Low molecular weight carboxylic acids in an urban atmosphere: Winter measurements in São Paulo City, Brazil

Abstract

Atmospheric gas and particle-phase carboxylic acids were measured during July 1996, Winter, in an urban area of São Paulo, a highly polluted Latin American city. Ion chromatography and capillary electrophoresis techniques were used to determine the species. As oxalic (36.2±21.4%), pyruvic (15.0±7.9%), β-hydroxy-butyric (9.15±9.00%) and glycolic (3.55±2.26%) acids were determined in aerosol particles, formic and acetic acids were determined both in the gaseous (4.36±2.70 and 3.66±2.63 ppbv, respectively) and particulate phases (17.8±12.4 and 18.2±9.8%, respectively). Approximately 98% of the total acetic and formic acids were in the gas-phase and the gas–aerosol equilibrium was influenced by high levels of relative humidity. Gaseous formic-to-acetic ratios fell in the 0.94–1.85 range. Photochemical production appeared to be a very likely source of the gaseous acetic and formic acid levels found in this investigation. Direct emissions, mainly motor exhaust of vehicles also contributed to their presence in air. The observed amounts of formic and acetic acids in the particle phase were comparable with those observed in other urban sites. Results from aerosol particles indicated lower concentrations of the carboxylic acids at night, but their diurnal and nocturnal variation were similar. Using a correlation matrix, it was possible to suggest some sources for the carboxylic acids in the particulate phase. During daytime, vehicular emission appeared to be the primary source of acetic acid, whereas formic and pyruvic acids should be formed photochemically. Moreover, emissions from biogenic primary sources appeared to be an important contribution to atmospheric concentrations of formic and glycolic acids. Presumably, the photooxidation of pyruvic and glycolic acids gave rise to the oxalic acid. No source for acetic and pyruvic acids at nighttime was possible to suggest. However, direct vehicular and biogenic emissions might be major sources of TOC in nocturnal measurements. Oxalic acid might result from vehicular emission, glycolic acid from biogenic emission and formic acid from both sources.