Table_1_Impact of the COVID-19 Lockdown on Air Quality and Resulting Public Health Benefits in the Mexico City Metropolitan Area.DOCX

Meteorology and long-term trends in air pollutant concentrations may obscure the results from short-term policies implemented to improve air quality. This study presents changes in CO, NO2, O3, SO2, PM10, and PM2.5 based on their anomalies during the COVID-19 partial (Phase 2) and total (Phase 3) lockdowns in Mexico City (MCMA). To minimise the impact of the air pollutant long-term trends, pollutant anomalies were calculated using as baseline truncated Fourier series, fitted with data from 2016 to 2019, and then compared with those from the lockdown. Additionally, days with stagnant conditions and heavy rain were excluded to reduce the impact of extreme weather changes. Satellite observations for NO2 and CO were used to contrast the ground-based derived results. During the lockdown Phase 2, only NO2 exhibited significant decreases (p < 0.05) of between 10 and 23% due to reductions in motor vehicle emissions. By contrast, O3 increased (p < 0.05) between 16 and 40% at the same sites where NO2 decreased. During Phase 3, significant decreases (p < 0.05) were observed for NO2 (43%), PM10 (20%), and PM2.5 (32%) in response to the total lockdown. Although O3 concentrations were lower in Phase 3 than during Phase 2, those did not decrease (p < 0.05) from the baseline at any site despite the total lockdown. SO2 decreased only during Phase 3 in a near-road environment. Satellite observations confirmed that NO2 decreased and CO stabilised during the total lockdown. Air pollutant changes during the lockdown could be overestimated between 2 and 10-fold without accounting for the influences of meteorology and long-term trends in pollutant concentrations. Air quality improved significantly during the lockdown driven by reduced NO2 and PM2.5 emissions despite increases in O3, resulting in health benefits for the MCMA population. A health assessment conducted suggested that around 588 deaths related to air pollution exposure were averted during the lockdown. Our results show that to reduce O3 within the MCMA, policies must focus on reducing VOCs emissions from non-mobile sources. The measures implemented during the COVID-19 lockdowns provide valuable information to reduce air pollution through a range of abatement strategies for emissions other than from motor vehicles.

气象条件与空气污染物浓度的长期趋势，可能会掩盖旨在改善空气质量的短期政策的实施效果。本研究基于墨西哥城都会区（MCMA）新冠疫情局部封控（第二阶段）与全面封控（第三阶段）期间的污染物异常值，分析了一氧化碳（CO）、二氧化氮（NO2）、臭氧（O3）、二氧化硫（SO2）、可吸入颗粒物（PM10）与细颗粒物（PM2.5）的浓度变化。为尽可能削弱污染物浓度长期趋势的影响，本研究以2016-2019年的监测数据为基准，采用截断傅里叶级数（truncated Fourier series）计算污染物异常值，并与封控期间的异常值进行对比。此外，为降低极端气象变化的干扰，本研究剔除了静稳天气与强降雨时段的数据。本研究还利用二氧化氮与一氧化碳的卫星观测数据，与地面监测得到的结果进行交叉验证。在封控第二阶段期间，仅二氧化氮浓度出现10%~23%的显著下降（p<0.05），这源于机动车排放的削减；与之相对，在二氧化氮浓度下降的同一监测点位，臭氧浓度上升了16%~40%，且具有统计学显著性差异（p<0.05）。在封控第三阶段期间，二氧化氮（下降43%）、可吸入颗粒物（下降20%）与细颗粒物（下降32%）的浓度均出现显著下降（p<0.05）。尽管第三阶段的臭氧浓度低于第二阶段，但在全面封控期间，所有监测点位的臭氧浓度均未较基准值出现显著下降（p<0.05）。二氧化硫仅在第三阶段的道路邻近环境中出现浓度下降。卫星观测数据证实，全面封控期间二氧化氮浓度下降、一氧化碳浓度趋于稳定。若未考虑气象条件与污染物浓度长期趋势的影响，封控期间的空气污染物浓度变化幅度可能被高估2~10倍。尽管臭氧浓度有所上升，但封控期间机动车与细颗粒物排放的削减使得空气质量显著改善，为墨西哥城都会区居民带来了健康收益。一项健康评估结果显示，封控期间约有588例与空气污染暴露相关的死亡得以避免。本研究结果表明，若要降低墨西哥城都会区的臭氧浓度，政策需聚焦于削减非移动源的挥发性有机物（VOCs）排放。新冠疫情封控期间实施的管控措施，为通过机动车以外的多种排放削减策略改善空气质量提供了宝贵参考。