Chemical characteristics of aerosols transported from Asia to the East China Sea: an evaluation of anthropogenic combined nitrogen deposition in autumn

Abstract

Deposition of terrestrial aerosols is one of the major means of fertilizing the oligotrophic ocean waters. In order to understand the biogeochemical significance of aerosols transported from the Asian continent, and deposited over the East China Sea, we studied their chemical properties by making continuous measurements on board R/V Hakuho Maru in autumn, a season of lower dust events. Outflows of anthropogenic substances were observed over the East China Sea. The average concentration of non-sea-salt (nss)-SO 4 2− in aerosols was 8.7±6.8 μg m −3 , a value typical for polluted atmosphere of a highly industrialized urban area in South China. Fine mode NH 4 + mainly occurred as (NH 4 ) 2 SO 4 and/or NH 4 HSO 4 while in coarse mode it was formed from intermodal coagulation of ammonium particles in fine and coarse aeorosols. On the other hand, the mean concentration of NO 3 − (1.8±1.2 μg m −3 ) was lower than that in the urban atmosphere. Non-sea-salt SO 4 2− were derived from combustion sources and marine biological processes, the latter accounting to 0–38%. Nitrate in coarse mode was associated with Ca 2+ and Na + perhaps due to replacement reactions. Our estimates of the annual atmospheric aerosol deposition fluxes of NH 4 + and NO 3 − to the East China Sea were 270 Gg N yr −1 and 160 Gg N yr −1 , respectively. These atmospheric inputs are comparable to the riverine inputs of the Changjiang River. The total combined nitrogen deposition could account for a biological fixation of about 2.5 Tg C (approximately 0.1–9% of the new production) in the East China Sea annually, which could actually be larger since the autumn season experience minimal dust storms.