First satellite mapping of nitrous acid (HONO) in wildfire plumes

Dr. Nicolas Theys,
Royal Belgian Institute for Space Aeronomy (BIRA-IASB)

"Nitrous acid (HONO) is a short-lived reactive gas and plays a key role in the atmosphere through its influence on the OH budget, and contributes to secondary aerosols and ozone formation. Laboratory experiments and aircraft field campaigns have revealed that biomass burning is a source of HONO. However, the global importance of pyrogenic HONO is poorly constrained. Here we present the first global measurements of HONO using the TROPOspheric Monitoring Instrument (TROPOMI) onboard Sentinel-5 Precursor. Using Differential Optical Absorption Spectroscopy (DOAS) applied in the UV, we analyzed one year of TROPOMI data and found that HONO is unambiguously detected in wildfire plumes from main biomass burning regions, consistent with volume mixing ratios of several ppbvs of HONO.

During July-September 2018, the University of Colorado participated to a field campaign (Biomass Burning Fluxes of Trace Gases and Aerosols (BB-Flux)), performed research flights near fire plumes in the US and successfully detected HONO for several of them using the CU-DOAS aircraft instrument. For some cases, flights were planned around the TROPOMI overpass. We exploit this unique opportunity to validate our satellite HONO retrievals. We discuss the comparison results and reasons for discrepancies. In particular, viewing observations are very different and can lead to air mass sampling differences in the presence of elevated aerosols. To overcome this problem, we compared the ratio HONO/NO2 (RHN), which is also a proxy for HONO production. We found that satellite and aircraft RHNs are in excellent agreement, within their mutual error estimates. From the global RHNs, we demonstrate that previous assessments underestimate pyrogenic HONO emissions by a factor of 2–4 across all ecosystem types.

Finally, we performed model simulations to assess the impact of HONO on atmospheric oxidants, such as OH and tropospheric ozone. We estimate that HONO emissions are responsible for two-thirds of the OH production in fresh wildfire plumes worldwide and act to accelerate oxidative plume chemistry and ozone production. Our findings suggest that pyrogenic HONO emissions have a substantial impact on atmospheric composition, which enhances regional ozone levels by up to 7 ppbv."

N. Theys, R. Volkamer, J.-F. Müller, K. Zarzana, N. Kille, L. Clarisse, I. De Smedt, C. Lerot, H. Finkenzeller, F. Hendrick, T. Koenig, C.F. Lee, C. Knote, H. Yu, M. Van Roozendae.

Please note that the seminar begins at 12:40. For the Zoom link email anne.handschy@colorado.edu