Chemical Variation of Submicron aerosols during the COVID-19 lockdown in multi-sites in Zhejiang, China

To prevent the spread of COVID-19, the Chinese government announced a complete lockdown in Wuhan on 23th January 2020. As one of the worst-affected provinces in China, Zhejiang declared the "Major public health emergencies first-level magnitude response" from 24^th Jan to 2^nd Mar. Stringent social distance and personal protective measures were taken, travels were banned and factories were mostly shutdown in all cities in Zhejiang to mitigate the impact of COVID-19 pandemic. In this study, we performed measurements by a suite of instrumentation in multi-sites across Zhejiang province, with a focus on the chemical composition of non-refractory submicron aerosols (NR-PM₁) measured by Aerosol Chemical Speciation Monitor (ACSM) from 1^st January to 30^th April, 2020 to evaluate the impact of COVID-19 lockdown on different atmospheric pollutants. Organic aerosol (OA, 29.3% to 41.2%) and nitrate (21.6% to 38.3%) dominated the aerosol concentrations at all sites. OA was resolved into hydrocarbon-like OA (HOA), biomass burning OA (BBOA) and two oxygenated OAs (OOA), including less oxidized OOA (LO-OOA) and more oxidized OOA (MO-OOA) by positive matrix factorization (PMF) analysis. As the sharp decline of human activities during the 1^st-level response, non-refractory PM₁ (NR-PM₁) decreased 11.2% to 53.9% and the diurnal patterns of most of the chemical species were flattened. Among the different components, the traffic-related pollutants, including nitrate, HOA and NOx, dramatically decreased by 36.1% to 64.2%, 43.8% to 81.1% and 39.0% to 71.6%, respectively. However, secondary formed species including MO-OOA and sulfate decrease a little or had no obvious change. O₃ showed a significant increasing trend possibly due to the variation of NOx/VOCs and the light intensity increased due to the decreasing of NR-PM₁, the secondary formed O₃ increased significantly. In addition, we found that the decline degrees for species varied greatly across sites and the concentrations of some species still remained high with a very low anthropogenic emissions level, suggesting more studies are needed to get a comprehensive understanding of interaction of different atmospheric species and their formation and evolution mechanisms.