Measurement Report: Seasonal variation and anthropogenic influence on cloud condensation nuclei (CCN) activity in the South China Sea: Insights from shipborne observations during summer and winter of 2021

Abstract. Understanding seasonal variation in cloud condensation nuclei (CCN) activity and the impact of anthropogenic emissions in marine environments is crucial for assessing climate change. In this study, two shipborne observations in the South China Sea (SCS) during the summer and winter of 2021 were conducted. During summer, higher particle number concentrations but lower mass concentrations of non-refractory submicron particles (NR-PM₁) were observed. These differences were attributed to the dominance of particles in the Aitken mode during summer and in the accumulation mode during winter. Moreover, particles during summer were more hygroscopic with higher activation ratios (ARs) at all supersaturation (SS). Based on backward trajectory analysis, the whole campaign was classified into terrestrial and mixed air mass influence periods. Particles measured during the terrestrial period consistently exhibited lower hygroscopicity values. Additionally, minor variations were shown for all NR-PM₁ components under different air mass influences during summer, while the mass fraction of nitrate increased significantly under terrestrial influence during winter. Particle number size distribution (PNSD) exhibited unimodal distribution during terrestrial period and bimodal distribution during mixed air mass influence period, with winter displaying a more pronounced bimodal pattern than summer. The impact of PNSD on AR was greater than on aerosol hygroscopicity in summer, and vice versa in winter. During terrestrial period, significant variations in PNSD were observed with the offshore distance, and the largest variation was seen in Aitken mode during both summer and winter. Meanwhile, aerosol hygroscopicity shows an increasing trend with the offshore distance, which is primarily attributed to the increase of sulfate fraction during summer and the decrease of the black carbon fraction during winter. Using a single parameterized PNSD in the N_CCN prediction can lead to errors exceeding 100 % during both summer and winter, with dominant terrestrial air masses in the SCS atmosphere, while using a constant hygroscopicity parameter would lower the errors in the N_CCN prediction (~15 % during winter and ~10 % during summer). Our study shows significant differences in aerosol properties between winter and summer seasons and highlights the influence of anthropogenic emissions on the CCN activity in the SCS.