Investigation of the glyoxal tropospheric column variability observed from space during wildfire events

Wildfires release in the atmosphere large amounts of aerosols and ozone precursors and have a significant impact on air quality and the climate. Global warming leads to more frequent and intense wildfires such as those that occurred in Australia in January 2020, in Siberia in 2021 or in California in the last few years. The spaceborne TROPOspheric Monitoring Instrument (TROPOMI) launched in October 2017 onboard the Sentinel-5 Precursor platform provides invaluable information on a series of key trace gases (NO_2, HONO, CO, HCHO, CHOCHO)and on aerosols. Its daily global coverage and high spatial resolution are ideal to monitor wildfire emissions and to characterize their spatial extent and temporal evolution.

In this work, we present an analysis of the trace gas distributions observed by TROPOMI for various selected intense wildfire events, with a specific focus on glyoxal (CHOCHO). Primarily emitted gases such as NO₂ show intense signals near the fire sources with limited spatial extent. On contrary, other species like formaldehyde, carbon monoxide and glyoxal are mostly produced via secondary processes, which contribute to extend significantly their spatial spread. Those TROPOMI spatial patterns are consistent with the current knowledge of the different production mechanisms. However, we report here the identification of a very strong reduction of glyoxal slant columns in presence of very high clouds or aerosols, while other gases do not show such behavior. Uptake on aerosols or cloud droplets is a known destruction mechanism for glyoxal and is the most likely cause for this signal reduction. We hypothesize that, owing to its high solubility in water, glyoxal is transferred into the liquid phase within the convective cells of (pyro)cumulus clouds and that (contrary to formaldehyde) it is not degassed upon freezing and therefore remains in the condensed phase in the upper troposphere. We investigate the conditions in which this process is observed by correlating the glyoxal level with e.g. the fire intensity, the presence of high clouds and their altitude (pyrocumulonimbus), atmospheric conditions (temperature and humidity), the nature of the burning eco-system, etc.