Determination of enhancement ratios of HCOOH relative to CO in biomass burning plumes by the Infrared Atmospheric Sounding Interferometer (IASI)

Abstract. Formic acid (HCOOH) concentrations are often underestimated by models, and its chemistry is highly uncertain. HCOOH is, however, among the most abundant atmospheric volatile organic compounds, and it is potentially responsible for rain acidity in remote areas. HCOOH data from the Infrared Atmospheric Sounding Interferometer (IASI) are analyzed from 2008 to 2014 to estimate enhancement ratios from biomass burning emissions over seven regions. Fire-affected HCOOH and CO total columns are defined by combining total columns from IASI, geographic location of the fires from Moderate Resolution Imaging Spectroradiometer (MODIS), and the surface wind speed field from the European Centre for Medium-Range Weather Forecasts (ECMWF). Robust correlations are found between these fire-affected HCOOH and CO total columns over the selected biomass burning regions, allowing the calculation of enhancement ratios equal to 7.30  ×  10⁻³ ± 0.08  ×  10⁻³ mol mol⁻¹ over Amazonia (AMA), 11.10  ×  10⁻³ ± 1.37  ×  10⁻³ mol mol⁻¹ over Australia (AUS), 6.80  ×  10⁻³ ± 0.44  ×  10⁻³ mol mol⁻¹ over India (IND), 5.80  ×  10⁻³ ± 0.15  ×  10⁻³ mol mol⁻¹ over Southeast Asia (SEA), 4.00  ×  10⁻³ ± 0.19  ×  10⁻³ mol mol⁻¹ over northern Africa (NAF), 5.00  ×  10⁻³ ± 0.13  ×  10⁻³ mol mol⁻¹ over southern Africa (SAF), and 4.40  ×  10⁻³ ± 0.09  ×  10⁻³ mol mol⁻¹ over Siberia (SIB), in a fair agreement with previous studies. In comparison with referenced emission ratios, it is also shown that the selected agricultural burning plumes captured by IASI over India and Southeast Asia correspond to recent plumes where the chemistry or the sink does not occur. An additional classification of the enhancement ratios by type of fuel burned is also provided, showing a diverse origin of the plumes sampled by IASI, especially over Amazonia and Siberia. The variability in the enhancement ratios by biome over the different regions show that the levels of HCOOH and CO do not only depend on the fuel types.