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.

Spatiotemporal Investigations of Multi-Sensor Air Pollution Data over Bangladesh during COVID-19 Lockdown

“…Therefore, it would be better to compare with the most recent year (2019) rather than the long-term average, which may represent considerably poorer air quality than in 2019 and 2020. While this may potentially overestimate any improvement effects of the COVID lockdown on air quality, the improvements were reported only for particulate pollution, and changes over a single year are considered to have a little impact [47].…”

Subtle Changes or Dramatic Perceptions of Air Pollution in Sydney during COVID-19

“…The hourly values are smoothed as a 25-point running mean of hourly data to reflect roughly daily averages. It is always difficult to ascertain whether year-to-year differences arise from weather change or emissions [5], so caution is needed in subsequent interpretation. Nevertheless, the figure suggests that the period of lockdown in 2020 has concentrations that are a little lower.…”

“…While such improvements in air quality were occasionally welcomed with positive and sometimes poetic phrases [7,[18][19][20], the improvements are not evenly spread [21]. These are likely more nuanced than reported in the media [2,5], and not all cities experienced declines in air pollution [22,23]. Some authors have drawn attention to potential reductions in premature deaths as a result of improved air quality [24], though under lockdown it seems likely that the exposure would not be well-represented by outdoor measurements and there is a possibility of higher exposures indoors under lockdown [25][26][27].…”

“…Limiting travel and human activity, during lockdown, reduced pollutant emissions. Concentration reductions, especially of nitrogen dioxide (NO 2 ), were observed in many places: China [4][5][6][7][8][9][10], Korea [11], Europe [12,13], the United States [14] and South America [15][16][17]. While such improvements in air quality were occasionally welcomed with positive and sometimes poetic phrases [7,[18][19][20], the improvements are not evenly spread [21].…”

“…Peak air pollution concentrations might remain [32] and increased ozone levels that have accompanied the declines in nitrogen oxides (NOx) have been widely discussed [33][34][35]. Enhanced ozone has also offered the possibility that secondary aerosol concentrations would additionally increase, reminding us that emission and pollution is often linked by a complicated chemistry [5,35]. Some authors, additionally suggest that emissions might increase, as public transport is restricted [36].…”