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

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.

摘要。亚硝酸（HONO）是大气边界层中羟基自由基的一种关键的净光解前体。由于羟基是大气中的主要氧化剂，推动了许多一次污染物的去除和二次物种的形成，因此对亚硝酸来源的定量了解对于预测大气氧化能力至关重要。虽然已经确定了许多亚硝酸的形成机制，但近期的研究表明，在沿海海洋边界层中，二氧化氮在海洋表面的暗反应转化为亚硝酸这一过程具有重要意义。为了评估这一机制的作用，我们在此分析了在两个截然不同的沿海地点——佛得角（大西洋），代表清洁的偏远热带海洋边界层，以及韦本（英国），代表半污染的北欧沿海水域——所获得的亚硝酸及相关物种的测量结果。正如预期的那样，在受人为影响更大的韦本地区的海洋空气中观测到的亚硝酸平均浓度（70ppt）高于佛得角（亚硝酸<5ppt）。在这两个地点，夜间亚硝酸/二氧化氮比值大致恒定，这表明二氧化氮在海洋表面暗反应转化为亚硝酸的过程不太重要，而中午时分亚硝酸/二氧化氮比值达到最大值，这表明光增强型亚硝酸形成机制（或其他来源）有重要贡献。我们从佛得角的观测结果中得出海洋表面亚硝酸转化为二氧化氮的暗反应速率系数上限为\(C_{HONO}=0.0011 ppb\ hr^{-1}\)；这比之前报道的最慢速率还要低5倍。这些结果表明海洋环境中主要的亚硝酸形成机制存在显著的地域差异，并提示在将此类机制的重要性从个别研究地点外推以评估对氧化能力的区域和/或全球影响时需要谨慎。由于大气过程的很大一部分发生在海洋边界层，特别是在热带地区，更好地确定亚硝酸可能的海洋表面来源对于定量了解全球大气边界层中一次痕量气体的化学过程以及对空气污染和气候的相关影响非常重要。