The 2020 Chinese COVID lockdown produced a measurable radiative forcing change that affected atmospheric circulation
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Peer-reviewed literature indicates that the Chinese COVID-19 lockdown produced measurable radiative forcing changes, but the retrieved evidence does not establish the subsequent effect on atmospheric circulation.
Efforts to stem the transmission of coronavirus disease 2019 (COVID-19) led to rapid, global ancillary reductions in air pollutant emissions. Here, we quantify the impact on tropospheric ozone using a multiconstituent chemical data assimilation system. Anthropogenic NO <sub><i>x</i></sub> emissions dropped by at least 15% globally and 18 to 25% regionally in April and May 2020, which decreased free tropospheric ozone by up to 5 parts per billion, consistent with independent satellite observations. The global total tropospheric ozone burden declined by 6TgO<sub>3</sub> (∼2%) in May and June 2020, largely due to emission reductions in Asia and the Americas that were amplified by regionally high ozone production efficiencies (up to 4 TgO<sub>3</sub>/TgN). Our results show that COVID-19 mitigation left a global atmospheric imprint that altered atmospheric oxidative capacity and climate radiative forcing, providing a test of the efficacy of NO <sub><i>x</i></sub> emissions controls for co-benefiting air quality and climate.
The global total tropospheric ozone burden declined by 6TgO 3 (∼2%) in May and June 2020, largely due to emission reductions in Asia and the Americas that were amplified by regionally high ozone production efficiencies (up to 4 TgO 3 /TgN). Our results show that COVID-19 mitigation left a global atmospheric imprint that altered atmospheric oxidative capacity and climate radiative forcing, providing a test of the efficacy of NO x emissions controls for co-benefiting air quality and climate.
Analysis of the atmospheric composition response to COVID-19 lockdown measures thus provides important information on effective environmental policy-making aimed at improving air quality. Most relevant for this study, tropospheric ozone and aerosols also affect radiative forcing; therefore, their response to changing emissions also sheds light on air quality–climate cobenefits ( 16 ). This study quantifies the response of global tropospheric ozone to the unprecedented NO x emission reductions associated with COVID-19.
For China, however, where the first government-imposed lockdown occurred earlier than in the rest of the world, the difference in emissions on January 10 is used to obtain the BAU emissions. Therefore, the 2020 COVID-19 emission anomaly, estimated as difference between the BAU and COVID-19 emissions, does not include the influence of climatological seasonal changes in anthropogenic emissions, such as use of winter heating, nor does it include interannual changes from years before 2020 (see Materials and Methods and Fig. 1 for further information).
This bias has been partly attributed to a negative bias in the cloud height in FRESCO (fast retrieval scheme for clouds from the oxygen A band) and affected the country-scale emission analysis. Nevertheless, these systematic errors likely change only slightly before and after the COVID lockdown and likely not substantially affect the magnitude and temporal evolution of the COVID emission anomaly. Aerosols can also have an impact on the NO 2 retrievals, depending on the type of aerosol and distribution.
Tropospheric ozone response Using the BAU emissions and 2020 emissions with the same meteorological conditions allows us to evaluate tropospheric ozone concentration changes directly linked to COVID-19 emission declines while accounting for the “observed” meteorology (as filtered through a reanalysis system; see Materials and Methods). This approach is in contrast to studies that evaluate atmospheric composition anomalies in 2020 directly from comparisons between 2020 conditions and previous years ( 7 , 11 ).
Increased surface ozone was seen in our estimates over parts of northern Europe, China, and South Africa, as has already been reported for northern China during the lockdown ( 10 , 21 ). Nevertheless, the obtained ozone production efficiency (OPE, mass of ozone produced per unit mass of NO x emitted) for the monthly mean tropospheric ozone burden (TOB, in TgO 3 unit, integrated from the surface to the tropopause globally) based on the regional emission changes was mostly positive throughout the analysis period (i.e., NO x emission declines reduced TOB), as seen in other modeling studies ( 39 ). Fig. 5 Monthly ozone changes due to the COVID NO x emission reductions in May 2020.
Applying the average satellite-derived TOB trend over the past two decades (+0.71 TgO 3 /year, from −2.15 to +2.85 TgO 3 /year for different satellite sensors) ( 42 ), the COVID-19 TOB reductions of 6 to 9 TgO 3 are equivalent to going back in time to TOB values for 2007–2011. These TOB reductions correspond to a tropospheric ozone radiative forcing of 233 to 350 mWm −2 based on the global mean normalized tropospheric ozone radiative forcing of 42 mWm −2 DU −1 from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) simulation results ( 43 ).
The COVID-related ozone anomaly is widespread in the NH and is substantial even in the SH, especially downwind of megacities in South America. Overall, the pandemic led to a modeled 6-TgO 3 (∼2%) decrease in TOB in May and June. Decreased concentrations of PAN and OH suggest highly nonlocal impacts of the lockdowns and substantial changes in the tropospheric chemistry system. The results described here demonstrate the strong impacts of the worldwide restrictions on human activity on global tropospheric chemistry and radiative forcing.
The quality of the reanalysis fields for 2005–2018 has been evaluated on the basis of comparisons against ozonesondes and independent aircraft and satellite observations for various chemical species on regional and global scales, as well as for seasonal, yearly, and decadal scales, from the surface to the lower stratosphere ( 20 ). The emissions for 2020 constrained by TROPOMI NO 2 at 0.56° horizontal resolution have already been used to evaluate the air quality response to the Chinese COVID-19 lockdown ( 21 ).
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 (NO<sub>2</sub>) 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
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 (NO 2 ) 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
At the end of 2019, the sudden outbreak of the coronavirus disease 2019 (COVID-19) pandemic produced serious impacts in China that had never been seen before. In order to curb the spread of the virus and protect human health, Chinese government firstly implemented an urban lockdown policy in Hubei province, and later extended it to nationwide ( Tian et al., 2020 ). Strict restrictions on human activity also reduced anthropogenic emissions significantly in EC during this period ( Kraemer et al., 2020 ).
Furthermore, changes in atmospheric aerosol loading due to declines in air pollution have far-reaching effects on both regional weather and climate. Wang et al. (2020) noted that unfavorable meteorological conditions over the North China Plain can offset part of the benefits of emission reductions and subsequently trigger air pollution events. Su et al. (2020) found that the spring season in China arrived 8.4 days earlier in 2020, implying that short-term changes brought about by COVID-19 had established a relatively rapid climatic response on a regional scale.
Specifically, the aims of this study were to quantify (1) the changes in AOD and anthropogenic emissions as an indicator of tropospheric NO 2 over three stages; (2) the changes in the spatial and temporal distribution of natural AOD over three stages; (3) the differences in the factors contributing to AOD changes in EC and its typical sub-regions; (4) the potential effects of meteorological conditions; and (5) the radiative response to declining air pollution in the COVID-19 period. 2. Data and methods 2.1. Study period The COVID-19 lockdown period was defined in this study as lasting from 1 February to 31 May 2020 (referred to simply as the COVID-19 period).
Meanwhile, in eastern and central regions, AOD continued to decrease, with a −10.7 % decrease in YRD and a −1.3 % decrease in CC. Fig. 3 Fractional changes (%) in AOD, tropospheric NO 2 , dust AOD, and carbonaceous AOD over EC and five typical regions during (a) and after (b) the COVID-19 period. Fig. 3 In general, the significant reduction in AOD with anthropogenic emissions due to the COVID-19 lockdown was mainly observed in the eastern region, especially in NCP and YRD, where the population is the densest, and hence produced the highest anthropogenic emissions over China. 3.2.
Less aerosol loading in the atmosphere during lockdown period led to more solar radiation reaching the near-surface and less staying in the atmosphere, thus breaking the radiation balance within the boundary layer and affecting the regional climate ( Liu et al., 2019 ). In summary, this paper provides comprehensive and valuable information on aerosol loading changes in the Chinese region during COVID-19. In the future, the development of emission reduction measures should not only limit anthropogenic emissions, but regional transport from neighboring regions or countries must also be taken into account.
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