Impact of the COVID-19 lockdown on air quality and resulting public health benefits in the Mexico City metropolitan area

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, NO 2 , O 3 , SO 2 , PM 10 , and PM 2.5 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 NO 2 and CO were used to contrast the ground-based derived results. During the lockdown Phase 2, only NO 2 exhibited significant decreases ( p < 0.05) of between 10 and 23% due to reductions in motor vehicle emissions. By contrast, O 3 increased ( p < 0.05) between 16 and 40% at the same sites where NO 2 decreased. During Phase 3, significant decreases ( p < 0.05) were observed for NO 2 (43%), PM 10 (20%), and PM 2.5 (32%) in response to the total lockdown. Although O 3 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. SO 2 decreased only during Phase 3 in a near-road environment. Satellite observations confirmed that NO 2 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 NO 2 and PM 2.5 emissions despite increases in O 3 , 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 O 3 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.