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We evaluated the relative importance of dry deposition of ammonia (NH3) gas at several headwater areas of the Susquehanna River, the largest single source of nitrogen pollution to Chesapeake Bay, including three that are remote from major sources of NH3 emissions (CTH, ARN, and KEF) and one (HFD) that is near a major agricultural source. We also examined the importance of nitrogen dioxide (NO2) deposition at one of these sites. Over the past decade, increasing evidence has suggested that NH3 deposition, in particular, may be an important contributor to total nitrogen deposition and to downstream nitrogen pollution. We used Ogawa passive samplers to measure NH3 concentrations over several years (2006-2011) for CTH, and primarily in 2008 and 2009 for the other sites. NO2 was measured at CTH mainly in 2007. Chamber calibration studies for NH3 and NO2, and field comparisons with annular denuders for NH3, validated the use of these passive samplers over a range of temperatures and humidity observed in the field, if attention is given to field and laboratory blank issues. The annual mean NH3 concentrations for the forested sites were 0.41 +/- A 0.03, 0.41 +/- A 0.06 and 0.25 +/- A 0.08 A mu g NH3/m(3) for CTH, ARN and KEF, respectively. NO2 passive sampler mean annual concentration was 3.19 +/- A 0.42 A mu g NO2/m(3) at CTH. Direct comparison of our measured values with the widely used Community Multiscale Air Quality (CMAQ) model (v4.7.1) show reasonably good agreement. However, the model-based estimates tend to be lower than our measured average NH3 concentration, by 8 % at our best studied site where we measured moderately low concentration, and up to 60 % at our site with the lowest concentrations and lowest sampling frequency. CMAQ NO2 concentration estimates were substantially higher than our measured values. Along a transect of sites near a source of NH3 emissions from animal agriculture, we found NH3 concentrations to be far higher than predicted for this area by the CMAQ model. This is not surprising, since the CMAQ model integrates over a relatively wide area. The higher NH3 concentrations we measured were generally within 1 km of the agricultural source. Such locally high atmospheric concentrations leading to locally high deposition may be ecologically significant. Analysis of such issues requires more locally scaled estimates than can be provided from the 12 km grid scale estimates of CMAQ used in this study. We estimated deposition of NH3 and NO2 using our concentration data and modified (concentration-weighted) deposition velocities derived from the CMAQ model. We estimate dry gaseous NH3 deposition as 2.0 +/- A 0.3 (CTH), 2.2 +/- A 0.4 (ARN) and 1.4 +/- A 0.7 kg N/ha-year (KEF). NO2 deposition at CTH is estimated to be 0.16 kg N/ha-year. NO2 deposition is a very small component of total nitrogen deposition at this site. On the other hand, NH3 deposition is either the largest or the second largest form of dry deposition at our sites, depending on how total N deposition is estimated. Based on total deposition best estimates of 9.2 kg N/ha for CTH and 8.6 kg N/ha for KEF, NH3 contributes between 16 and 22 % of total nitrogen deposition.

Such deposition has normally not been measured through traditional national monitoring programs, yet is significant as a source of nitrogen pollution to areas such as the highly nitrogen-sensitive Chesapeake Bay ecosystem.