Atmospheric HONO Observed Over Global Biomass Burning Regions Using Satellite Observations of TROPOMI and GEMS

Abstract Nitrous acid (HONO) is a key tropospheric species primarily due to ist role as a source of the hydroxyl radical (OH), which is pivotal in breaking down pollutants and greenhouse gases and is also a key ingredient to photochemical air pollution. Recent HONO measurements from space in fresh biomass burning plumes offer the potential to inform about HONO formation mechanisms globally. However, research is needed to further develop, interpret, and evaluate the satellite retrievals. Here, we present a new global HONO column data set of 5.5 years from the TROPOspheric Monitoring Instrument (TROPOMI). We leverage the Covariance‐Based Retrieval Algorithm to significantly improve the sensitivity to weak HONO signals over all biomass burning regions. Radiative transfer simulations for retrieving HONO columns indicate a strong dependence on plume height and smoke aerosols. Such information is mostly inaccessible from space for thick plumes but can be obtained from suborbital measurements during dedicated campaigns. We compare the TROPOMI HONO columns to aircraft observations from the BB‐FLUX campaign. When explicitly accounting for aerosols, the satellite and aircraft data are in good agreement albeit with significant comparison uncertainty. We also evaluate the TROPOMI retrievals against HONO columns measured by IASI and discuss the differences. Next, we demonstrate the potential of geostationary satellites like the Geostationary Environment Monitoring Spectrometer to provide temporally resolved information on pyrogenic HONO. Finally, we find a close relationship between satellite HONO detections and fire intensity both in space and time, highlighting the likely dominance of HONO production during the flaming phase of the fires.