Size partitioning of particulate inorganic nitrogen species between the fine and coarse mode ranges and its implication to their deposition on the surface ocean

Abstract

In order to discuss the dry deposition fluxes of atmospheric fixed nitrogen species, observations of aerosol chemistry including nitrate (NO 3 −) and ammonium (NH 4 +) were conducted at two islands, Rishiri Island and Sado Island, over the Sea of Japan. Although the atmospheric concentrations of particulate NH 4 +–N showed higher values than those of particulate NO 3 −–N at both sites, the dry deposition fluxes of the particulate NO 3 −–N were estimated to be higher than those of the particulate NH 4 +–N. This was caused by the difference of particle sizes between the particulate NO 3 − and NH 4 +; NH 4 + was almost totally contained in fine particles ( d < 2.5 μm) with smaller deposition velocity, whereas NO 3 − was mainly contained in coarse particles ( d > 2.5 μm) with greater deposition velocity. Fine mode NO 3 − was strongly associated with fine mode sea-salt and mineral particles, of which higher concentrations shifted the size of particulate NO 3 − toward the fine mode range. This size shift would decrease the dry deposition flux of the fixed nitrogen species on coastal waters and accelerate atmospheric transport of them to the remote oceanic areas.