Formic and acetic acid over the central Amazon region, Brazil: 1. Dry season

We have determined the atmospheric concentrations of formic and acetic acid in the gas phase, in aerosols, and in rain during the dry season (July–August 1985) in the Amazonia region of Brazil. At ground level the average concentrations of gas phase formic and acetic acid were 1.6±0.6 and 2.2±1.0 ppb, respectively. The diurnal behavior of both acids at ground level and their vertical distribution in the forest canopy point to the existence of vegetative sources as well as to production by chemical reactions in the atmosphere. Dry deposition of the gaseous acids appears to be a major sink. The concentrations of formic and acetic acid in the gas phase were about 2 orders of magnitude higher than concentrations of the corresponding species in the atmospheric aerosol. About 50–60% of the aerosol (total) formate and acetate were in the size fraction below 1.0 μm diameter. The highest levels of aerosol formate and acetate were found in haze layers derived from biomass burning. In precipitation, (total) formate and acetate represented about one half of the anion equivalents. This is in contrast to the atmospheric aerosol, where they contributed less than 10% of the soluble anionic equivalents. Furthermore, the precipitation contained considerable acidity (average 36 μeq L−1 during the study period), again in contrast to the aerosol, which was acid-base neutral. The mean hydrogen ion concentration in rain was about 21–26 μeq L−1 (pH 4.6–4.7). Most of the precipitation acidity can be attributed to the organic acids, with sulfuric and nitric acids contributing only about 10–20% of the hydrogen ion concentration. Aerosol scavenging can explain only a small fraction of the observed amounts of formate and acetate in rain. The observed levels of these ions in rain are most likely the result of a combination of chemical reactions in hydrometeors and scavenging of the gaseous acids by cloud droplets.

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