Impact of COVID-19 lockdown on the optical properties and radiative effects of urban brown carbon aerosol

Intensive measurements were conducted in Xi'an, China before and during a COVID-19 lockdown period to investigate how changes in anthropogenic emissions affected the optical properties and radiative effects of brown carbon (BrC) aerosol. The contribution of BrC to total aerosol light absorption during the lockdown (13%–49%) was higher compared with the normal period (4%–29%). Mass absorption cross-sections (MACs) of specific organic aerosol (OA) factors were calculated from a ridge regression model. Of the primary OA (POA), coal combustion OA (CCOA) had the largest MACs at all tested wavelengths during both periods due to high molecular-weight BrC chromophores; that was followed by biomass burning OA (BBOA) and hydrocarbon-like OA (HOA). For secondary OA (SOA), the MACs of the less-oxidized oxygenated OA (OOA) species (LO-OOA) at λ = 370–590 nm were higher than those of more-oxidized OOA (MO-OOA) during both periods, presumably due to chromophore bleaching. The largest contributor to BrC absorption at the short wavelengths was CCOA during both periods, but BrC absorption by LO-OOA and MO-OOA became dominant at longer wavelengths during the lockdown. The estimated radiation forcing efficiency of BrC over 370–600 nm increased from 37.5 W·g⁻¹ during the normal period to 50.2 W·g⁻¹ during the lockdown, and that enhancement was mainly caused by higher MACs for both LO-OOA and MO-OOA. This study provides insights into the optical properties and radiative effects of source-specific BrC aerosol when pollution emissions are reduced.