Individual particle analysis of marine aerosols collected during the North–South transect cruise in the Pacific Ocean and its marginal seas

Sea-salt particles are major aerosol constituents in the marine boundary layer (MBL) and are chemically modified by acidic substances from anthropogenic, volcanic, and biogenic sources. When these acidic substances react with sea-salt particles, they can be easily scavenged from the MBL. This scavenging process influences the concentration of cloud condensation nuclei in the MBL. In this study, differences in chemical compositions of sea-salt particles collected over the Pacific Ocean and its marginal seas were investigated based on an individual particle analysis. The sources of the acidic substances that modified sea-salt particles were also estimated. Approximately 70% of the analyzed particles were fresh sea-salt particles. However, a sample collected near Guam was dominated by sulfates and was affected by polluted air masses that originated over the Asian continent. The aerosols collected around the Aleutian Islands included a large number of sulfates and modified sea-salt particles. Individual particle composition analyses indicate that these sea-salt particles were modified by methanesulfonic acid produced from marine biota and sulfuric acid from volcanic gas from the Kamchatka Peninsula. The gravitational deposition velocities of sulfates and modified sea-salt particles were calculated based on measured particle sizes. The results suggested that if acidic substances react with sea-salt particles, their lifetime in the atmosphere could be shortened due to dry deposition. Therefore, it is necessary to consider the scavenging effect of sea-salt particles for estimating the climate cooling effect by clouds over the ocean.