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 (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 h−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.