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Is the ocean surface a source of nitrous acid (HONO) in the marine boundary layer?

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JournalAtmospheric Chemistry and Physics
DateAccepted/In press - 8 Nov 2021
DatePublished (current) - 16 Dec 2021
Issue number24
Volume21
Number of pages13
Pages (from-to)18213-18225
Original languageEnglish

Abstract

Nitrous acid, HONO, is a key net photolytic precursor to OH radicals in the atmospheric boundary layer. As OH is the dominant atmospheric oxidant, driving the removal of many primary pollutants and the formation of secondary species, a quantitative understanding of HONO sources is important to predict atmospheric oxidising capacity. While a number of HONO formation mechanisms have been identified, recent work has ascribed significant importance to the dark, ocean-surface-mediated conversion of NO2 to HONO in the coastal marine boundary layer. In order to evaluate the role of this mechanism, here we analyse measurements of HONO and related species obtained at two contrasting coastal locations - Cabo Verde (Atlantic Ocean, denoted Cape Verde herein), representative of the clean remote tropical marine boundary layer, and Weybourne (UK), representative of semi-polluted northern European coastal waters. As expected, higher average concentrations of HONO (70ppt) were observed in marine air for the more anthropogenically influenced Weybourne location compared to Cape Verde (HONO <5ppt). At both sites, the approximately constant HONO/NO2 ratio at night pointed to a low importance for the dark, ocean-surface-mediated conversion of NO2 into HONO, whereas the midday maximum in the HONO/NO2 ratios indicated significant contributions from photo-enhanced HONO formation mechanisms (or other sources). We obtained an upper limit to the rate coefficient of dark, ocean-surface HONO-to-NO2 conversion of CHONOCombining double low line0.0011ppbh-1 from the Cape Verde observations; this is a factor of 5 lower than the slowest rate reported previously. These results point to significant geographical variation in the predominant HONO formation mechanisms in marine environments and indicate that caution is required when extrapolating the importance of such mechanisms from individual study locations to assess regional and/or global impacts on oxidising capacity. As a significant fraction of atmospheric processing occurs in the marine boundary layer, particularly in the tropics, better constraint of the possible ocean surface source of HONO is important for a quantitative understanding of chemical processing of primary trace gases in the global atmospheric boundary layer and associated impacts upon air pollution and climate.

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Funding Information:
Financial support. This research has been supported by the Natural Environment Research Council (grant nos. NE/K012169/1, NE/M013545/1, NE/M010554/1, NE/S000518/1 and NE/K004069/1).

Funding Information:
Acknowledgements. The authors would like to thank Luis Neves, Instituto Nacional de Meteorologia e Geofisica (INMG), Cape Verde, for day-to-day running of the CVAO NOx measure- ments. The University of Leicester authors thank Roberto Som-mariva for his technical and organisational input into the WAO campaign and Roland Leigh (now at EarthSense Systems Ltd) for processing the spectral radiometer data from WAO. This work was supported by the Natural Environment Research Council (NERC), UK, through projects ICOZA (Integrated Chemistry of Ozone in the Atmosphere, NE/K012169/1), SNAABL (Sources of Nitrous Acid in the Atmospheric Boundary Layer, NE/M013545/1 (Birmingham) and NE/M010554/1 (Leicester)), Atmospheric Reactive Nitrogen Cycling over the Ocean (NE/S000518/1), and Assessment of ClNO2 as a missing oxidant in the UK atmosphere (NE/K004069/1).

Publisher Copyright:
© 2021 Leigh R. Crilley et al.

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