Changes in source contribution to particle number concentrations during the 2022 COVID-19 Lockdown on southern edge...

The COVID-19 lockdown was a typical example of extreme emission reduction, providing an opportunity to study the impact of lockdown measures on air pollution. Particle number concentrations (PNC) originate from direct emissions or through new particle formation events. However, their variations during the lockdown period are under investigation. This study focuses on Luohe, a city on the southern edge of the North China Plain, analyzing the changes in PNC and its sources before, during, and after the COVID-19 lockdown. From March 25^th to May 31^st, 2022, real-time PNC measurements were conducted using a Scanning Mobility Particle Sizer for particle size. Results showed an 11.2 % decrease in PNC during the lockdown compared to pre-lockdown and a 3.6 % decrease compared to post-lockdown, indicating reduced local emissions and weakened regional transportation during the lockdown. Positive Matrix Factorization analysis identified six sources contributing to the total PNC, including photochemical nucleation, aged photochemical nucleation, gasoline vehicle emissions, diesel vehicle emissions, coal and biomass combustion, and secondary aerosols. The significant changes in source emissions indicate a substantially reduced traffic volume after the implementation of lockdown measures (2644.8 #/cm³, 2202.2 #/cm³, 2792.7#/cm³). Concurrently, photochemical nucleation (310.1 #/cm³, 306.3 #/cm³, 393.1 #/cm³) and photochemical nucleation aging (592.8 #/cm³, 744.1 #/cm³, 810.7 #/cm³) exhibited increasing trends, while coal/biomass combustion (1656.6 #/cm³, 1586.2 #/cm³, 980.0 #/cm³) and secondary sources (999.4 #/cm³, 791.1 #/cm³, 804.1 #/cm³) showed decreasing trends. In summary, the contributions of traffic emissions to PNC highlight the potential for targeted traffic management strategies to improve urban air quality.