Extensive field measurements of nitrous acid in the marine atmosphere
Scientists have shed new light on the ‘self-cleaning’ capacity of the atmosphere.
Field measurements around Cabo Verde in the tropical Atlantic have confirmed that nitrous acid (HONO) is widespread in the lower layers of the atmosphere above the oceans. Evidence for the release of this substance from photolysis of nitrate aerosols is now presented by an international research team led by the University of York (UK) in Science Advances, the open access journal of SCIENCE. Nitrous acid (HONO) plays a central role in tropospheric chemistry as an important source of the hydroxyl radical (OH) - in both polluted and clean environments.
Using a combination of aircraft and ground-based observations, scientists were able to confirm the widespread presence of nitrous oxide (HONO) in the remote Atlantic troposphere formed by so-called “renoxification”, whereby photolysis of aerosol nitrate returns nitrogen oxides (NOx) and HONO to the marine atmosphere.
Historically, aerosol nitrate had been considered a permanent sink for NOx. This new process could increase the ability of the atmosphere to self-cleanse on a global scale.
Scientists say the findings, published in Sciences Advances, could be highly significant for atmospheric chemistry and largely reconcile widespread uncertainty on the importance of renoxification.
With funding from the Natural Environmental Research Council (NERC), scientists from the Wolfson Atmospheric Chemistry Laboratories (WACL) - a collaborative venture between the University of York and the National Centre for Atmospheric Science (NCAS) - led extensive aircraft and ground-based observations in and around Cape Verde in the Atlantic Ocean in August 2019 and February 2020. The ARNA (Atmospheric Reactive Nitrogen over the remote Atlantic) field campaigns used the British atmospheric research aircraft FAAM BAe-146-301. Twelve flights were conducted with in situ measurements including Nitric Oxide (NO), Nitrogen dioxide (NO2), Nitrous acid (HONO), Ozone (O3), and aerosol surface area. Particulate nitrate (pNO-3) was determined from aerosol filters. Photolysis rates and OH radical concentrations were modelled using the global three-dimensional atmospheric chemistry model GEOS-Chem. At the Cape Verde Atmospheric Observatory (CVAO), Nitrogen oxide (NOx), Particulate nitrate (pNO-3), Ozone (O3) and photolysis rates are measured routinely. These were supplemented by HONO measurements during the ARNA campaigns.
Lead author, Professor Lucy Carpenter said: “Importantly, the observations showed that the efficiency of renoxification increased with relative humidity and decreased with the concentration of nitrate. This observation reconciled the very large discrepancies in the rates of renoxification found across multiple laboratory and field studies. It was also consistent with renoxification occurring on the surface of aerosols, rather than within their bulk, a new and exciting finding with implications for how this fundamental process is controlled and parameterised in models.”
It is also noted that nitrate aerosols are becoming increasingly important in the atmosphere as precursor ammonia emissions increase and ammonium sulphate aerosols decrease. "Thus, the formation of nitrogen oxides from nitrate aerosols could have important, increasing and previously unexplored effects on the trends and distributions of atmospheric oxidants," adds Dr Khanneh Wadinga Fomba of TROPOS, who has been researching aerosols on Cabo Verde for many years.
Recycling of nitrogen oxides on nitrate aerosol could have important, increasing, and as yet unexplored implications for the trends and distributions of atmospheric oxidants such as tropospheric ozone, an important greenhouse gas.
Simone T. Andersen, Lucy J. Carpenter, Chris Reed, James D. Lee, Rosie Chance, Tomás Sherwen, Adam R. Vaughan, Jordan Stewart, Pete M. Edwards, William J. Bloss, Roberto Sommariva, Leigh R. Crilley, Graeme J. Nott, Luis Neves, Katie Read, Dwayne E. Heard, Paul W. Seakins, Lisa K. Whalley, Graham A. Boustead, Lauren T. Fleming, Daniel Stone, Khanneh Wadinga Fomba (2023). Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols. Sci. Adv. 9, eadd6266 (2023). https://doi.org/10.1126/sciadv.add6266
The research was funded by the National Environmental Research Council (NERC, grant NE/S000518/1), the National Centre for Atmospheric Research (NCAS), the SPHERES Natural Environment Research Council (NERC) Doctoral Training Partnership (DTP, grant NE/L002574/1), the European Research Council (ERC) under the European Union’s Horizon 2020 programme (project O3-SML; grant agreement no. 833290).
Dr. Khanneh Wadinga Fomba
Department Chemistry of the Atmosphere, Leibniz Institute for Tropospheric Research (TROPOS), Leipzig
Phone: +49 341 2717-7033
Professor Lucy Carpenter
UK principal investigator of the Cape Verde Observatory (CVAO)
Department of Chemistry, University of York, Heslington (UK)
Phone: +44 1904 324588
Tilo Arnhold, Public relations, TROPOS
Phone: +49 341 2717-7189
Long-term observation of aerosol at CVAO (Cape Verde Atmospheric Observatory)
Cape Verde Atmospheric Observatory (CVAO)
ARNA (Atmospheric Reactive Nitrogen over the remote Atlantic)
Facility for Airborne Atmospheric Measurements (FAAM)
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