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 wave-lengths 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 lessoxidized oxygenated OA(OOA)species(LO-OOA)atλ=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·gduring the normal period to 50.2 W·gduring 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.

Aerosol composition,sources,and secondary processing during autumn at a regional site in the Beijing-Tianjin-Hebei region

Air pollution is serious during autumn in the Beijing-Tianjin-Hebei(BTH)region,but there are few studies that have utilized real-time observations and source apportionment of the autumn submicron aerosols in this region.In this study,a quadrupole aerosol chemical speciation monitor(Q-ACSM)was deployed for the real-time measurement of the non-refractory compositions of submicron aerosols(NR-PM1)at a regional site(Xianghe)from October 3 to November 14,2017.The results showed that nitrate was the largest inorganic aerosol,and the oxygenated organic aerosol(OOA)was the largest organic aerosol in Xianghe.Hydrocarbon-like OA(HOA)was the largest organic aerosol When the NR-PM1 mass concentrations increased from the lowest to the highest bins,nitrate and biomass burning OA(BBOA)showed increasing trends in the suburban area.Enhanced nitrate formation during the pollution epi-sodes resulted from both photochemical and aqueous processing.To reduce the particulate matter(PM2.5)concentrations and eliminate heavy pollution episodes,control measures should focus on reducing NO_(x),NH_(3),and volatile organic compound(VOCs)emissions.