Atmospheric deposition of nitrogen along the Connecticut coastline of Long Island Sound: a decade of measurements

Abstract

Four monitoring stations were established along the Connecticut coastline to study the atmospheric deposition of nitrogen into the Long Island Sound (LIS). Two stations at Bridgeport and Hammonasset were set up in 1991. Two additional ones have been in full operation since 1997 at Old Greenwich and Avery Point. Measurements were taken continuously from 1991 through 1994, and from 1997 through 1999. Nitrogen species including (nitrate/nitrite), ammonium, nitric acid gas and total nitrogen (since 1997) in air and precipitation were collected and used to infer ambient concentrations and dry and wet deposition flux densities, with independently collected meteorological data. Statistical procedures were employed to analyze the spatial and temporal variations of the weekly data and quarterly means for the two sampling periods of 1991–1994 and 1997–1999. The deposition flux densities of total nitrogen during 1991–1994 were 6.80 and 24.36 mg N m −2 wk −1, respectively, for dry and wet forms averaged over the two sites of Bridgeport and Hammonasset. During 1997–1999, the overall mean weekly wet and dry depositions of total nitrogen over the four sites were 4.28 and 23.64 mg N m −2 wk −1, respectively. In dry deposition, nitric acid gas was predominant and explained more than 70% of the total dry flux, while in wet deposition nitrate/nitrite was the major contributor (40–60%). Most of the collected nitrogen quantities were higher during warm months, especially from June through September, indicating higher emission rate, scavenging efficiency, and surface uptaking at warmer temperature. The ambient concentration and dry deposition peaked at Bridgeport, and the maximal wet deposition was observed at Old Greenwich. The spatial variation of the dry deposition quantities was affected by potential local emission sources, while the geographic gradient of the wet deposition was largely attributed to the proximities of the sites to the New York–New Jersey metropolitan area. Comparisons of nitrogen quantities between the two sampling periods revealed that the dry and total depositions at Hammonasset decreased, while there were no discernible changes at Bridgeport.