Atmospheric ammonia measurements at low concentration sites in the northeastern USA: implications for total nitrogen deposition and comparison with CMAQ estimates
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We evaluated the relative importance of dry deposition of ammonia (NH?) 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 NH?emissions (CTH, ARN, and KEF) and one (HFD) that is near a major agricultural source. We also examined the importance of nitrogen dioxide (NO?) deposition at one of these sites. Over the past decade, increasing evidence has suggested that NH?deposition, in particular, may be an important contributor to total nitrogen deposition and to downstream nitrogen pollution. We used Ogawa passive samplers to measure NH?concentrations over several years (2006–2011) for CTH, and primarily in 2008 and 2009 for the other sites. NO?was measured at CTH mainly in 2007. Chamber calibration studies for NH?and NO?, and field comparisons with annular denuders for NH?, 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 NH?concentrations for the forested sites were 0.41 ± 0.03, 0.41 ± 0.06 and 0.25 ± 0.08 µg NH?/m³for CTH, ARN and KEF, respectively. NO?passive sampler mean annual concentration was 3.19 ± 0.42 µg NO?/m³at 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 NH?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 NO?concentration estimates were substantially higher than our measured values. Along a transect of sites near a source of NH?emissions from animal agriculture, we found NH?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 NH?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 NH?and NO?using our concentration data and modified (concentration-weighted) deposition velocities derived from the CMAQ model. We estimate dry gaseous NH?deposition as 2.0 ± 0.3 (CTH), 2.2 ± 0.4 (ARN) and 1.4 ± 0.7 kg N/ha-year (KEF). NO?deposition at CTH is estimated to be 0.16 kg N/ha-year. NO?deposition is a very small component of total nitrogen deposition at this site. On the other hand, NH?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, NH?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.