Research Institute for Humanity and Nature, Kyoto, Japan.
Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan.
Sci Rep. 2021 May 7;11(1):9800. doi: 10.1038/s41598-021-87673-2.
COVID-19 related restrictions lowered particulate matter and trace gas concentrations across cities around the world, providing a natural opportunity to study effects of anthropogenic activities on emissions of air pollutants. In this paper, the impact of sudden suspension of human activities on air pollution was analyzed by studying the change in satellite retrieved NO concentrations and top-down NOx emission over the urban and rural areas around Delhi. NO was chosen for being the most indicative of emission intensity due to its short lifetime of the order of a few hours in the planetary boundary layer. We present a robust temporal comparison of Ozone Monitoring Instrument (OMI) retrieved NO column density during the lockdown with the counterfactual baseline concentrations, extrapolated from the long-term trend and seasonal cycle components of NO using observations during 2015 to 2019. NO concentration in the urban area of Delhi experienced an anomalous relative change ranging from 60.0% decline during the Phase 1 of lockdown (March 25-April 13, 2020) to 3.4% during the post-lockdown Phase 5. In contrast, we find no substantial reduction in NO concentrations over the rural areas. To segregate the impact of the lockdown from the meteorology, weekly top-down NOx emissions were estimated from high-resolution TROPOspheric Monitoring Instrument (TROPOMI) retrieved NO by accounting for horizontal advection derived from the steady state continuity equation. NOx emissions from urban Delhi and power plants exhibited a mean decline of 72.2% and 53.4% respectively in Phase 1 compared to the pre-lockdown business-as-usual phase. Emission estimates over urban areas and power-plants showed a good correlation with activity reports, suggesting the applicability of this approach for studying emission changes. A higher anomaly in emission estimates suggests that comparison of only concentration change, without accounting for the dynamical and photochemical conditions, may mislead evaluation of lockdown impact. Our results shall also have a broader impact for optimizing bottom-up emission inventories.
COVID-19 相关限制措施降低了世界各地城市的颗粒物和痕量气体浓度，为研究人为活动对空气污染物排放的影响提供了一个自然机会。在本文中，通过研究卫星反演的 NO 浓度和新德里城乡地区的自上而下的 NOx 排放的变化，分析了人类活动突然停止对空气污染的影响。选择 NO 是因为它的寿命很短，在行星边界层中只有几个小时，是最能指示排放强度的。我们提出了一种可靠的臭氧监测仪器（OMI）在锁定期间反演的 NO 柱密度与反事实基线浓度的时间比较，该基线浓度是根据 2015 年至 2019 年观测期间的长期趋势和季节性循环分量外推得到的。新德里市区的 NO 浓度经历了异常的相对变化，从锁定第一阶段（2020 年 3 月 25 日至 4 月 13 日）的 60.0%下降到锁定第五阶段的 3.4%。相比之下，我们发现农村地区的 NO 浓度没有明显下降。为了将锁定的影响与气象分开，我们根据稳态连续性方程得出的水平平流，从高分辨率的对流层监测仪器（TROPOMI）反演的 NO 中估算每周的自上而下的 NOx 排放量。与锁定前的正常业务阶段相比，新德里市区和电厂的 NOx 排放量在第一阶段分别下降了 72.2%和 53.4%。市区和电厂的排放估算与活动报告显示出很好的相关性，表明这种方法可用于研究排放变化。排放估算的更高异常表明，仅比较浓度变化，而不考虑动力学和光化学条件，可能会导致对锁定影响的评估出现偏差。我们的结果也将对优化自上而下的排放清单产生更广泛的影响。
