Geographical distribution of particle number density in the accumulation mode range over the North Pacific Ocean

From 1998 to 2002, seven research cruises were conducted over the widespread areas of the North Pacific Ocean to investigate particle number densities in the accumulation mode range with the diameter of 0.1–2.5 µm. The densities over the Bering Sea, and the northeastern and southwestern North Pacific Ocean showed median values of 69.1, 122.8, and 66.9 cm− 3, respectively, in the summer. These measurements would be representatives of a background marine atmosphere over the remote North Pacific Ocean. Asian outflows bring the particles to the northwestern area of the North Pacific Ocean, especially to the coastal seas close to the land, in the transitional seasons of spring and autumn. Even in the summer, the influences of the outflows on the densities over the northwestern area of the North Pacific Ocean were found, although a high marine productivity would also contribute to an increase in the densities. Transport of combustion-derived particles from the land is a critical factor controlling the densities in the continentally modified atmosphere with a short transport time of the air masses from the land. In the background marine atmosphere, on the other hand, secondary production processes over the ocean would be a main factor for the densities due to the reduced influence of continental-derived particles.