Ratios of atmospheric formic acid and acetic acid seen from space

Abstract : Formic acid (HCOOH) and acetic acid (CH3COOH) are ubiquitous atmospheric trace gases and the most abundantcarboxylic acids in the global troposphere. They have a substantial impact on the atmospheric aqueous-phasechemistry and are major sources of cloud and precipitation acidity. Together, they account for > 60% of the rain-water acidity in remote regions (e.g., over tropical and boreal forests). Sources of formic and acetic acids includedirect emissions (e.g., from biomass burning, fossil fuel, plants) and secondary production from sunlight-induceddegradation of a suite of other volatile organic compounds. However, several investigations have pointed to largeinconsistencies between measurements and model simulations, suggesting key gaps in our understanding of theirsources and the likely existence of so far unidentified sources.Here we use a neural network-based approach to retrieve total columns of formic and acetic acids from theIASI (Infrared Atmospheric Sounding Interferometer) satellite observations, and to produce daily global andregional pictures of both species. With this dataset we characterize their respective spatial distributions, seasonalvariability as well as their transport patterns from emission sources. We focus on their emission regions to studythe variability and evolution of the HCOOH-to-CH3COOH ratios. Their respective enhancement ratios withrespect to CO total columns from IASI are also investigated in fire plumes. This study contributes to improvingour understanding of the emission sources of these dominant carboxylic acids.