Airborne Measurements of Ozone Precursor Fluxes - Proof of Concept

绝大多数空气污染物是由地球表面的活动直接排放到大气中的。这些活动可能是人为的，也可能是自然（生物）过程。与空气质量特别相关的是氮氧化物（NOx）和挥发性有机化合物（VOCs）的排放。氮氧化物或一氧化氮（NO）和二氧化氮（NO2）的总和，是由车辆、发电站和许多其他工业活动在燃烧化石燃料时排放的。挥发性有机化合物（VOCs）可作为未燃烧或部分燃烧的化石燃料，通过溶剂和其他工业化学品的蒸发排放到大气中，也可作为生物源过程由植物排放。氮氧化物和挥发性有机化合物通过光化学反应在大气中结合，形成另外两种极其重要的污染物——臭氧和颗粒物。氮氧化物、某些挥发性有机化合物、臭氧和颗粒物等物质都受欧盟空气质量指令的管制。虽然人们对氮氧化物和挥发性有机化合物的化学反应和大气过程已经相当了解，并且可以通过一定的技巧建立模型，但由于与初始排放率相关的不确定性，在模型中产生了很大的不确定性。Defra负责英国非常详细的排放清单（国家大气排放清单NAEI），这是大多数空气质量大气模拟的起点。NAEI的构建是为了给出空间分解的排放量，根据国家活动数据集和排放因子计算，然后在空间上分解到1x1km网格。NAEI运作的框架本身受到监管报告程序的严格限制。近年来，很明显，某些污染物（如二氧化氮）的测量趋势与清单预测的趋势不符。与此同时，其他研究表明，生物源异戊二烯等物种目前也没有被NAEI很好地复制。持续超出某些空气污染目标是负责任的政府部门Defra的重大关切，他们已经确定减少与排放相关的不确定性是关键的证据需求。排放清单基本上是基于纸张的可能排放量计算，在大空间尺度上，用真实世界的排放测量来挑战这些计算并不简单。在这个项目中，我们将应用并演示一种新的“自上而下”的实验方法来测量城市或景观范围内的污染通量。我们将在NERC的轻型研究飞机——多尼尔228上安装快速响应的氮氧化物和挥发性有机化合物传感器，并以尽可能低和尽可能慢的速度在大伦敦和东安格利亚农村上空飞行。这架飞机已经配备了测量大气湍流的仪器，通过将湍流探测器的数据与化学传感器的数据相结合（使用称为涡动相关和虚拟分离涡动相关的数据分析技术），我们的目标是证明我们可以在这些尺度上对氮氧化物和挥发性有机化合物进行自上而下的通量测量。然后，我们将实验得出的数据与国家大气排放清单产生的数据进行比较，作为了解差异发生的地方和原因的起点。这项技术如果成功，将对Defra具有相当大的战略意义，Defra是该项目的共同资助方。然而，在城市领域之外也有非常重要的应用。了解遥远和原始环境中气体的排放通量是了解地球系统的关键因素。在此开发的技术有可能转移到森林和海洋生物地球化学气体交换等领域。

The vast majority of air pollutants are emitted directly into the atmosphere from activities occurring at the Earth's surface. These activities may be anthropogenic in origin or may be natural (biogenic) processes. Of particular relevance to air quality are the emissions of oxides of nitrogen (NOx) and volatile organic compounds (VOCs). NOx or the sum of nitric oxide, NO and nitrogen dioxide, NO2, is emitted by vehicles, power stations and many other industrial activities during the combustion of fossil fuels. Volatile organic compounds (VOCs) may be emitted into the atmosphere as unburnt or partially burnt fossil fuels, by the evaporation of solvents and other industrial chemicals and may also be emitted by plants as a biogenic process. In combination NOx and VOC combine through photochemical reactions in the atmosphere leading to the formation of two other extremely important pollutants - ozone and particulate matter. Species such as NOx, certain VOCs, Ozone and particulate matter are all regulated by EU Air Quality directives. Whilst the chemical reactions and atmospheric processing of NOx and VOcs is reasonably well understood, and can be modelled with some skill, large uncertainties arise in models from uncertainty associated with the initial rate of emissions. Defra is responsible for a highly detailed emissions inventory for the UK (the National Atmospheric Emissions Inventory NAEI), which is the starting point for most atmospheric simulations of air quality. The NAEI is constructed to give spatially resolved emissions, calculated from national activity datasets and emissions factors which are then spatially disaggregated to 1x1km grids. The framework under which the NAEI operates is itself tightly constrained by regulated procedures for reporting. In recent years it has become clear that measured trends in certain pollutants, for example NO2, have not followed trends predicted by inventories. In parallel, other studies have shown that species such as biogenic isoprene are also not currently well reproduced by the NAEI. Continued exceedences of certain air pollution targets is of significant concern to the responsible Government department Defra, who have identified reducing this uncertainty associated with emissions as a key evidence need. Emissions inventories are essentially paper-based calculations of likely emissions, and it is not straightforward to challenge these with real-world emissions measurements, on large spatial scales. In this project we will apply and demonstrate a novel "top down" experimental method for measuring pollution fluxes at the city-wide or landscape scale. We will fit fast-response sensors for NOx and VOCs on NERC's light research aircraft, a Dornier 228, and fly the instruments over Greater London and over rural East Anglia as low and as slow as possible. This aircraft is already fitted with an instrument to measure atmospheric turbulence, and by combining the data from the turbulence probe with that from the chemical sensors (using data analysis techniques known as eddy covariance and virtual disjunct eddy covariance) we aim to demonstrate that we can make top down flux measurements for NOx and VOCs at these scales. We will then compare the experimentally derived data with that produced by the National Atmospheric Emissions Inventory, as a starting point for understanding where and why difference occur.The technique, if successful, would be of considerable strategic importance to Defra, who are co-funding this project. There are however very important applications outside of the urban domain. Understanding emission fluxes of gases in remote and pristine environments is a key element of understanding the earth system. The techniques to be developed here have potential transfers in to fields such as forest and marine biogeochemical gas exchange.