Atmospheric inorganic nitrogen input via dry, wet, and sea fog deposition to the subarctic western North Pacific Ocean

Aerosol, rainwater, and sea fog water samples were collected during the cruise conducted over the subarctic western North Pacific Ocean in the summer of 2008, in order to estimate dry, wet, and sea fog deposition fluxes of atmospheric inorganic nitrogen (N). During sea fog events, mean number densities of particles with diameter larger than 0.5 µm decreased by 12–78%, suggesting that particles with diameters larger than 0.5 µm could act preferentially as condensation nuclei (CN) for sea fog droplets. Mean concentrations of nitrate (NO₃^–), methanesulfonic acid (MSA), and non sea-salt sulfate (nss-SO₄^2–) in sea fog water were higher than those in rainwater, whereas those of ammonium (NH₄⁺) in both sea fog water and rainwater were similar. These results reveals that sea fog scavenged NO₃^– and biogenic sulfur species more efficiently than rain. Mean dry, wet, and sea fog deposition fluxes for atmospheric total inorganic N (TIN; i.e., NH₄⁺ + NO₃^–) over the subarctic western North Pacific Ocean were estimated to be 4.9 ± 2.6 µmol m^–2 d^–1, 33 ± 47 µmol m^–2 d^–1, and 7.8 ± 8.7 µmol m^–2 d^–1, respectively. While NO₃^– was the dominant inorganic N species in dry and sea fog deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH₄⁺. The contribution of dry, wet, and sea fog deposition to total deposition flux for TIN (46 ± 48 µmol m^–2 d^–1) were 11%, 72%, and 17%, respectively, suggesting that ignoring sea fog deposition would lead to underestimate of the total influx of atmospheric inorganic N into the subarctic western North Pacific Ocean, especially in summer periods.