Impact of meteorological condition changes on air quality and particulate chemical composition during the COVID-19 lockdown

Stringent quarantine measures during the Coronavirus Disease 2019(COVID-19) lockdown period(January 23,2020 to March 15,2020) have resulted in a distinct decrease in anthropogenic source emissions in North China Plain compared to the paralleled period of 2019.Particularly,22.7% decrease in NO2and 3.0% increase of O3was observed in Tianjin,nonlinear relationship between O3generation and NO2implied that synergetic control of NOxand VOCs is needed.Deteriorating meteorological condition during the COVID-19 lockdown obscured the actual PM2.5reduction.Fireworks transport in 2020 Spring Festival(SF) triggered regional haze pollution.PM2.5during the COVID-19 lockdown only reduced by 5.6% in Tianjin.Here we used the dispersion coefficient to normalize the measured PM2.5(DN-PM2.5),aiming to eliminate the adverse meteorological impact and roughly estimate the actual PM2.5reduction,which reduced by 17.7% during the COVID-19 lockdown.In terms of PM2.5chemical composition,significant NO3~-increase was observed during the GOVID-19 lockdown.However,as a tracer of atmospheric oxidation capacity,odd oxygen(Ox=NO2+O3)was observed to reduce during the COVID-19 lockdown,whereas relative humidity(RH),specific humidity and aerosol liquid water content(ALWC) were observed with noticeable enhancement.Nitrogen oxidation rate(NOR) was observed to increase at higher specific humidity and ALWC,especially in the haze episode occurred during 2020 SF,high air humidity and obvious nitrate generation was observed.Anomalously enhanced air humidity may response for the nitrate increase during the COVID-19 lockdown period.