Does reduction of emissions imply improved air quality?

Abstract

Abstract. The air pollution in China is among the highest in the world. However, legislation to reduce pollutant emissions have been successful and concentrations of SO2 (since 2007), aerosols (since 2011) and NO2 (since 2012) have decreased substantially as deduced from satellite and ground-based observations. The strong reduction of the emissions by the end of January 2020, first caused by the Spring Festival holidays and enhanced and extended by the COVID-19 lockdown, offered an opportunity to study the effects on air quality across the country. In particular the reduction of NO2 concentrations observed using the TROPOspheric Monitoring Instrument (TROPOMI) on board the Copernicus Sentinel-5 Precursor satellite was used to quantitatively estimate the lockdown effect. To determine a baseline for the expected concentrations in 2020, we used Ozone Monitoring Instrument (OMI) NO2 TVCD time series from 2011 to 2019 and determined trends for different regions, noticing a possible halt of the decrease in recent years, but for different periods in the south than in the north of east China. Neglecting this leveling leads to overestimation of the lockdown effect on the concentrations in the south, neglecting the trends may lead to underestimation in the north. We also looked at the temporal resolution used in studies on the reduction of the concentrations and noticed the gradual decrease ahead of the Spring Festival, which actually continued during 3 weeks into the lockdown. Using satellite observations of other species and ground-based monitoring data, it was noticed that the expected improvement of air quality due to the reduction of NO2 concentrations was offset by the increase of the concentrations of aerosols and O3 ascribed to meteorological influences and complex chemistry. In the current study we use TROPOMI tropospheric NO2 vertical column densities (TVCDs) together with ground-based monitoring data for NO2, SO2, CO, O3, PM2.5 and PM10 in 11 regions around large cities to evaluate the evolution of their concentrations during 19 weeks after the Spring Festival and their effect on air quality. For comparison, and in an attempt to average out short term (e.g., meteorological) influences, ground based monitoring data were used for the same period in 2015–2019. The results confirm that the strong reduction of NO2 does not always imply good air quality because concentrations of other pollutants may increase. The study shows the different behavior in city clusters in the north and south of China, and inland in the Sichuan and Guanzhong basins. Effect of other holidays and events are small, except in Beijing where the air quality in 2020 was notably better than in previous years. This study was undertaken for east China, but the methodology can also be used for other areas and part of the conclusions are generally applicable.