Number-Size Distribution and CCN Activity of Atmospheric Aerosols in the Western North Pacific During Spring Pre-Bloom Period: Influences of Terrestrial and Marine Sources

Abstract

A cruise observation of atmospheric aerosols and gaseous air/seawater compounds was performed in the western North Pacific during the spring pre-bloom period. The aerosol size distribution and cloud condensation nuclei (CCN) activity were characterized according to the source type: terrestrial/pollution, marine, and mixed. The CCN activity was represented by the aerosol hygroscopicity parameter (ĸCCNC) from the activation diameter (dact) in water-vapor supersaturation (SS), which is controlled by the composition of particles with sizes around dact. For marine type, whereas a high hygroscopicity parameter value of 0.86 for accumulation mode (ĸaccum) suggests a strong influence of sea salt and sulfate, a lower value of 0.49 in Aitken mode (ĸAit) suggests the contribution of biogenic organic matter (OM). Lower ĸAit and ĸaccum for terrestrial/pollution type (0.12 and 0.62, respectively) suggests greater contributions of OM. Under the influence of marine sources, ĸCCNC at a dact of ∼110 nm (low SS) was not responsive to the chlorophyll a (Chl-a) concentration, whereas ĸCCNC at ∼50 nm (high SS) correlated negatively with Chl-a. The hygroscopicity parameter of OM was estimated as 0.13–0.29 considering the aerosol composition. During a new particle formation event, the ĸCCNC at 0.60% SS increased; nucleated particles grew in size to become CCN. Sensitivity analysis indicated that changes in the size distribution and chemical composition of the aerosol contributed to the increase in CCN concentration. This study highlights the high variability in size distribution and CCN activity of off-coast aerosols from different sources, including marine biological activities.