Direct measurements of NOx emissions in central London: How are they changing and how can they be used to inform government policy?

空气污染是目前英国人口最大的环境健康压力源。目前，城市中心的主要污染物是二氧化氮（NO2）和直径小于2.5微米的颗粒（PM2.5），以及郊区和农村环境中的臭氧。据估计，空气污染对英国的影响相当于每年4万人过早死亡，每年给英国经济造成100亿至200亿英镑的损失。然而，由于我们的运输系统、供暖方式、能源供应、溶剂的使用和农业系统都在发生变化，我们知道，污染物排放的深刻变化很可能在未来几年发生，而且实际上已经在发生。由于从2016年开始引入的新车辆排放技术（欧6/VI标准）和其他政策干预（如扩大伦敦超低排放区（ULEZ）），这是预期减排的关键时刻。以前的标准未能在“真实世界”的驾驶条件下实现预期的排放减少，而对单个车辆尾气和更广泛区域的总排放进行直接测量的独特组合将有助于评估新技术的有效性，并更好地估计清单中的氮氧化物交通来源。因此，迫切需要改进对城市氮氧化物排放的直接评估，以测试这些排放是否如预测的那样对英国车辆的变化和新政策的实施做出反应。该项目将使用在伦敦中部BT塔测量的氮氧化物，使用涡流相关方差技术计算排放量。然后将使用足迹模型将这些局部通量与地表联系起来，使它们能够与排放清单进行比较，并有助于对来源进行分配。之前的氮氧化物排放测量是在2013年春季和2017年分别进行一个月，然后从2020年秋季开始连续进行。这些时期的数据表明，英国国家大气排放清单（NAEI）将白天的测量排放量低估了1.5倍，尽管它们在夜间非常一致)（Drysdale et al., 2022）。此外，在2020年COVID-19封锁期间，流量比2017年（大流行限制措施和超低排放区（ULEZ）引入之前）的测量值减少了75%。这似乎与仅减少20%的二氧化碳排放量和32%的交通负荷不一致，这种明显的异常部分归因于伦敦市中心空气质量政策的成功，但关键是由于大流行减少了流动性导致的拥堵大幅减少。2020年通量的空间映射表明，在交通减少期间，伦敦市中心主要是点源热和发电排放。在该项目期间所采取的测量将继续进行，将用于进一步评估氮氧化物来源的混合，因为伦敦的车辆越来越电气化，因此废气排放低或零。这些数据对于调查哪些氮氧化物来源（例如来自家庭/商业燃烧或工业）将在未来占主导地位至关重要，从而告知空气污染需要如何应对这些变化的来源。

Air pollution is currently the largest environmental health stressor on the UK population. At present the main pollutants of concern in urban centres are nitrogen dioxide (NO2) and particles less than 2.5 micro meters in diameter, measured by their mass (PM2.5), alongside ozone in suburban and rural environments. It is estimated that air pollution has an effect equivalent to 40,000 premature deaths/year in England and costs the UK economy between £10 billion and £20 billion/year. However, since our transport systems, the way we heat our homes, our energy supply, our use of solvents and our agricultural systems are all changing, we know that profound changes in pollutant emissions are likely in the coming years and indeed are already taking place. It is a critical time with respect to expected reductions in emissions due to new vehicle emission technologies introduced from 2016 onwards (Euro 6/VI standards) and other policy interventions such as the extended London ultra-low emission zone (ULEZ). Previous standards have failed to deliver the expected reductions in emissions under 'real world' driving conditions and the unique combination of direct emission measurements from both individual vehicle exhaust and the total emitted from a wider area will help assess the effectiveness of the new technologies and provide better estimates of the NOx traffic source in the inventories. As a result, there is a critical research need for improved direct assessments of urban NOx emissions to test whether these are responding as predicted to vehicle fleet changes and implementation of new policies in the UK.This project will use measurements of NOx made at the BT tower in central London to calculate emissions using the eddy covariance technique. A footprint model will then be used to link these local fluxes to the surface allowing them to be compared with emission inventories and to aid in source apportionment. Previous measurements of NOx emissions were made for a month each in spring 2013 and 2017 and then have been made continuously since autumn 2020. Data from these periods showed that the UK National Atmospheric Emissions Inventory (NAEI), underestimated measured emissions during the daytime by a factor of ~1.5, although they agreed well overnight) (Drysdale et al., 2022). In addition, in 2020 during COVID-19 lockdowns, fluxes were shown to be 75% less than those measured in 2017, prior to the pandemic restrictions and the introduction of the Ultra-Low Emissions Zone (ULEZ). This was seemingly at odds with only a 20% reduction in CO2 emissions and 32% reduction in traffic load, with the apparent anomaly attributed in part to the success of air quality policy in central London, but crucially due to the substantial reduction in congestion that resulted from pandemic reduced mobility. Spatial mapping of the fluxes in 2020 suggested that central London was dominated by point source heat and power generation emissions during the period of reduced mobility. Measurements taken during this project, which will continue for its duration, will be used to further assess the mix of sources of NOx as the vehicle fleet in London becomes increasingly electrified and thus having low or zero exhaust emissions. Such data is crucial for investigating which sources of NOx (e.g. from domestic / commercial combustion or industry) will dominate in future and therefore inform how air pollution needs to respond to those changing sources.