Evolution of PM2.5 chemical composition during summer new particle formation in a coastal city in the Yangtze River Delta: insights from pollution episodes, machine learning, and air mass origins.

To investigate the influence of O3 on new particle formation (NPF) events and associated changes in PM2.5 chemical composition, an observation campaign was conducted in suburban Wuzhen, Jiaxing from August to September 2024. A total of 13 NPF events were observed (frequency: 21.31%). The mean SO2 concentration during NPF events in early August and early September was 57.93% and 30.67% higher than those observed on non-NPF days within the same months, respectively. Furthermore, the average relative humidity (RH) during mid-August NPF events declined by 22.05% relative to that on non-event days in August, indicating that high SO2 concentration and low relative humidity favor the occurrence of NPF events. In addition, precipitation can promote NPF formation by lowering PM2.5 concentrations. On NPF days with O3 concentrations above 160 μg/m3, the mean particle formation and growth rates were 7.00 ± 5.41 cm-3 s-1 and 17.78 ± 2.03 nm h-1, respectively. By contrast, on NPF days with O3 levels below 160 μg/m3, the respective values were 2.62 ± 1.90 cm-3 s-1 and 16.56 ± 3.14 nm h-1, and NPF initiation occurred approximately one hour earlier compared with the former scenario. Machine learning identified a critical O3 threshold of 128.1 μg/m3 for 3-10 nm particles, below which O3 promoted NPF and above which it inhibited NPF. During NPF, total water-soluble ions decreased by ∼31.5%, with NH4+ and NO3- showing the largest reductions (41%-43% and 65%-71%, respectively). The O3-Polluted non-NPF group exhibited the highest total ion concentration (18.89 μg/m3), sulfur oxidation ratio (SOR = 66.3%), nitrogen oxidation ratio (NOR = 22.4%), and secondary organic carbon (3.70 μg/m3), indicating enhanced secondary transformation under excessively high O3 despite NPF suppression. Diurnal patterns of NH4+ shifted from synchrony with NO3- (non-NPF) to synchrony with SO42- (NPF). Air mass origin was critical: clean marine air favoured NPF, while polluted continental air from the Yangtze River Delta promoted O3 formation and suppressed NPF.