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Abstract. Aerosol particles in marine atmosphere have been shown to significantly affect cloud formation, atmospheric optical properties, and climatechange. However, high temporally and spatially resolved atmospheric measurements over the sea are currently sparse, limiting our understanding ofaerosol properties in marine atmosphere. In this study, a ship-based cruise campaign was conducted over the northern South China Sea (SCS) region duringsummertime 2018. The chemical composition of non-refractory PM 1 (NR-PM 1 ), the particle number size distribution (PNSD), and size-resolved cloudcondensation nuclei (CCN) activity were measured by a time-of-flight aerosol chemical speciation monitor (ToF-ACSM) and the combination of a cloudcondensation nuclei counter (CCNc) and a scanning mobility particle sizer (SMPS). Overall, aerosol particles exhibited a unimodaldistribution centering at 60–80  nm and the chemical composition of the NR-PM 1 was dominated by sulfate ( ∼  46 %), whichlikely originated from anthropogenic emissions rather than dimethyl sulfide (DMS) oxidation. Two polluted episodes (P1 and P2) were observed, and both were characterized by high particle number concentrations ( NCN ) which originated from local emissionsand from emissions in inland China via long-range transport. The concentrations of trace gases (i.e., O3 , CO, NOx ) andparticles ( NCN and NCCN at ss  =  0.34 %) were elevated during P2 at the end of the campaign and decreased with offshore distance, further suggesting important impacts of anthropogenic emissions from the inland Pearl River Delta (PRD) region. Two relativelyclean periods (C1 and C2) prior to and after tropical storm Bebinca were classified and the air was affected by air masses from the southwest and from the Indo-Chinese Peninsula, respectively. Chemical composition measurements showed an increase in organic mass fraction during P2 compared to C2; however,no obviously different κ  values were obtained from the CCNc measurements, implying that the air masses carried pollutants from local sourcesduring long-range transport. We report an average value of about 0.4 for the aerosol hygroscopicity parameter  κ , which falls within the literaturevalues (i.e., 0.2–1.0) for urban and remote marine atmosphere. In addition, our results showed that the CCN fraction( NCCN∕NCN, tot ) and the κ  values had no clear correlation either with the offshore distance or with concentrations ofthe particles. Our study highlights dynamical variations in particle properties and the impact of long-range transport from continental China and the Indo-Chinese Peninsula on the northern SCS region during summertime.

Effects of continental emissions on cloud condensation nuclei (CCN) activity in the northern South China Sea during summertime 2018