Measurement report: The importance of biomass burning in light extinction and direct radiative effect of urban aerosol during the COVID-19 lockdown in China

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 (b_ext) 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 b_ext (29.7 %) was entirely attributed to the sharp reductions in anthropogenic emissions. Chemical calculation of b_ext based on the ridge regression analysis showed that organic aerosol (OA) was the largest contributor to b_ext in both periods (45.1–61.4 %), and contributions of two oxygenated OAs to b_ext increased by 3.0–14.6 % during the lockdown. A hybrid environmental receptor model combining with chemical and optical variables identified six sources of b_ext. It was found that b_ext 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 b_ext 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 b_ext and DRE from anthropogenic sources, and the results implied the importance of biomass burning for tackling climate change in China in the future.