Marine aerosol distributions from shipborne observations over the South China Sea: diurnal variation characteristics and their controlling factors

Marine aerosols critically influence Earth's radiation budget and climate dynamics through their spatial distributions and composition due to their production and transport processes. However, in situ observational datasets remain limited, particularly in the South China Sea (SCS). Based on our comprehensive shipborne measurements, this study presents a quantitative analysis of marine aerosol distributions and compositional variations between the offshore and pelagic regions over the SCS. Our data demonstrate a 120 % increase in offshore aerosol number concentrations (NCs, Dp < 10.37 µm) relative to pelagic baselines, featuring 120 % higher accumulation-mode particles (Dp ≤ 1.981 µm) and 70 % higher coarse-mode particles (1.981 µm < Dp < 10.37 µm), quantitatively confirming continental transport affects spatial distribution of marine aerosols. In contrast, in the pelagic regions, marine aerosols are virtually unaffected by continental source and distinctly represent characteristics of the local production. Meteorological analyses identified wind speed (WS) and sea surface temperature (SST) as primary regulators of NC. However, observed NC variations at fixed WS and SST values suggest additional controlling factors. We demonstrate that sea-air temperature differentials (SST-T2 m) exhibit a stronger correlation (r=-0.82, p < 0.01) with NC than the other meteorological parameters, where increased SST-T2 m led to decreased marine aerosol production. This temperature gradient effect drives pronounced diurnal NC variations, with maximum differences of 35 % observed between daytime, nighttime, and transition periods. These findings provide concrete evidence for the spatial and diurnal variability in marine aerosol distributions over the SCS, thereby further improving understanding of marine aerosol transport and production.