Isotopic signature of nitrate in the remote troposphere

大气硝酸(HNO3)对酸雨的贡献以及与之相关的对植物、土壤和建筑物的负面影响是众所周知的。硝酸是大气氮氧化的最终产物，但本身相对稳定。因此，它只有在湿沉积或干沉积(降水或直接转移到地球表面)时才会丢失。硝酸气体可在沉积前转化或吸附到颗粒(气溶胶)上，这通常需要将硝酸分解为H+和硝酸根离子。这里，我们只考虑硝酸盐物种的总和，即气态和颗粒状HNO3加上颗粒状硝酸盐。大气硝酸盐的前体是氮氧化物(NOx=NO+NO2)。由于化石燃料燃烧、生物质燃烧和飞机排放，全球范围内的NOx水平正在上升。较高的NOx水平会导致硝酸盐沉积增加。尽管硝酸盐是一种重要的植物营养物质，但过量的硝酸盐会导致河流、湖泊和沿海地区的藻类大量繁殖。大气中的硝酸盐沉积还会造成硝酸盐化肥对地下水的污染，这可能会导致饮用水中硝酸盐的有毒水平(例如，导致“蓝婴综合症”)。要了解人类扰动对氮循环的影响，重要的是要确定土壤和闪电产生自然氮氧化物的速率。不幸的是，没有任何东西可以从化学上区分自然和人为的氮氧化物来源。然而，微量气体和气溶胶的稳定同位素组成可以提供有关它们在化学和生物过程中的来源和去向的独特信息。同位素是同一元素的不同物种，它们的化学反应方式相同，但速度略有不同。此外，化合物的“同位素特征”的变化有助于追踪其在自然界中的路径。本研究的目标之一是建立以自然NOx来源为主的硝酸盐的同位素特征。因此，我们选择分析北大西洋和南大西洋船只的一组气溶胶样本。人为NOx在北半球的贡献较大，但遥远的南大西洋气溶胶的同位素组成应揭示天然NOx的特征。此外，热带大西洋的NOx产生以闪电为主。大气中HNO3的形成有不同的途径，尽管NOx的来源编码在硝酸盐的氮同位素中，但这些途径的相对重要性(尽管不是绝对值)可以用氧同位素来研究。NOx继承了臭氧的同位素异常。根据硝酸生成途径的不同，这种异常在臭氧中有不同程度的表现。不同的路径在白天和晚上占据主导地位，我们希望在北诺福克一个受污染的海岸遗址的白天研究中找到它们相对贡献的证据。我们还假设，硝酸盐的同位素特征将帮助我们区分对硝酸盐浓度日循环的不同解释。在进行上述测量之前，我们必须建立一种适合大气硝酸盐同位素分析的方法。对于这些分析，使用了一种特定类型的质谱仪，它只能采集气体样品。因此，我们必须将硝酸盐转化为气体。直接转换成元素是困难的。相反，我们计划使用一种细菌菌株，可以将硝酸盐转化为笑气(N2O)。它比任何化学方法都更有效地做到这一点。JK已经在普林斯顿大学(美国)的实验室中学会了使用这种方法，该实验室以首次使其适应环境样本而闻名。JK将其进一步发展，用于分析硝酸盐的氧同位素异常。他计划在晚些时候使用这种方法直接研究硝酸盐的形成反应，并可能研究旧雪样本和极地冰芯中的硝酸盐，以限制工业化前和冰川时期的大气化学。

Atmospheric nitric acid (HNO3) is mostly known for its contribution to acid rain and the associated negative effects on plants, soils and buildings. HNO3 is the end-product of atmospheric nitrogen oxidation, but relatively stable itself. Therefore, it is only lost by wet or dry deposition (precipitation or direct transfer to the Earth's surface). HNO3 gas can be converted to or adsorbed onto particles (aerosol) before deposition, which often involves splitting HNO3 into H+ and nitrate ions. Here, we only consider the sum of nitrate species, i.e., gaseous and particulate HNO3 plus particulate nitrate. The precursors to atmospheric nitrate are nitrogen oxides (NOx = NO + NO2). NOx levels are rising on a global scale, due to fossil fuel combustion, biomass burning and aircraft emissions. Higher NOx levels lead to increased nitrate deposition. Even though nitrate is an important plant nutrient, too much of it can cause algal blooms in rivers, lakes and coastal areas. Atmospheric nitrate deposition also contributes to ground water pollution by nitrate fertilisers, which can lead to toxic levels of nitrate in drinking water (causing, e.g., 'blue baby syndrome'). To understand the impacts of human perturbations of the nitrogen cycle, it is important to establish the rate of natural NOx production from soils and lightning. Unfortunately, there is nothing to distinguish natural and anthropogenic NOx sources chemically. However, the stable isotope composition of trace gases and aerosols can provide unique information on their origin and fate in chemical and biological processes. Isotopes are different species of the same element, which react in the same way chemically, but at slightly different speeds. In addition, changes in the 'isotopic signature' of a compound help tracing its way in nature. One of the goals of the present study is to establish the isotopic signature of nitrate dominated by natural NOx sources. We therefore chose to analyse a set of aerosol samples from ships across the North and South Atlantic. The contribution of anthropogenic NOx is large in the Northern hemisphere, but the isotopic composition of aerosol in the remote South Atlantic should reveal the signature of natural NOx. Also, NOx production in the tropical Atlantic is dominated by lightning. There are different pathways of HNO3 formation in the atmosphere, and whereas the source of NOx is encoded in the nitrogen isotopes of nitrate, the relative importance of these pathways (albeit not the absolute magnitudes) can be studied using the oxygen isotopes. NOx inherits an isotopic anomaly from ozone. Depending on the pathways of HNO3 formation, this anomaly is expressed to various degrees in ozone. Different pathways dominate during day and night and we hope to find evidence of their relative contributions in diurnal studies at a polluted coastal site in North Norfolk. We also hypothesise that the isotopic signature of nitrate will help us distinguish between different explanations for the diurnal cycle of nitrate concentrations. Prior to the measurements outlined above, we have to build a suitable method for isotopic analysis of atmospheric nitrate. For these analyses, a specific kind of mass spectrometer is used that can only take gaseous samples. We therefore have to convert nitrate into a gas. The direct conversion to the elements is difficult. Instead we plan to use a bacterial strain that can convert nitrate to laughing gas (N2O). It does this much more efficiently than any chemical method. JK has learnt to use this method in the lab at Princeton University (USA) that is renowned for first adapting it to environmental samples. JK has further developed it to analyse the oxygen isotope anomaly of nitrate. He plans to use this method at a later stage for direct studies of nitrate formation reactions and, possibly, nitrate in old snow samples and polar ice cores, as a constraint on pre-industrial and glacial atmospheric chemistry.