Photochemical Production of Carboxylic Acids in a Remote Continental Atmosphere

Abstract

Model calculations are conducted to investigate the production of carboxylic acids from photochemical decomposition of isoprene, one of the main natural hydrocarbons emitted from vegetation. Both gas-phase and aqueous-phase chemical reaction pathways are examined. A simple dynamical model is proposed to simulate the boundary layer of the Amazon rain forest, and model predictions are compared to measurements made in that region in July 1985. It is found that formic acid, methacrylic acid, and pyruvic acid can be produced in significant quantities by gas-phase decomposition of isoprene. In the Amazon basin, this source may yield concentrations of these acids in the order of 1 ppb, 0.1 ppb, and 0.02 ppb, respectively. Production of formic acid in cloud by aqueous-phase oxidation of CH2O does not greatly increase the formic acid concentration predicted from the gas-phase mechanism; cloud droplets with pH > 4 are actually expected to constitute net sinks for formic acid. No significant production of acetic acid is expected from the photochemical decomposition of isoprene. Comparisons of model predictions with field data indicates that isoprene could be a major source of formic acid and pyruvic acid observed in the gas phase and in rainwater; however, acetic acid must originate from another source.