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

Abstract

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.

Dominant Processes of HONO Derived from Multiple Field Observations in Contrasting Environments

“…Diurnal HONO data from 46 observational data sets at 31 sites, including 17 inland (urban or rural) sites, six coastal or island sites, four mountain/forest sites, and four marine/remote sites, were collected from published literature and our measurements (Figure S1 and Table S1). More details about these data sets can be found in the original literature. ,,,,− ,− In general, 21 of the observational data sets were extracted from observations conducted inside China and most of the rest from observations conducted inside North America or Europe. Our group conducted 13 observations with more available parameters, which are used to explore daytime HONO sources further.…”

HONO Measurement by Catalytic Conversion to NO on Nafion Surfaces

“…However, based on our surface area study, the limited information we have on Nafion drier geometry, and assuming a gas sample with a HONO/NO ratio of 1, it is possible that a HONO interference caused by a Nafion dryer inlet may result in measured NO mixing ratios that are 15–100% higher than the actual value. Higher bias would be observed in situations where NO and HONO concentrations are similar in magnitude, such as in indoor or marine environments . In the past, commercial CL instruments have been used along with LOPAP to measure NO and HONO emissions from soil. ,, If the CL instruments used were equipped with Nafion dryers, it is possible that a fraction of the observed NO signal could be due to HONO-to-NO conversion on the in-line Nafion dryer.…”

“…Higher bias would be observed in situations where NO and HONO concentrations are similar in magnitude, such as in indoor 73 or marine environments. 74 In the past, commercial CL instruments have been used along with LOPAP to measure NO and HONO emissions from soil. 16,18,75 If the CL instruments used were equipped with Nafion dryers, it is possible that a fraction of the observed NO signal could be due to HONO-to-NO conversion on the in-line Nafion dryer.…”

Comparison of Simultaneous Measurements of Indoor Nitrous Acid: Implications for the Spatial Distribution of Indoor HONO Emissions

“…For the sequential chlorine mopping experiment on June 10, a weighted fit of the measurements results in a slope of 1.20 ± 0.06 and an R 2 value of 0.72. Although this interference is unlikely to be significant in most forested and urban environments where the ratio of HOCl to HONO is very low, future LP/LIF measurements will require a detailed characterization of the HOCl interference during bleach cleaning experiments, or measurements in marine environments where the expected HONO mixing ratios are only a few ppt 43 compared to HOCl mixing ratios as high as 1 ppb. 44 Enhanced Ventilation Experiments.…”