Preprints
https://doi.org/10.5194/acp-2021-532
https://doi.org/10.5194/acp-2021-532

  07 Jul 2021

07 Jul 2021

Review status: this preprint is currently under review for the journal ACP.

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

Leigh Crilley1,a, Louisa Kramer1,b, Francis Pope1, Chris Reed2,c, James Lee2, Lucy Carpenter2, Lloyd Hollis3, Stephen Ball3, and William Bloss1 Leigh Crilley et al.
  • 1School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, United Kingdom
  • 2Wolfson Atmospheric Chemistry Laboratory (WACL), Department of Chemistry, University of York, Heslington, York, United Kingdom
  • 3School of Chemistry, University of Leicester, Leicester, United Kingdom
  • anow at: Department of Chemistry, York University, Toronto, ON, Canada
  • bnow at: Ricardo Energy & Environment, Harwell, Oxfordshire, UK
  • cnow at: FAAM Airborne Laboratory, Building 146, Cranfield University, Cranfield, UK

Abstract. Nitrous acid, HONO, is a key net photolytic precursor to OH radicals in the atmospheric boundary later. 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 – Cape Verde (Atlantic Ocean), representative of the clean remote tropical marine boundary layer, and Weybourne (United Kingdom), representative of semi-polluted Northern European coastal waters. As expected, higher average concentrations of HONO (70 ppt) were observed in marine air for the more anthropogenically influenced Weybourne location compared to Cape Verde (HONO < 5 ppt). 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 CHONO = 0.0011 ppb hr−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.

Leigh Crilley et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-532', Anonymous Referee #1, 20 Jul 2021
  • RC2: 'Comment on acp-2021-532', Anonymous Referee #2, 25 Aug 2021

Leigh Crilley et al.

Leigh Crilley et al.

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Short summary
Nitrous acid (HONO) is a key source of atmospheric oxidants. We evaluate if the ocean surface is a source of HONO into the marine boundary layer, using measurements from two contrasting coastal locations. We observed no evidence for a night-time ocean surface source, in contrast to previous work. This points to significant geographical variation in the predominant HONO formation mechanisms in marine environments, reflecting possible variability in the sea-surface microlayer composition.
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