Sources and fluxes of organic nitrogen in precipitation over the southern East Sea/Sea of Japan

Abstract. We measured total dissolved reactive nitrogen in precipitation samples collected at Uljin, a Korean coastal site upwind of the southern East Sea/Sea of Japan (EJS), selected as a representative study site of atmospheric deposition over the northwestern Pacific margin. NO₃^- was found to be the most abundant nitrogen species, followed by NH₄⁺ and dissolved organic nitrogen (DON). Air-mass back-trajectory (AMBT) analysis revealed that a significant fraction of the inorganic nitrogen (NO₃^- and NH₄⁺) originated from mainland Asia, whereas the DON was primarily derived from Korea. Using varimax-rotated factor analysis in combination with major ions as tracers, agricultural activities in Korea were identified as the primary sources of DON in these samples. In addition, a positive correlation was found at Uljin between the size of organic fraction in total reactive nitrogen and nitrogen to carbon atomic ratio in organic matter. This correlation has also been observed at other locations worldwide, implying the utilization potential of atmospheric organic nitrogen might increase with its proportion in total nitrogen. Combining wet deposition measurements in this study with literature values for dry deposition observed at a remote island in the EJS, the total atmospheric depositional flux of reactive nitrogen was estimated to be 115 mmol N m⁻² yr⁻¹ over the southern EJS. Our study sheds new light on the potentially significant contribution to primary productivity of the northwestern Pacific Ocean by atmospheric deposition of nitrogen, especially the organic fraction.