The occurrence of lower-than-expected bulk NCCN values over the marginal seas of China - Implications for competitive activation of marine aerosols

In this study, we combined the measured bulk particle number concentration (NCN), particle number size distribution (PNSD) and bulk cloud condensation nuclei concentration (NCCN) at various supersaturation (SS) levels to investigate competitive activation of aerosols in the marine atmospheres over the marginal seas of China during two winter campaigns Campaign A (December 9-19, 2019) and Campaign B (December 28, 2019-January 16, 2020). During the two campaigns, we observed various categories of aerosols, i.e., long-range transport continental aerosols, clean marine aerosols, grown new particles ranging from nucleation mode to larger sizes, and grown pre-existing particles ranging from Aitken mode to accumulation mode size, etc. We found that the measured NCCN increased by only approximately 30 % with increases in SS levels from 0.2 % to 1.0 %, e.g., (1.8 ± 1.4) × 103 cm−3 at SS = 0.2 % and (2.4 ± 1.4) × 103 cm−3 at SS = 1.0 % during Campaign A. We further calculated the hygroscopicity parameter kappa (κ) by combining simultaneously measured PNSD and bulk NCCN to explore the causes. The calculated κ values were below 0.1 at SS = 0.4 % during the 72 % (or 88 %) period of Campaign A (or Campaign B). When κ values below 0.1 (or 0.2) were excluded, the remaining κ values were apparently reasonable, with an average of 0.22 (or 0.36) and a standard deviation of 0.10 (or 0.21) at SS = 0.4 % during Campaign A (or Campaign B). The unexpectedly lower κ values were discussed in terms of competitive activation of aerosols in marine atmospheres together with its net contribution to lowering the measured bulk NCCN below the expected value. [Display omitted] •The bulk NCCN increased by only approximately 30 % with increases in SS from 0.2 % to 1.0 % over the marginal seas of China.•The kappa values were lower than expected and negative correlated to the geometric median diameter of atmospheric particles.•The competitive activation of aerosols may substantially lower NCCN, which needs to be urgently resolved.