Changes in aerosol loading before, during and after the COVID-19 pandemic outbreak in China: Effects of anthropogenic and natural aerosol.

Anthropogenic emissions reduced sharply in the short-term during the coronavirus disease pandemic (COVID-19). As COVID-19 is still ongoing, changes in atmospheric aerosol loading over China and the factors of their variations remain unclear. In this study, we used multi-source satellite observations and reanalysis datasets to synergistically analyze the spring (February-May) evolution of aerosol optical depth (AOD) for multiple aerosol types over Eastern China (EC) before, during and after the COVID-19 lockdown period. Regional meteorological effects and the radiative response were also quantitatively assessed. Compared to the same period before COVID-19 (i.e., in 2019), a total decrease of -14.6 % in tropospheric TROPOMI nitrogen dioxide (NO2) and a decrease of -6.8 % in MODIS AOD were observed over EC during the lockdown period (i.e., in 2020). After the lockdown period (i.e., in 2021), anthropogenic emissions returned to previous levels and there was a slight increase (+2.3 %) in AOD over EC. Moreover, changes in aerosol loading have spatial differences. AOD decreased significantly in the North China Plain (-14.0 %, NCP) and Yangtze River Delta (-9.4 %) regions, where anthropogenic aerosol dominated the aerosol loading. Impacted by strong wildfires in Southeast Asia during the lockdown period, carbonaceous AOD increased by +9.1 % in South China, which partially offset the emission reductions. Extreme dust storms swept through the northern region in the period after COVID-19, with an increase of +23.5 % in NCP and + 42.9 % in Northeast China (NEC) for dust AOD. However, unfavorable meteorological conditions overwhelmed the benefits of emission reductions, resulting in a +20.1 % increase in AOD in NEC during the lockdown period. Furthermore, the downward shortwave radiative flux showed a positive anomaly due to the reduced aerosol loading in the atmosphere during the lockdown period. This study highlights that we can benefit from short-term controls for the improvement of air pollution, but we also need to seriously considered the cross-regional transport of natural aerosol and meteorological drivers.

The Different Impacts of Emissions and Meteorology on PM2.5 Changes in Various Regions in China: A Case Study

Emissions and meteorology are significant factors affecting aerosol pollution, but it is not sufficient to understand their relative contributions to aerosol pollution changes. In this study, the observational data and the chemical model (GRAPES_CUACE) are combined to estimate the drivers of PM2.5 changes in various regions (the Beijing–Tianjin–Hebei (BTH), the Central China (CC), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD)) between the first month after COVID-19 (FMC_2020) (i.e., from 23 January to 23 February 2020) and the corresponding period in 2019 (FMC_2019). The results show that PM2.5 mass concentration increased by 26% (from 61 to 77 µg m−3) in the BTH, while it decreased by 26% (from 94 to 70 µg m−3) in the CC, 29% (from 52 to 37 µg m−3) in the YRD, and 32% (from 34 to 23 µg m−3) in the PRD in FMC_2020 comparing with FMC_2019, respectively. In the BTH, although emissions reductions partly improved PM2.5 pollution (−5%, i.e., PM2.5 mass concentration decreased by 5% due to emissions) in FMC_2020 compared with that of FMC_2019, the total increase in PM2.5 mass concentration was dominated by more unfavorable meteorological conditions (+31%, i.e., PM2.5 mass concentration increased by 31% due to meteorology). In the CC and the YRD, emissions reductions (−33 and −36%) played a dominating role in the total decrease in PM2.5 in FMC_2020, while the changed meteorological conditions partly worsened PM2.5 pollution (+7 and +7%). In the PRD, emissions reductions (−23%) and more favorable meteorological conditions (−9%) led to a total decrease in PM2.5 mass concentration. This study reminds us that the uncertainties of relative contributions of meteorological conditions and emissions on PM2.5 changes in various regions are large, which is conducive to policymaking scientifically in China.

The dominant mechanism of the explosive rise of PM2.5 after significant pollution emissions reduction in Beijing from 2017 to the COVID-19 pandemic in 2020

The Chinese government implemented strict emission reduction measures of air pollution between 2013 and 2017. However, from the winter of 2017 until February 2020, during the COVID-19 pandemic, the twenty explosive rise (ER) events of PM2.5 mass in twelve heavy aerosol pollution episodes (HPEs) still appeared in Beijing and its vicinity (BIV). To explore the controlling mechanism for the ER under the condition of drastically reduced emissions, the vertical structure of meteorological elements by L-band second-level sounding and aerosol properties by Lidar were investigated associating with the analysis of surface concentration in PM2.5 mass, its main precursor gases, as well as black carbon (BC) by seven-wavelength Aethalometer. The planetary boundary layer height (BLH) was also estimated together with an analysis of the unfavorable meteorological index (PLAM) that can quantify the impact of unfavorable meteorological conditions to cause the change of PM2.5 concentration. The results suggested that the ER reoccurrence's fundamental cause is that the emissions have not yet fallen sufficiently to a level to decouple HPEs from unfavorable meteorological conditions. During the ER period, the BLH dropped significantly. The fact that PM2.5, its precursor gases, and black carbon increased almost in a similar proportion, indicating that the boundary layer structure change caused by aerosol accumulation is the dominant reason for the ER phenomenon compared to the chemical conversion factor. The two-way feedback effect between the further worsened meteorological conditions and the accumulation of PM2.5 typically interpreted 54%–93% of the ER. An HPE starting 8 Feb. 2020 during the COVID-19 period underwent one of the worst meteorological conditions, quantified by PLAM, in BIV since 2013. However, with a similar level of unfavorable meteorological conditions, the average PM2.5 concentration during the HPE in 2020 was only about 66% of that of a similar HPE in 2016. It shows that the substantial reduction of emissions reduces the PM2.5 pollution level primarily as before when facing an equivalent level of unfavorable meteorological conditions. These results combined suggest that China's continuous efforts to reduce emissions proceed in the right direction and have achieved the desired results.