Résumé
COVID-19 pandemic responses affected atmospheric composition and climate. These effects depend on the background emissions, climate, and season in which they occur. Although using multiple scenarios is common in explorations of long-term climate change, they are rarely used to explore atmospheric composition or climate changes in response to transient emission perturbations on the scale of COVID-19 lockdowns. We used the ModelE Earth system model to evaluate how atmospheric and climate impacts depend on the decade and season in which lockdowns occurred. Global COVID-19-related anomalies in aerosols and trace gases differed by up to an order of magnitude or more when comparing lockdowns in 1980, 2008, 2020, and 2051. Regional atmospheric composition anomalies tended to be largest when emissions were near a historical peak: 1980 in Europe and temperate North America, 2008 or 2020 in eastern Asia, and 2051 in south Asia. Regional aerosol direct effect anomalies were almost always less than 0.1 W m( -2) during the first pandemic year, but over 0.1 W m (-2) in Europe and exceeded 0.2 W m(-2) in Europe and temperate North America in 1980, generally changing in tandem with regional emissions. In contrast, direct effect anomalies in Asia were positive in 1980 and negative in 2008, suggesting they may be primarily determined by exogenous emission anomalies. Shifting COVID-19 onset in 2020 by 3, 6, or 9 months also altered atmospheric composition on the order of 2%-25% globally. In all scenarios, changes in surface temperature or precipitation appeared unrelated to local atmospheric compositional changes.
Résumé
We examined daily level-3 satellite retrievals of Atmospheric Infrared Sounder (AIRS) CO, Ozone Monitoring Instrument (OMI) SO2 and NO2, and Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) over eastern China to understand how COVID-19 lockdowns affected atmospheric composition. Changes in 2020 were strongly dependent on the choice of background period since 2005 and whether trends in atmospheric composition were accounted for. Over central east China during the 23 January-8 April lockdown window, CO in 2020 was between 3 % and 12 % lower than average depending on the background period. The 2020 CO was not consistently less than expected from trends beginning between 2005 and 2016 and ending in 2019 but was 3 %-4 % lower than the background mean during the 2017-2019 period when CO changes had flattened Similarly for AOD, 2020 was between 14 % and 30 % lower than averages beginning in 2005 and 14 %-17 % lower compared to different background means beginning in 2016. NO2 in 2020 was between 30 % and 43 % lower than the mean over different background periods and between 17 % and 33 % lower than what would be expected for trends beginning later than 2011. Relative to the 2016-2019 period when NO2 had flattened, 2020 was 30 %-33 % lower. Over southern China, 2020 NO2 was between 23 % and 27 % lower than different background means beginning in 2013, the beginning of a period of persistently lower NO2. CO over southern China was significantly higher in 2020 than what would be expected, which we suggest was partly because of an active fire season in neighboring countries. Over central east and southern China, 2020 SO2 was higher than expected, but this depended strongly on how daily regional values were calculated from individual retrievals and reflects background values approaching the retrieval detection limit. Future work over China, or other regions, needs to take into account the sensitivity of differences in 2020 to different background periods and trends in order to separate the effects of COVID-19 on air quality from previously occurring changes or from variability in other sources.