Impact of Siberian Wildfires on Ice-Nucleating Particle Concentrations over the Northwestern Pacific

Ice-nucleating particles (INPs) significantly influence aerosol-cloud precipitation interactions at regional and global scales. However, information regarding the concentrations and origins of INPs over the open ocean, particularly at high latitudes, remains insufficient due to access difficulties. In this study, we investigated the concentrations and origins of INPs over the western North Pacific to the Arctic Ocean through ship-borne observations conducted in the early autumn of 2016. The number concentrations of INPs (N_INPs) active at -25 °C (N_INPs(-25 °C)) and -15 °C (N_INPs(-15 °C)) varied from 0.034 to 41.2 L^-1 and <0.0005 to 0.11 L^-1, respectively, and those over the Arctic Ocean (≥70°N) were the lowest. Comparisons of the observed N_INPs variation with chemical compositions and autofluorescent properties of ambient aerosol particles indicated that N_INPs(-25 °C) and N_INPs(-15 °C) were largely influenced by mineral and biological materials of terrestrial origin, respectively. We further observed higher N_INPs over the Bering Sea and the Northwestern Pacific (40-60°N) at the return cruise than those at the outward cruise. Aerosol composition and backward trajectory analyses indicated that particles originating from Siberian wildfires could significantly contribute to the observed high N_INPs. These results suggest a substantial role of boreal wildfires in supplying INPs onto the oceans, including high latitudes, depending on the transportation and emission conditions.