Composition and supply of inorganic and organic nitrogen species in dry and wet atmospheric deposition: Use of organic nitrogen composition to calculate the Ocean's external nitrogen flux from the atmosphere

Abstract

Wet and dry depositions in the southern East China Sea (ECS) differ substantially in their relative proportion of high-molecular-weight organic nitrogen (HMW-ON) and low-molecular-weight organic nitrogen (LMW-ON). By distinguishing between ON with different molecular weights, this study proposes an improved calculation of the contributions of dry and wet depositions to the new production in surface seawater in the southern ECS. This study used 54 rainwater samples and 12 sets of size-fractionated atmospheric particles from the southern ECS, collected from January to December 2014. Dissolved or water-soluble samples were subjected to an ultrafiltration method to separate ON into HMW-ON and LMW-ON. The overall average concentration of ON was 56.5 ± 18.8 μM in wet deposition and 117.0 nmol m−3 in dry deposition. LMW-ON and HMW-ON constituted 82 ± 9% and 18 ± 9% of the total wet deposition, respectively; the respective percentages in dry deposition were 58 ± 11% and 42 ± 11%. In this study, we used C/N: 6.625 in the Redfield ratio, and assumed a HMW and LMW ON utilization rates of 20% and 80%, respectively. The new production potentially supported by the ocean's external nitrogen supply provided through atmospheric input was calculated to be equivalent to 17.5 g C m−2 yr−1; specifically, atmospheric deposition could contribute 26.6% of the total new production in the southern ECS.