Atmospheric reactive nitrogen cycling over the ocean

大气化学处理推动排放污染物的去除，导致臭氧和二次气溶胶的形成，对人类和环境健康有害，并有助于气候强迫。对这种化学的定量了解对于准确预测当前的空气质量和未来的大气成分是至关重要的。在对流层中，气体化学处理严重依赖于氮氧化物(NOx，NO+NO2)的丰度，它调节着大气的氧化能力、臭氧的形成和许多气溶胶粒子的主要成分。在全球范围内，NOx的主要来源都是大陆(交通、发电、工业、土壤NO排放)；这些在一些地方得到了很好的理解，但在发展中国家，特别是非洲特大城市，这些来源非常不确定，而且迅速增加。一旦进入大气，NOx就会转化为PAN等储藏化合物，在运输后可能会释放出NOx，最终在几天的时间尺度上转化为硝酸(HNO3)。目前的理解是，硝酸是大气中氮氧化物的最终汇，通过沉积从大气中移除。因此，在距离海岸几天的偏远海洋地点，我们预计NOx水平将非常低，无机氮收支将以非反应性输送的HNO3为主。最近的观察结果挑战了这一理解：热带大西洋上空观测到了令人惊讶的高水平NOx物种和HONO(一种寿命只有几分钟的NOx前体)。这指向了HONO和NOx的一个缺失来源。人们假设，粒子结合的硝酸盐光解转化为气态HONO和NO2可能解释了这些观察到的现象，并形成了缺失的NOx源--一种称为“再氧化”的机制(Ye等人，“自然”2016)。我们在热带大西洋的佛得角观测站对HONO和NOx水平进行了概念验证测量和建模，我们发现这符合这一机制(Reed等人，ACP，2017)--然而，关于颗粒硝酸盐光解速度和产物的数量级不确定性仍然存在，并且缺乏其在主要气溶胶物种(尘埃、硫酸盐气溶胶)上存在的观测证据，这意味着对全球规模的影响尚不清楚。该项目旨在通过将现有的地面、飞机和卫星观测与有针对性的新的实地和实验室研究相结合来解决这些不确定性。我们将重点放在一个自然实验室--热带大西洋地区，在那里我们将探索从西非发展中地区向大西洋中部清洁海洋环境流出的受污染空气中氮物种的排放和演变(佛得角天文台，CVO)。具体地说，我们将(1)利用热带大西洋作为一个天然实验室，研究不同季节和气溶胶状况下的再氧化作用，同时进行实验室研究，以将颗粒硝酸盐光解过程参数化；(2)将这一新认识纳入全球化学输送模式，以评估NOx在输送过程中的循环和转化，从而评估这些过程对热带大西洋的影响，以及对我们对全球大气化学处理的理解。

Atmospheric chemical processing drives the removal of emitted pollutants, and leads to the formation of ozone and secondary aerosol, which are harmful to human and environmental health, and contribute to climate forcing. Quantitative understanding of such chemistry is essential for the accurate prediction of current air quality and future atmospheric composition.In the troposphere, gaseous chemical processing is critically dependent upon the abundance of nitrogen oxides (NOx, NO + NO2), which regulate atmospheric oxidising capacity, ozone formation and the major components of many aerosol particles. Globally, the dominant NOx sources are all continental (traffic, power generation, industry, soil emissions of NO); these are well understood in some locations, but are very uncertain and rapidly increasing in developing nations, particularly African megacities. Once in the atmosphere, NOx is converted to reservoir compounds such as PAN, which may release NOx after transport, and ultimately into nitric acid (HNO3) on timescales of days. Current understanding is that HNO3 is the final atmospheric sink for NOx, and is removed from the atmosphere by deposition. Consequently, at remote marine sites a number of days transit time from the coast, we would expect NOx levels to be very low, and the inorganic nitrogen budget to be dominated by unreactive transported HNO3.Recent observations challenge this understanding: surprisingly high levels of NOx species, and HONO (a NOx precursor with a lifetime of a few minutes) have been observed over the tropical Atlantic ocean. This points to a missing source of HONO and NOx. It has been hypothesised that the photolytic conversion of particle-bound nitrate to gaseous HONO and NO2 may account for these observations and form the missing NOx source - a mechanism termed "renoxification" (Ye et al., Nature 2016). We have performed proof-of-concept measurements and modelling of HONO and NOx levels at the Cape Verde observatory in the tropical Atlantic, which we have found to be consistent with this mechanism (Reed et al., ACP, 2017) - however, order of magnitude uncertainties over the rate and products of particle nitrate photolysis remain, and observational evidence for its occurrence on dominant aerosol species (dust, sulfate aerosol) is missing, meaning that impacts on the global-scale are unknown.This project aims to address these uncertainties, through integrating existing ground-based, aircraft and satellite observations with targeted new field and laboratory studies. We will focus upon a natural laboratory, the tropical Atlantic region where we will probe the emissions and evolution of nitrogen species in the outflow of polluted air from the developing regions of West Africa to the clean marine environment of the mid Atlantic (Cape Verde Observatory, CVO). Specifically, we will (1) Use the tropical Atlantic as a natural laboratory to study renoxification during different seasons and aerosol regimes, alongside laboratory studies to parameterise the particulate nitrate photolysis process; (2) integrate this new understanding into a global chemistry-transport model to evaluate the recycling and transformations of NOx during transport, and hence the impacts of these process in the tropical Atlantic ocean, and upon our understanding of atmospheric chemical processing globally.