Highly concentrated atmospheric inorganic nitrogen deposition in an urban, coastal region in the US

Much of our understanding of atmospheric nitrogen deposition (AD) is based on important monitoring networks, such as the National Atmospheric Deposition Program (NADP) in the United States (US). However, it is likely that rural-based monitoring data are unrepresentative of reactive nitrogen concentrations found in urban areas. In this study, event-based precipitation was collected over one-year in Providence, RI - a mid-sized city situated at the head of Narragansett Bay, the largest estuary in the northeastern US Significant concentration enhancements in inorganic nitrogen (i.e., ammonium and nitrate) were found for wet deposition in the urban area compared to nearby NADP rural and non-urban coastal monitoring sites. Moreover, the annual ratio of ammonium to nitrate was highest in urban precipitation (>1.0). From 164 precipitation events, the mean volume-weighted ammonium and nitrate concentrations were 32.9 mu mol l(-1)and 28.4 mu mol l(-1), respectively, with total inorganic nitrogen concentrations exceeding 100 mu mol l(-1)in at least 30 events. AD data for Narragansett Bay is limited and outdated: our results suggest inorganic nitrogen is at least 2.5 times greater than currently estimated in policy decision-making related to improving water quality and ecosystem services. This underestimation is due to an increase (by as much as 6 times) in ammonium wet deposition since 1990, likely due to an increase in urban sources of precursor ammonia.