We have determined the atmospheric concentrations of formic and acetic acid in gas and particulate phases and in precipitation over a 15-month time period at a site in eastern Virginia. These atmospheric species occurred principally in the gas phase (•>98%), with a marked annual seasonality. Atmospheric concentrations of formic and acetic acid in the gas phase averaged 1890 ___ 1235 and 1310 ___ 910 ppt during the growing season, compared to 695 ___ 405 and 700 _+ 375 ppt over the nongrowing season, respectively. Our data support the hypothesis that biogenic emissions from vegetation are important sources of atmospheric formic and acetic acid during the local growing season. The same general time series trends were also evident in precipitation, but the seasonality was not defined nearly as well as in the gas phase. Atmospheric concentrations of aerosol formate and acetate showed substantial temporal variability and generally remained in the 5-to 25-ppt range throughout the year. Our measurements indicate that the near-surface (< 10 m altitude) diurnal behavior of formic and acetic acids in the gas phase is as follows: the concentrations are lowest prior to sunrise, increase steadily thereafter, and begin to decline by late afternoon, again reaching very low levels in the early morning hours. We have observed a diurnal variation with an amplitude of up to tenfold in atmospheric concentration for formic acid. This pronounced diurnal variation is presumably related to mixed layer dynamics, in addition to daytime biogenic and photochemical source inputs and gaseous dry deposition at nighttime. The removal of organic acids from the nocturnal boundary layer may be facilitated by uptake of their vapors into dew. A striking feature of our data is the distinct differences in both gas phase and precipitation ratios of formic-to-acetic acid between the growing and nongrowing seasons. In the gas phase this ratio had a mean value of 1.33 during the growing season, decreasing to 0.90 in the nongrowing season. Likewise, the ratio averaged 1.43 and 1.09 in precipitation at this site during the growing and nongrowing season, respectively. Our measurements indicated that direct emissions from motor vehicles and biomass combustion processes are important sources of atmospheric acetic acid and that these sources have formicto-acetic ratios much less than 1.0. We hypothesize that the seasonal variability in formic-to-acetic acid ratios in ambient air is due to a shift in the relative dominance of biogenic versus anthropogenic sources. During the nongrowing season, anthropogenic inputs appear to be the principal source of acetic acid to the atmosphere. However, it remains unclear as to what the major source of formic acid is to the wintertime atmosphere. 1988a]. Carboxylic acids are also common constituents of global precipitation [Galloway et al., 1982-1, with formic and acetic acids generally constituting the majority of the free acidity in remote regions [Keene et al., 1983; Keene and Galloway, Paper number 7D0900. 0148-0227/88/00...

We have determined the gas phase concentrations of formic (HCOOH), acetic (CH3COOH), and pyruvic (CH3C(O)COOH) acids in the forest canopy, boundary layer, and free troposphere over the central Amazon Basin during the April–May segment of the 1987 wet season. At 150‐m altitude in the boundary layer the daytime average concentrations were 430±225, 340±155, and 25± 5 ppt for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. These values were fivefold lower than those observed in the 1985 dry season. Concentrations measured near canopy top were not significantly different from boundary layer values (P = 0.10), while concentrations in the lower canopy were significantly less. Concentrations in the free troposphere (5 km) were lower than in the boundary layer and averaged 170±40, 210±40, and 15±15 ppt for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. Fivefold enhancements of CH3C(O)COOH concentrations were observed in convective outflows at 5‐ to 6‐km altitudes. Aerosol carboxylate concentrations were usually below our detection limit of 5–10 ppt. Preliminary branch enclosure measurements indicated significant direct emission of carboxylic acids by vegetation. A one‐dimensional photochemical model for the canopy and the boundary layer was used to examine the contributions from various sources to the carboxylic acid budgets. Model results indicate that direct emissions from vegetation can account for most of the concentrations observed in the canopy. These emissions peak during the daytime hours, and 24‐hour average upward fluxes at canopy top are 4.4×109, 3.7×109, and 2.8×108 molecules cm−2 s−1 for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. However, direct emissions from vegetation can account for only a small fraction of the observed carboxylic acid concentrations in the boundary layer, suggesting a large contribution from atmospheric sources. The atmospheric reactions previously suggested in the literature as sources of carboxylic acids (gas phase decomposition of isoprene, CH3CO3 + peroxy, aqueous phase oxidation of CH2O) appear to be too slow to explain the observed concentrations. Other atmospheric reactions, so far unidentified, could make a major contribution to the carboxylic acid budgets.

The distribution and chemistry of the atmospheric aerosol over the Amazon Basin during the April-May segment of the 1987 wet season are described using ground-and aircraft-based data. Wet season aerosol concentrations and composition are variable in contrast to the remarkably uniform distribution and composition of the predominantly biogenic aerosol that we observed during the 1985 dry season. Four distinct intervals of enhanced aerosol concentration coincided with 3-to 5-day periods of extensive rainfall over central Amazonia. It is hypothesized that a major source of aerosols to the basin was the direct input of northern hemispheric air laden with variable mixtures of Saharan dust, marine aerosol, and possibly biomass combustion products. The enhanced aerosol concentrations over Amazonia were reduced in 1-3 days to 5-10% of their peak levels by large-scale changes in the circulation field with subsequent decoupling from the source region, frequent precipitation, and intermixing of northern and southern hemispheric air masses. The intrusion of northern hemispheric air into the Amazon Basin is linked to the establishment and persistence of the West African Subtropical High (WASH) in a limited region over west Africa. Marine aerosols may be intermixed with the soil dust during transit across the Atlantic or within the sea breeze regime along the northeast coast of South America. It is proposed that a principal source of NO•-and SO]-associated with the dust is biomass burning south of the Sahara in western Africa.

Rain samples from three sites in central Amazonia were collected over a period of 6 weeks during the 1987 wet season and analyzed for ionic species and dissolved organic carbon. A continuous record of precipitation chemistry and amount was obtained at two of these sites, which were free from local or regional pollution, for a time period of over 1 month. The volume-weighted mean concentrations of most species were found to be about a factor of 5 lower during the wet season compared with previous results from the dry season. Only sodium, potassium, and chloride showed similar concentrations in both seasons. When the seasonal difference in rainfall amount is taken into consideration, the deposition fluxes are only slightly lower for most species during the wet season than during the dry season, again with the exception of chloride, potassium, and sodium. Sodium and chloride are present in the same ratio as in sea salt; rapid advection of air masses of marine origin to the central Amazon Basin during the wet season may be responsible for the observed higher deposition flux of these species. Statistical analysis suggests that sulfate is, to a large extent, of marine (sea salt and biogenic) origin, but that long-range transport of combustion-derived aerosols also makes a significant contribution to sulfate and nitrate levels in Amazonian rain. Organic acid concentrations in rain were responsible for a large fraction of the observed precipitation acidity; their concentration was strongly influenced by gas/liquid interactions. INTRODUCTION Almost a third of the rain which falls on the Earth's continents is deposited within the narrow equatorial belt between 10øN and 10øS latitude. However, in spite of the resulting importance of tropical rainfall for global biogeochemical cycling, there are relatively few studies on precipitation chemistry and wet deposition rates in the wet tropics [Visser, 1961; Wetselaar and Hutton, 1963; Stallard and Edmond, 1981; Lewis, 1981; Galloway et al., 1982; Keene et al., 1983; Hendry et al., 1984; Junk and Furch, 1985; Likens et al., 1987; Lacaux et al., 1987; Ayers and Gillett, 1988]. Furthermore, some of the former studies have used techniques (e.g., long-term bulk collection) which do not produce reliable data on rain chemistry and deposition rates. The precipitation chemistry measurements reported here are a component of our studies on the cycles of sulfur and organic acids over the Amazon Basin [Andreae and Andreae, 1988; Andreae et al., 1988c, this issue; Talbot et al., this issue (b)]. In these investigations, our goal is to obtain data which enable us to estimate simultaneously the source fluxes, atmospheric transformation rates, and deposition rates of the relevant species. At the high rainfall rates characteristic of Amazonia, wet deposition fluxes are particularly useful in this type of study, since they are the dominant sink for many species and thus provide an estimate for the integrated removal flux of chemical species from the atmosphere. These deposition fluxes can then be c...

During June 1986, eight systems for measuring vapor phase and four for measuring particulate phase concentrations of formic acid (HCOOH) and acetic acid (CH3COOH) were intercompared in central Virginia. HCOOH and CH3COOH vapors were sampled by condensate, mist, Chromosorb 103 GC resin, NaOH-coated annular denuders, NaOH impregnated quartz filters, K2CO3 and Na2CO3 impregnated cellulose filters, and Nylasorb membranes. Atmospheric aerosol was collected on Teflon and Nuclepore filters using both hi-vol and 10-vol systems to measure particulate phase concentrations. Samples were collected during 31 discrete day and night intervals of 0.5-2 hour duration over a 4-day period. Performance of the mist chamber and K2CO3 impregnated filter techniques were also evaluated using zero air and ambient air spiked with HCOOHg, CH3COOHg , and formaldehyde (CH2Og) from permeation sources. Results of this intercomparison show significant systematic and episodic artifacts among many currently deployed measurement systems for HCOOHg and CH3COOHg. The spiking experiments revealed no significant interferences for the mist chamber technique and results generated by the mist chamber and denuder techniques were statistically indistinguishable. The condensate technique showed general agreement with the mist chamber and denuder methods, but episodic bias between these systems was inferred from large and significant differences observed during the first day of sampling. Nylasorb membranes are unacceptable for collecting carboxylic acid vapors as they did not retain HCOOHg and CH3COOHg quantitatively.Strong base impregnated filter and GC resin sampling techniques are prone to large positive interferences apparently resulting, in part, from reactions involving CH2Og to generate HCOOH and CH3COOH subsequent to collection. Significant bias presumably associated with differences in postcollection handling was observed for particulate phase measurements by participating groups. Analytical bias did not contribute significantly to differences in vapor and particulate phase measurements.