Growing role of secondary organic aerosol in the North China Plain from 2014 to 2024

Abstract

Since the Clean Air Act was implemented in 2013, China has witnessed a reduction of over 50 % in the annual average concentration of fine particulate matter (PM2.5). Despite emission cuts, the formation mechanism of secondary organic aerosols (SOA), a crucial constituent of PM2.5, remains inadequately understood. In this study, we performed a model-assisted analysis of field sampling data collected in Shijiazhuang. The results show that, compared to 2014, the contribution of SOA to the total organics (from 27 % in 2014 to 75 % in 2024) exceeded that of primary organic aerosol (POA) during the winter haze in 2024. The current model, which includes SOA formation pathways involving aromatic oxidation, aqueous formation, and SVOC/IVOC oxidation, underestimates the measured SOA levels by 60 % in 2024. However, incorporating the transformation of transported POA into SOA (accounting for up to 50 % of SOA) under high relative humidity (RH) conditions helps bridge the gap between model predictions and field measurements. The pathway of SOA formation via POA aging is consistent with the identified POA aging factor derived from biomass burning and coal combustion. The increase in SOA contribution occurred amidst large emission reductions, which accounted for 70 % of the decline in POA levels, while meteorological factors contributed an additional 10 %. Increased contribution of SOA was also found in other North China Plains areas, which underscores the pressing necessity for coordinated regional initiatives to effectively mitigate SOA levels across the NCP, thereby tackling the transboundary nature of air pollution.