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

To explore the impacts of anthropogenic emissions on aerosol optical properties and direct radiative effect (DRE), a series of real-time measurements was conducted in an urban area of China before and during the lockdown of Coronavirus Disease 2019. The generalized addictive model analysis shows that the light extinction coefficient (bext) decreased largely during the lockdown due to the sharp reductions in anthropogenic emissions. Organic aerosol was the largest contributor to bext based on the ridge regression analysis. A hybrid environmental receptor model combining with chemical and optical variables shows that bext from biomass burning increased in the lockdown due to the undiminished needs of residential cooking and heating in winter. Biomass burning contributed the largest positive effect to aerosol DRE in the atmosphere during the lockdown, highlighting the importance of controlling biomass burning for mitigating climate change in China in the future.