| Atmospheric deposition is a substantial source of nutrient nitrogen to coastal water bodies in the mid-Atlantic region. Fluxes of nitrogen via wet deposition to these systems are fairly well-quantified, but those for dry deposition remain uncertain. This is primarily due to the existence of only a small number of coastal sampling stations in addition to a lack of collection techniques and modeling strategies appropriate for estimating dry nitrogen deposition to coastal waters. The first phase of this work established reliable methods for collecting, storing, and analyzing atmospheric particulate nitrogen species.{09}Specifically, the chemical stability of particulate nitrogen species on atmospheric size-resolved aerosols collected and stored frozen at −10°C for approximately 8 weeks was tested. Results indicated that for nitrogen compounds that are commonly present in large concentrations in the coastal zone (NH4+ and NO3 −), immediate freezing and subsequent storage was clearly adequate for accurate quantification.; The second phase of this work applied the developed methods to measure concentrations of atmospheric nitrogen species (including gaseous NH 3 and HNO3 and particulate NH4+, NO3−, NO2−, and organic nitrogen) at Lewes, DE during a two-week sampling intensive in the summer. The corresponding deposition fluxes of each of the nitrogen species were modeled and controls on deposition, including pH-dependent phase changes that occur in the coastal atmosphere, were investigated. Results indicated that during the sampling period, dry deposition contributed approximately 43% to total deposition. Under all flow conditions, dry deposition of NH 3(g) accounted for the largest fraction of total nitrogen dry deposition (averaging 60%); HNO3(g) and NO3− also contributed considerably (averaging 25% and 8%, respectively). During onshore flow, scavenging of HNO3 by sea-salt aerosols resulted in relatively greater concentrations of particulate NO3 − in aerosol solutions. Consequently, particulate NO3 − deposition accounted for larger fractions of total NO 3 (HNO3 + particulate NO3−) deposition under these flow regimes. Because the large sea-salt particles exhibited mass-weighted deposition velocities similar to those of HNO 3, this phase change did not alter the overall dry deposition velocity for total NO3. In light of these results, the following changes to current dry deposition monitoring programs at the coast are recommended: (1) NH3(g) deposition should be considered; (2) supermicron particles (with which most particulate NO3− is associated) should be reliably sampled; (3) models of particulate nitrogen deposition should be consistent with the ambient size distributions of nitrogen compounds observed in coastal air. New data generated from these changes will enable a more accurate picture of nitrogen dry deposition to the mid-Atlantic coast to be developed on a long-term basis so that more informed decisions regarding mitigation of deposition effects, such as eutrophication, can be made. |
