Atmospheric reactive nitrogen cycling over the ocean

大气中的化学处理推动了排放污染物的清除，并导致臭氧和二次气溶胶的形成，这对人类和环境健康有害，并导致气候强迫。在对流层中，气态化学过程主要取决于氮氧化物（NOx，NO + NO2）的丰度，氮氧化物调节着大气的氧化能力、臭氧的形成以及许多气溶胶颗粒的主要成分。在全球范围内，主要的氮氧化物来源都是大陆性的（交通、发电、工业、土壤排放的氮氧化物）;这些在一些地方是很好理解的，但在发展中国家，特别是非洲的特大城市，非常不确定，而且迅速增加。一旦在大气中，NOx被转化为储库化合物，如PAN，其可在运输后释放NOx，并最终在数天的时间尺度上转化为硝酸（HNO 3）。目前的理解是，HNO 3是NOx的最终大气汇，并通过沉积从大气中去除。因此，在距离海岸数天的远程海洋站点，我们预计NOx水平非常低，并且无机氮收支由未反应的输送HNO 3主导。最近的观测挑战了这种认识：在热带大西洋上观察到了令人惊讶的高水平NOx物质和HONO（一种寿命为几分钟的NOx前体）。这表明HONO和NOx的来源缺失。已经假设，颗粒结合的硝酸盐到气态HONO和NO2的光解转化可以解释这些观察结果并形成缺失的NOx源-一种称为“再氧化”的机制（Ye等人，Nature 2016）。我们已经在热带大西洋的佛得角天文台对HONO和NOx水平进行了概念验证测量和建模，我们发现这与这种机制一致（Reed等人，ACP，2017年）-然而，颗粒硝酸盐光解的速率和产物的数量级不确定性仍然存在，其发生在主要气溶胶物种上的观测证据（灰尘，硫酸盐气溶胶）缺失，这意味着对全球范围的影响是未知的。本项目旨在解决这些不确定性，通过整合现有的地基，飞机和卫星观测，并进行有针对性的新实地和实验室研究。我们将把重点放在一个自然实验室，热带大西洋地区，我们将探测从西非发展中地区到大西洋中部清洁海洋环境的污染空气流出中氮物质的排放和演变（佛得角天文台，CVO）。具体来说，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制度下的再氧化作用，同时进行实验室研究，以参数化颗粒硝酸盐光解过程;（2）将这一新的认识纳入全球化学传输模式，以评估氮氧化物在传输过程中的再循环和转化，从而评估这些过程在热带大西洋中的影响，以及我们对全球大气化学过程的理解。

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.