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

空气污染目前是英国民众面临的最大环境健康压力。目前，城市中心令人担忧的主要污染物是二氧化氮(NO2)和以质量衡量直径小于2.5微米的颗粒物(PM2.5)，以及郊区和农村环境中的臭氧。据估计，空气污染对英国造成的影响相当于每年4万人过早死亡，给英国经济造成的损失在100亿到200亿GB/年之间。然而，由于我们的交通系统、我们取暖的方式、我们的能源供应、我们对溶剂的使用和我们的农业系统都在发生变化，我们知道，污染物排放的深刻变化可能在未来几年内发生，而且实际上已经在发生。从2016年起引入新的机动车排放技术(欧盟6/VI标准)和其他政策干预措施，如扩大伦敦超低排放区(ULEZ)，目前是预期排放量减少的关键时刻。以前的标准未能在“现实世界”的驾驶条件下实现预期的减排，从单个车辆尾气的直接排放测量和从更广泛地区排放的总排放量的独特组合将有助于评估新技术的有效性，并提供对清单中NOx交通来源的更好估计。因此，迫切需要改进对城市NOx排放的直接评估，以测试这些评估是否如预测的那样对英国车队的变化和新政策的实施做出反应。该项目将使用在伦敦市中心的BT塔进行的NOx测量，使用涡流协方差技术计算排放量。然后，将使用足迹模型将这些当地通量与地表联系起来，以便将它们与排放清单进行比较，并帮助进行来源分配。之前对NOx排放量的测量分别在2013年春季和2017年进行了一个月，然后从2020年秋季开始连续进行。这些时期的数据显示，英国国家大气排放清单(NAEI)将白天测量的排放量低估了约1.5倍，尽管他们在一夜之间就很好地达成了一致(Drysdale等人，2022年)。此外，在2020年新冠肺炎封锁期间，通量被显示比2017年大流行限制和引入超低排放区(ULEZ)之前的测量结果减少了75%。这似乎与仅减少了20%的二氧化碳排放和减少了32%的交通负荷不符，这种明显的异常现象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机动性，导致拥堵的大幅减少。2020年通量的空间分布图表明，在机动性降低的时期，伦敦市中心主要是点源热能和发电排放。在该项目期间进行的测量将被用来进一步评估NOx来源的组合，因为伦敦的车队越来越电气化，因此废气排放量较低或为零。这些数据对于调查未来NOx的主要来源(例如，来自家庭/商业燃烧或工业)至关重要，从而为空气污染需要如何应对这些不断变化的来源提供信息。

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.