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 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.

An investigation into the chemistry of HONO in the marine boundary layer at Tudor Hill Marine Atmospheric Observatory in Bermuda

“…4a), with maxima of ∼ 3 pptv around solar noon and minima near or below ∼ 1 pptv during the early morning hours before sunrise. The observed diurnal trends of HONO were in good agreement with those observed from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer (Crilley et al, 2021;Kasibhatla et al, 2018;Reed et al, 2017;Ye et al, 2017a). At night, HONO concentrations reached steady-state concentrations of ∼ 1 and 0.7 pptv for the spring and summer season, respectively; these nocturnal HONO concentration steady states were consistent with previous reports in the clean marine boundary layer (Crilley et al, 2021;Kasibhatla et al, 2018;Reed et al, 2017;Ye et al, 2017a).…”

“…4). Nighttime steady states in HONO concentrations were previously reported in a clean (Crilley et al, 2021;Kasibhatla et al, 2018;Reed et al, 2017;Ye et al, 2017a) and polluted marine environment (Wojtal et al, 2011). Several recent research works showed nighttime HONO accumulations in polluted marine environments (Wen et al, 2019;Wojtal et al, 2011;Yang et al, 2021;Zha et al, 2014).…”

“…To date, few studies have investigated HONO distribution and its chemical cycling in the marine boundary layer (MBL), even though the oceans cover > 70 % of the earth surface (Crilley et al, 2021;Kasibhatla et al, 2018;Reed et al, 2017;Wojtal et al, 2011;Ye et al, 2016b;Zha et al, 2014). Several studies observed substantial HONO concentrations (∼ 3.5-11 pptv) during the day in clean, well-aged marine air (Crilley et al, 2021;Kasibhatla et al, 2018;Reed et al, 2017;Ye et al, 2016b). Evidence from both field observations and laboratory experiments suggested that pNO 3 photolysis might contribute to a major fraction of the observed level of HONO in the marine atmosphere (Ye et al, 2016b).…”

Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols

“…Nitrous acid (HONO) has a pivotal role in tropospheric chemistry as an important source of the hydroxyl radical (OH) (1)(2)(3)(4)(5)(6)(7). It has also been proposed as a substantial source of NO x (NO and NO 2 ) to the remote marine environment (8)(9)(10). NO x regulates the abundance of atmospheric oxidants and is essential for the formation of secondary atmospheric aerosols, and OH controls the self-cleansing capacity of the atmosphere via degradation of pollutants and greenhouse gases such as methane (1).…”

Snowmelt Leads to Seasonal Nitrous Acid Formation Across Northwestern China

“…It is now generally accepted that HONO can be directly emitted into the atmosphere from traffic, soil emissions, etc. (Kirchstetter et al., 1996; Su et al., 2011; Xu et al., 2015), while it can also be formed through gas‐phase or heterogeneous reactions, including the gas‐phase reaction of NO with • OH (Kleffmann, 2007; Lee et al., 2016), heterogeneous reactions and photosensitized conversion of NO 2 on various surfaces (Crilley et al., 2021; Finlayson‐Pitts et al., 2003; George et al., 2005; Stemmler et al., 2006), photolysis of nitric acid and nitrate anions (NO 3 − ) (Scharko et al., 2014; Wang, Dalton, et al., 2020; Zhou et al., 2003, 2011), neonicotinoid nitenpyram ozonolysis (Wang, Ezell, et al., 2020) and nitroaromatic compounds photolysis (Bejan et al., 2006; Li et al., 2020; Meusel et al., 2017; You et al., 2022). Besides, local micrometeorological processes, such as the formation of an atmospheric stable decoupled layer at the ground level, can significantly influence the HONO concentrations in the air (Wang, Li, et al., 2021).…”