Phase partitioning and dry deposition of atmospheric nitrogen at the mid‐Atlantic U.S. coast

Abstract

The deposition of atmospheric nitrogen contributes significantly to total nitrogen (TN) entering U.S. coastal water bodies. Nitrogen inputs via wet deposition are well quantified but dry‐deposition fluxes are uncertain. In this study, dry fluxes of major atmospheric nitrogen species (including gaseous NH3 and HNO3 and particulate NH4+, NO3−, NO2−, and organic nitrogen (ON)) were quantified during a 2‐week summer sampling period at Lewes, Delaware, on the mid‐Atlantic U.S. coast. Results indicate that dry deposition contributed approximately 43% to total atmospheric nitrogen deposition. Under all flow conditions, NH3(g) accounted for the largest fraction of TN dry deposition (averaging 60%); HNO3(g) and NO3− also contributed considerably (averaging 25 and 8%, respectively). During onshore flow, scavenging of HNO3(g) by sea‐salt aerosols shifted the phase partitioning and relative dry fluxes of total NO3 (HNO3(g) + NO3−) toward particulate NO3−. The mass‐weighted deposition velocities for particulate NO3− (associated primarily with sea‐salt size fractions) were similar to those of HNO3(g). Consequently, phase changes did not substantially alter the dry‐deposition fluxes of total NO3. In light of these results, dry‐deposition monitoring programs at coastal locations should (1) quantify NH3(g) deposition; (2) reliably sample supermicron particles (with which most particulate NO3− is associated); and (3) apply deposition models for particulate nitrogen that are consistent with corresponding size distributions.