Isoprene Versus Monoterpenes As Gas-Phase Organic Acid Precursors In The Atmosphere

Gas-phase organic acids are ubiquitous throughout the atmosphere and are tracers for volatile organic compound oxidation, although their abundance and sources are poorly characterized. Several studies have pointed to the oxidation of isoprene and monoterpenes as major sources of formic and acetic acids. Here, we quantify >100 gas-phase organic acids produced from isoprene and a-pinene oxidation in a series of laboratory chamber experiments. Overall, gas-phase organic acids constitute 1-28% of the initial organic carbon reacted in the experiments. Isoprene is a precursor for detected organic acids in amounts at least 2-3x greater than a-pinene from OH oxidation. We compare laboratory observations with two summertime field sites.one dominated by isoprene emissions (Southern Oxidant and Aerosol Study-SOAS) and one by monoterpene emissions (Seasonal Particles in Forests Flux studY-SPiFFY). Daytime organic acid mixing ratios were 4x higher at SOAS than SPiFFY, likely because of the substantial isoprene OH oxidation. In contrast to SOAS, factor analysis reveals separate daytime and nighttime organic acid sources at SPiFFY not obviously originating from photochemical oxidation. When available, isoprene is likely a dominant precursor for gas-phase organic acids.