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

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 these 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 87 % to in 2024) exceeded that of primary organic aerosol (POA) during the winter haze in 2024. Although the model underestimated the measured SOA levels, incorporating the transformation of transported POA into SOA under high relative humidity (RH) conditions helped bridge the gap between model predictions and on-site measurements. 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.