Abstract
To mitigate climate change in China, a better understanding of optical properties of aerosol is required due to the complexity in emission sources. Here, an intensive real-time measurement was conducted in an urban area of China before and during the lockdown of Coronavirus Disease 2019 (COVID-19), to explore the impacts of anthropogenic activities on aerosol light extinction and direct radiative effect (DRE). The mean light extinction coefficient (bext) reduced from 774.7 ± 298.1 Mm−1 during the normal period to 544.3 ± 179.4 Mm−1 during the lockdown period. The generalized addictive model analysis indicated that the large decline of bext (29.7 %) was entirely attributed to the sharp reductions in anthropogenic emissions. Chemical calculation of bext based on the ridge regression analysis showed that organic aerosol (OA) was the largest contributor to bext in both periods (45.1–61.4 %), and contributions of two oxygenated OAs to bext increased by 3.0–14.6 % during the lockdown. A hybrid environmental receptor model combining with chemical and optical variables identified six sources of bext. It was found that bext from traffic-related emission, coal combustion, fugitive dust, nitrate plus secondary OA (SOA) source, and sulfate plus SOA source decreased by 21.4–97.9 % in the lockdown, whereas bext from biomass burning increased by 27.1 % mainly driven by undiminished needs of residential cooking and heating. The atmospheric radiative transfer model was further used to illustrate that biomass burning instead of traffic-related emission became the largest positive effect (10.0 ± 10.9 W m−2) on aerosol DRE in the atmosphere during the lockdown. Our study provides insights into aerosol bext and DRE from anthropogenic sources, and the results implied the importance of biomass burning for tackling climate change in China in the future.
Highlights
The abrupt outbreak of Coronavirus Disease 2019 (COVID-19) since December of 2019 caused unprecedented economic and social disruption (Yao et al, 2020)
The mean values of bscat, babs, and bext during the normal period were 688.1 ±261.4 Mm-1, 86.6 ±43.0 Mm-1, and 774.7 ±298.1 Mm-1, respectively, which is consistent with the values (657.4 ±436.9 Mm-1, 104.0 ±69.6 Mm-1, and 761.4 ±506.5 Mm-1) reported previously in winter of 2009 in Xi’an (Cao et al, 2012), even though a series of nationwide air quality standards and long-term pollution control policies have been implemented in the 74 major cities since 2013 (Zheng et al, 2018)
The correlations of SSA and the ratio of LO-OOA to MO-OOA were established to reveal a more complex influence of secondary OA (SOA) on SSA (Figure S9c), which showed obviously negative relationships (R2 = 0.69–0.79). It indicated that SSA can be impacted by the degree of oxidation on aerosol, and higher scattering and lower absorption abilities are usually found for more 215 oxidized organic aerosol (OA) (Han et al, 2015; Lee et al, 2014)
Summary
The abrupt outbreak of Coronavirus Disease 2019 (COVID-19) since December of 2019 caused unprecedented economic and social disruption (Yao et al, 2020). Chinese government implemented the city lockdown and a series of strict restrictions on travel, transports, factories, and constructive activities for numerous cities in China to curb the virus 35 spread among humans. This provides a rare opportunity to investigate the impacts of anthropogenic activities on air pollution in China. Only a few studies are conducted to explore the link of chemical constituents in aerosol 40 with light absorption during the lockdown (Chen et al, 2020; Lin et al, 2021; Xu et al, 2020). The influences of reduced anthropogenic activities on the variations of aerosol optical properties and direct radiative effect (DRE) are less understood
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