Sources and sinks of formic, acetic, and pyruvic acids over central Amazonia: 2. Wet season

Abstract

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.

Urban and rural observations of carboxylic acids in rainwater in Southwest of China: the impact of urbanization

“…It was estimated the anthropogenic activity could be insignificant in this area. As a result, the seasonal variations of carboxylic acids indicate the biogenic emissions from vegetation dominated the sources of carboxylic acids in this site which was consistently with previous studies (Talbot et al 1990;Kuhn et al 2002;Peña et al 2002). However, the remarkable opposite seasonal pattern of carboxylic acids were observed in Guiyang (Fig.…”

“…On the contrary, the gas phase F/A ratio is more than 1 in Guiyang which may be caused by secondary sources in the atmosphere (Jordan et al 2009). Biomass burning 0.1∼0.5 (Talbot et al 1988) 0.2∼0.4 (Hartmann et al 1991) Vehicular emissions 0.4∼0.6 (Talbot et al 1988) 0.3∼0.5 (Grosjean 1992) Vegetation (tropical forests) 0.6 (Talbot et al 1990) Vegetation emissions 0.4 (Servant et al 1991) Secondary sources >1 (Talbot et al 1988) The unsaturated hydrocarbons (olefins, isoprene) are important factor to control the concentrations of carboxylic acids in the atmosphere which may originate from biogenic or anthropogenic sources (Lee et al 2006). …”

Isoprene photooxidation: new insights into the production of acids and organic nitrates

“…Since the main source of both acids in the chamber is unlikely to result from the ozonolysis of the alkenes, our study shows that additional channels for their formation should be included. The main identified sources (hydroxyacetone, glycolaldehyde, organic nitrates) are much longerlived than the ones currently included in global model which may help resolve part of the discrepancy between models (Jacob and Wofsy, 1988) and atmospheric observations (Andreae et al, 1988;Talbot et al, 1990).…”

Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic

“…During SONEX, formic acid was present in concentrations of =45 ppt in the UT and =120 ppt in the lower troposphere (Figure 9) , the process is quite slow, and this is at best a minor source of HOx radicals. In most cases, organic acids are removed from the atmosphere by wet and dry deposition processes, and their chemistry is at best highly uncertain [Talbot et al, 1990[Talbot et al, , 1995Jacob et al, 1996]. An accurate enough source inventory for these organic acids is not currently available to model their global distribution.…”