Spaceborne Measurements of Formic and Acetic Acid: A Global View of the Regional Sources

Formic (HCOOH) and acetic (CH3COOH) acids are the most abundant carboxylic acids in the Earth’s atmosphere and contribute to the acidity of rainwater and cloud. Current knowledge is however unable to explain their elevated concentration levels measured in the atmosphere since their sources remain poorly known and quantified. The first measurements of acetic acid atmospheric abundance have recently been obtained from space by the Infrared Atmospheric Sounding Interferometer (IASI) satellite instrument (Franco et al., 2020). These measurements allow the production of global maps, which reveal that formic and acetic acids exhibit similar abundance, distributions and seasonality, pointing to major common sources. Furthermore, their atmospheric abundance appear to be closely linked to the hydrocarbon emissions from the terrestrial vegetation as well as to the presence of wildfires, especially in the tropics. Over Africa, evidence is provided that residual smoldering combustion related to wildfires might be a major driver of the formic and acetic acid seasonality. In some regions, differences between these two compounds suggest that sources and production pathways specific to each species are also at play.

Top panel: Means (on a 0.5° × 0.5° grid) of the HCOOH and CH3COOH total columns from the 2007–2018 IASI/Metop–A observations over October. Middle panel: Correlation coefficients between the daily 0.5° × 0.5° gridded HCOOH and CH3COOH total columns and (d) CH3COOH:HCOOH column ratios, over October throughout the 2007–2018 time period. Bottom panel: Time series of monthly mean HCOOH and CH 3 COOH total column (blue and purple solid lines, respectively, MEGAN–MOHYCAN isoprene and monoterpenes emission fluxes (green dotted line) and cumulative MODIS FRP (orange dotted line) over areas of interest. (Franco et al., 2020)

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