Impacts of the COVID-19 lockdown on atmospheric oxidizing capacity and secondary aerosol formation over the Beijing-Tianjin-Hebei region in Winter-Spring 2020

By using WRF-Chem coupled with a heterogeneous reaction mechanism for sulfate formation, this study investigated the impact of meteorological condition and emission changes on chemical species, atmospheric oxidizing capacity (AOC), and secondary aerosol formation during the COVID-19 lockdown period from 23 January to April 8, 2020, focusing on a severe haze event on 7-14 February. The model with the new sulfate formation scheme reasonably reproduces the spatial-temporal distribution of meteorological variables and chemical species, and significantly improves predictions for both sulfate and SO2 concentrations, as well as for PM2.5, ammonium, and nitrate to some extent. It is found that the adverse meteorological conditions were the main cause for the haze event formation, whereas emission reduction due to the lockdown somewhat decreased PM2.5 concentration on average in the Beijing-Tianjin-Hebei (BTH) region. Compared with the same period in 2019, increased surface air temperature and relative humidity (RH) and decreased planetary boundary layer height (PBLH) facilitated accumulation of pollutants and formation of secondary aerosols during the haze episode in 2020, whereas the emission reduction due to the lockdown led to decreases in SO2, NO2, primary PM2.5 (PPM2.5), black carbon (BC), primary organic aerosols (POA), nitrate and ammonium concentrations, but increases in O3, sulfate and secondary organic aerosol (SOA) concentrations, due to the combined effect of changes in emissions and AOC. Gas and aqueous phase oxidation of SO2 accounted for approximately 24% of sulfate formation, while the heterogeneous reaction of Mn-catalytic oxidation of SO2 on aerosol surfaces dominated sulfate formation (76%) during the haze episode in the BTH region. Both adverse meteorological conditions and emission reductions increased heterogeneous sulfate formation rate mainly through altering aerosol surface area (ASA), pH, and Mn2+ concentration. Chemical species varied diversely during the three subperiods before (Period-1, 15-22 January) and during the lockdown (Period-2, 23 January to 5 March and Period-3, 6 March to 8 April) over the BTH. NO2 concentration firstly decreased and then rebounded, whereas O3 concentration increased gradually from the Period-1 to Period-3. All aerosols except SOA decreased throughout the lockdown period, whereas SOA peaked in the Period-2 due to its strong sensitivity to increasing AOC. Sulfate concentration decreased from the Period-1 to Period-2, mainly due to more adverse meteorological conditions in the Period-1, although sulfate increased slightly due to increasing AOC in the Period-2. The large difference in the direction and magnitude of species variations during the COVID-19 lockdown indicates the complex interplay among meteorology, emission, and chemistry.