Impact of the COVID-19 on the vertical distributions of major pollutants from a tower in the Pearl River Delta

The outbreak of the 2019 novel coronavirus (COVID-19) had a large impact on human health and socio-economics worldwide. The lockdown implemented in China beginning from January 23, 2020 led to sharp reductions in human activities and associated emissions. The declines in primary pollution provided a unique opportunity to examine the relationship between anthropogenic emissions and air quality. This study reports on air pollutant and meteorological measurements at different heights from a tall tower in the Pearl River Delta. These measurements were used to investigate the vertical scale response of pollutants to understand reductions in human activities. Compared to that in the pre-lockdown period (from December 16, 2019), the concentrations of surface layer nitric oxide (NO(x)), fine particulate matter (PM(2.5)), and daily maximum 8 h average ozone (MDA8O(3)) declined significantly during the lockdown by 76.8%, 49.4%, and 18.6%, respectively. Although the vertical profiles of NO(x) and O(3) changed during the lockdown period, those of PM(2.5) remained the same. During the lockdown period, there were statistically significant correlations between PM(2.5) and O(3) but not between PM(2.5) and NO(x) at four heights, indicating that the main composition of PM(2.5) have dramatically changed, during which the impact of NO(x) on PM(2.5) became insignificant. Additionally, O(3) concentrations were also insensitive to NO(x) concentrations during the lockdown, implying that O(3) levels were more of a representative of regional background level. In this case, local photochemical formation is no longer a significant ozone source. This evidence suggests that it is possible to mitigation of PM(2.5) and O(3) levels simultaneously by significant reductions in anthropogenic emissions.