Measurement report: Characteristics of airborne black carbon-containing particles during the 2021 summer COVID-19 lockdown in Yangzhou, China

Abstract

Abstract. Black carbon-containing (BCc) particles are pervasive in ambient atmosphere. The unexpected outbreak of the COVID-19 pandemic in 2021 summer prompted a localized and prolonged lockdown in Yangzhou City, situated in the YRD, China, which provides a unique opportunity to gain insights into the relationship between emission sources and BCc. Satellite and ground-level measurements both demonstrated that strict emission controls effectively reduced local gaseous pollutants. Meanwhile, single particle aerosol mass spectrometer (SPA-MS) analysis showed that the number fraction of freshly emitted BCc particles decreased to 28 % during the lockdown (LD), with that from vehicle emissions experiencing the most substantial reduction. However, the uncontrolled reductions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) likely contributed to increased ozone (O3) concentrations, increased the oxidizing capacity, which may in turn enhanced secondary PM2.5 formation and compensated the primary PM2.5 reduction. As a result, we did observe a slight increase of PM2.5 concentration (21.2 μg m-3) during the LD period compared to the period before the lockdown (BLD), and the increase of more aged BCc particles. Reactive trace gases (e.g., NOx, SO2, and VOCs) could form thick coatings on pre-existing particles likely via enhanced heterogeneous hydrolysis under high RH as well, resulting in significant BCc particle growth (~600 nm) during LD period. Furthermore, BCc source apportionment reveals that BCc particles were primarily of local origin (78 %) in Yangzhou during normal summertime. However, coal combustion (23 %) and vehicle emissions (21 %) were prominent non-local pollution sources, with the air mass originating from the southeast, along with biomass burning emissions (19 %) from the northeast, contributing significantly. Our research highlights that short-term, strict local emission controls may not effectively reduce PM pollution, due to the non-linear responses of PM2.5 to its precursors , further effective PM2.5 reduction requires a comprehensive and extensive approach with a regionally coordinated and balanced control strategy through joint regulation.