phase conc. (ppbv)

Dep. velocity (cm s- 1)

Rain conc. (/tM)

Rainfall (mm)

1.8 1.25 (1.5 3.0) (1.1 2.0)

0.8 0.7 (0.6 1.1) (0.5 1.0)

1.0 0.7 (0.8-1.3) (0.5-0.9)

1.0 0.8 (0.9-1.1) (0.7-1.0)

7.0 4.0 (3.0-9.0) (2.0-9.0)

1300 1300

Note: range values are given in parentheses.

4.2. Sinks of HCOOH and CH3COOH in Venezuela

4.2.1. Gas-phase reactions. Monocarboxylic acids absorb radiation in the range 180-240 nm (Calvert and Pitts, 1966) and therefore they do not photolyze in the troposphere. The rate constants with the OH radical are 3.2 and 6.0 x 10-13 cm 3 molecules- 1 s - x for HCOOH and CHaCOOH, respectively (Zetzsch and Stuhl, 1982). Taking for the tropics an average OH concentration of 2 x 106 molecules cm 3 (Crutzen, 1982; Crutzen and Zimmerman, 1991), lifetimes of 18 days for HCOOH and 10 days for CH3COOH are obtained. These lifetimes are longer than those estimated to result from deposition processes.

4.2.2. Removal by dry and wet depositions. The Diel cycles observed for HCOOH and CHaCOOH (e.g. Fig. 2) indicate that dry deposition to the surface is an important atmospheric sink for these acids. The diurnal variation also indicates (Fig. 3) that HCOOH is removed faster than CHaCOOH by dry deposition processes. Very little effort has been made to deter- mine the dry deposition velocities of formic and acetic acids. Several authors (e.g. Talbot et al., 1988; Gros- jean, 1989) used a value of 1 cms -1 to estimate the deposition of both acids; this value is in the range recommended by Wesely (1989). In Venezuelan eco- systems some estimations have been made (Hartmann et al., 1991; Sanhueza et al., 1992), indicating values ranging between 0.5 and 1.2 cm s-1. The dry deposition velocities and other data needed to estimate dry deposition rates in the Venezuelan savannah region are summarized in Table 5. The input data to calcu- late the rate of wet deposition are also given in this table.

The estimated rates of dry deposition are presented in Table 6. Since during nighttime, only ~ 5% of the column content of the acids is in contact with the surface, dry deposition rates were calculated assuming that deposition occurs only 12 h per day. The atmospheric lifetimes given in the table were obtained considering a boundary layer of 2000 m. The removal rates during the dry season (5 months, December- April), when wet deposition is negligible, are 28

and 17 / tmolm-2day -1 for formic and acetic acid, respectively. During this period atmospheric lifetimes of 6 days are estimated. During the wet season (7 months, May-November) the total removal rate (dry plus wet deposition) is for both acids ~ 1.8 times larger than in the dry season. Average lifetimes in the rainy season are estimated for both acids to be ~ 2 days. Dry deposition seems to play a major role in the organic acid budget in the tropical savannah region; the deposition rates listed in Table 6 indicate that around half of the annual removal occurs by dry deposition. However, at present the estimates are very crude and efforts to refine dry deposition velocities and other parameters involved in the process should be undertaken.

The atmospheric concentration of acids remain quite constant during a particular season, suggesting an equilibrium between sources and sinks. Therefore, the results indicate that the sources of HCOOH and CH3COOH should be ~ 1.8 higher during the wet season, in comparison with the dry season. Then, we can speculate that the higher regional levels of acids observed during the dry season are mainly due to a longer atmospheric lifetime during this period, rather than to larger sources of acids.

4.3. Sources of HCOOH and CHaCOOH

The lower levels observed in the coastal site (Los Caracas) indicate that there must be a significant continental source of these acids. According to the analysis made above, a larger source should exist during the wet season (HCOOH, 52.5/~molm -2 day-1; CH3COOH, 29.9/~mol m-2 day-1) compared with the dry season (HCOOH, 27.6 #mol m - 1 day- 1; CH3COOH, 16.9 #molm-2day-1) . In the following paragraphs an attempt to identify major regional sources of HCOOH and CH3COOH is made. Since the region is little affected by industrial or urban anthropogenic emissions (e.g. Sanhueza et al., 1988, 1992b), these sources are not included in the discussion.

4.3.1. Primary sources. Emissions from "dry" savannah soils have been proposed to be a significant

Atmospheric formic and acetic acids

Table 6. Wet and dry depositions, and lifetimes of formic and acetic acids in the Venezuelan savannah"

1871

Dry season Wet season Annual (pmolm-2day -1) (#molm-2day -1) (mmolm-2yr -1)

HCOOH Dry deposition 27.6 19.5 8.2 Wet deposition - - 33.0 9.1 Total deposition 27.6 52.5 17.3 Lifetime (days) b 5.8 1.7

CH3COOH Dry deposition 16.9 10.7 4.7 Wet deposition 19.2 5.2 Total deposition 16.9 29.9 9.9 Lifetime (days) a 6.5 2.1

a Based on mean values given in Table 5. b Assuming a boundary layer of 2000 m.

source of formic and acetic acids. A daily contribution to the atmospheric boundary layer of about 0.15 ppbv HCOOH and 0.07 ppbv CH3COOH was estimated at the Guri savannah site (Sanhueza and Andreae, 1991). This source could represent ~ 50% of the emission budget during the ,dry season. However, this source is likely negligible under wet soil conditions during the rainy season. Evaporation of dew may "recycle" back, to the atmosphere: the acids deposited during nighttime (Helas et al., 1992b); however, since organic acids are rapidly consumed by biological activity in surface dew-water (unpublished results), this should represent a minor source of atmospheric acids.

Direct emissions from vegetation have been proposed as a source of acids (Talbot et al., 1990); the emission has beert related with the photosynthetic activity of the plants (Sch~ifer et al., 1992). A study in Central Amazonia indicates that emissions of formic and acetic acids from vegetation can account for only a small fraction of the acids found in the boundary layer (Talbot et al., 1990). Since the tropical savannah region has sparse vegetation compared with the wet forest, and is mostly dry (dormant) during the dry season, it is unlikely that this source makes a major contribution to the organic acid budgets in the tropical savannah regien.

Biomass burning produces 4-10 times more acetic than formic acid (Talbot et al., 1988; Hartmann, 1990); however, during the dry-burning periods the atmospheric concentrations of HCOOH are higher than the levels of CH3COOH (Table 2). This, and the good correlation observed between the acids (Fig. 6), suggest that primary emissions from vegetation fires is a minor source of these gases. Ants have been proposed as a potential source of formic acids (Graedel and Eisner, 1988). Measurements made in Calabozo show that the air near large ants nests (thatch mounds) had simihtr levels of HCOOH than the back- ground air, and that concentrations in the tunnels (wet soil conditions) were below detections limits. In Arizona higher lew~ls of HCOOH were only observed

after a formicine ants colony was severely disturbed (Johnson and Dawson, 1993). It seems that ants do not contribute in a significant way to the HCOOH atmospheric budget.

4.3.2. Secondary sources. Production offormic and acetic acids by the atmospheric oxidation of hydrocarbons emitted by the vegetation has been proposed as a major source of these acids (Jacobs and Wofsy, 1988; Madronich et al., 1990; Martin et al., 1991). Laboratory evidences have been provided by Horie et al. (1994) and Neeb et al. (1994). The good correlation observed in this work between the boundary layer concentrations of both acids (Fig. 6) at very different sites (i.e. coastal, cloud forest, savannah) and different seasons, strongly suggest that they are "simultaneously" produced by a general process occur- ring everywhere, likely the atmospheric oxidation of reactive hydrocarbons. During the wet season hydrocarbons emitted from the soil-vegetation system should predominate, whereas emissions from biomass burning should make a significant contribution during the dry season. It is interesting to note that in the model used by Madronich et al. (1990), to explain the photochemical origin of these acids, HCOOH and CH3COOH correlate in all the kinetic assumptions tested. The excellent correlation obtained in the savannah region between the boundary layer concentrations of acids and precursor hydrocarbons (i.e. isoprene), and other oxidation products (i.e. peroxides, aldehydes) strongly support the existence of an important atmospheric source of these acids (Sanhueza et al., 1995).

5. CONCLUSIONS

Formic and acetic acids are important components of the Venezuelan atmosphere. Gas-phase levels and rain concentrations indicate that the acids are homogeneously distributed in the region. Loss of organic acids by depositional processes is greater than by


chemical loss mechanism. Dry deposi t ion contr ibutes ~ 5 0 % to the annua l a tmospher ic removal budget.

The sources of acids are larger dur ing the wet season and the higher regional levels observed dur ing the dry season are mainly due to a longer a tmospher ic lifetime, ra ther than to higher sources of acids. Likely the most impor t an t source of acids is their p roduc t ion in the oxidat ion of hydrocarbons ; soil emissions could make a significant con t r ibu t ion dur ing the dry season.

Acknowledgements--We appreciate important logistic support and facilities given at the Estaci6n Biolbgica de los Llanos by the Sociedad Venezolana de Ciencias Naturales and at Canaima by EDELCA. The Auyantepuy campaign was sponsored by the GEO Foundation (Germany) and Terramar (Venezuela). The chemical characterization of Venezuelan savannah rains is part of a larger project financed by the Environmental Protection Agency, Assist- ance Agreement CR 82175801.

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Atmospheric formic and acetic acids in Venezuela

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