Solubilities and deposition fluxes of atmospheric Fe and Cu over the Northwest Pacific and its marginal seas

Abstract Atmospheric input is an important exogenous source of Fe and Cu to the surface ocean. Here, we investigated temporal and spatial variations, solubilities, size distributions and deposition fluxes of aerosol Fe and Cu using a unique, new dataset including 3-years of observations at Huaniao Island and two cruises during the springtime of 2015 in the Northwest Pacific (NWP) and of 2017 in the Yellow and East China Seas (YECS). The mean concentrations of soluble aerosol Fe and Cu showed a seasonal variation over Huaniao Island with the highest and lowest values in winter and summer respectively, whilst the total Fe reached the maximum in spring. Their mean concentrations decreased significantly from the coastal island to the YECS and to the NWP. The solubility of aerosol Fe was inversely and positively correlated with crustal element Al and non-sea-salt-SO42- (nss-SO42-), respectively. Moreover, the size distribution of soluble Fe demonstrated major peaks in accord with nss-SO42- and a minor peak associated with total Fe. Based on the size distributions of total and soluble Fe, anthropogenic contribution and acid-enhanced dissolution were firstly estimated to be 61% and 32% of soluble Fe over Huaniao Island, 55% and 30% over the YECS, and 34% and 47% over the NWP, respectively. Unlike the Fe, the solubility of aerosol Cu was more correlated with relative humidity (RH). Our estimates of dry deposition fluxes of soluble Fe and Cu declined greatly from the coastal island 33.3 and 2.9 μg m−2 d−1 to the open Pacific 3.6 and 0.1 μg m−2 d−1, respectively. The fluxes of soluble Fe and Cu could vary asynchronously as a result of different origins and transport paths of air masses. Our concurrent study on aerosol Fe and Cu is very helpful for understanding the atmospheric deposition of bioavailable Fe and Cu to the ocean and their interactive effect on marine phytoplankton.