Measurement report: Cloud condensation nuclei (CCN) activity in the South China Sea from shipborne observations during the summer and winter of 2021 – seasonal variation and anthropogenic influence

Abstract

Understanding seasonal variations in cloud condensation nuclei (CCN) activity and the impact of anthropogenic emissions in marine environments is crucial for assessing climate change. This study presents findings from two shipborne observations conducted in the South China Sea (SCS) during the summer and winter of 2021. In summer, higher particle number concentrations but lower mass concentrations of non-refractory submicron particles (NR-PM1) were observed, driven by Aitken-mode particle dominance. In contrast, winter showed a more balanced distribution between Aitken and accumulation-mode particles. Summer particles were more hygroscopic, exhibiting higher activation ratios (ARs) at all supersaturation (SS) levels. Distinct air mass periods were identified: in summer, terrestrial air masses from Luzon (“Luzon” period) and the area formerly referred to as the Indochinese Peninsula (hereafter “Indochinese Peninsula” period) and marine air masses; in winter, periods were influenced by mainland China (hereafter “Mainland China” period), a mix of mainland China and marine air masses (“Mixed” period), and purely marine air masses. The Luzon period in summer exhibited the highest particle number concentration, especially in the Aitken mode, resulting in the highest CCN number concentration (NCCN). Aerosol hygroscopicity was higher during the Indochinese Peninsula period compared to the Luzon period, leading to a higher bulk AR due to the combination of higher hygroscopicity and a greater fraction of accumulation-mode particles. The Mainland China period in winter showed a high nitrate fraction in the NR-PM1, but the inorganic fraction was similar to it in the Luzon period, resulting in comparable hygroscopicity at low SS to the Luzon period. However, smaller particle hygroscopicity was significantly lower in the Mainland China period compared to summer. The Mixed period in winter exhibited a higher fraction of accumulation-mode particles, causing a higher bulk AR compared to the Mainland China period. Overall, summer terrestrial air masses increased the Aitken-mode particle and CCN concentration, while winter terrestrial air masses led to higher concentration of large particles and lower hygroscopicity of fine particles. CCN closure analysis, considering aerosol composition and mixing state, revealed that summer aerosols were primarily internally mixed, whereas smaller aerosols in winter were primarily externally mixed. The potential effect of undetected sea salt may lead to an underestimation of aerosol hygroscopicity in summer. This study highlights significant seasonal differences in aerosol properties and the impact of different types of terrestrial air masses on CCN activity in the SCS, contributing to our understanding of regional climate influences.