Atmospheric transport and wet deposition of ammonium in North Carolina

Wet deposition and transport analysis has been performed for ammonium (NH` 4 ) in North Carolina, USA. Multiple regression analysis is employed to model the temporal trend and seasonality in monthly volume-weighted mean NH` 4 concentrations in precipitation from 1983 to 1996 at six National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites. A signi"cant (p(0.01) increasing trend beginning in 1990, which corresponds to an annual concentration increase of approximately 9.5%, is detected at the rural Sampson County site (NC35), which is located within a densely populated network of swine and poultry operations. This trend is positively correlated with increasing ammonia (NH 3 ) emissions related to the vigorous growth of North Carolina's swine population since 1990, particularly in the state's Coastal Plain region. A source}receptor regression model, which utilizes weekly NH` 4 concentrations in precipitation in conjunction with boundary layer air mass back trajectories, is developed to statistically test for the in#uence of a particular NH 3 source region on NH` 4 concentrations at surrounding NADP/NTN sites for the years 1995}1996. NH 3 emissions from this source region, primarily evolving from swine and poultry operations, are found to increase NH` 4 concentration in precipitation at sites up to +80 km away. At the Scotland County (NC36) and Wake County (NC41) sites, mean NH` 4 concentrations show increases of at least 44% for weeks during which 25% or more back trajectories are in#uenced by this source region. ( 2000 Elsevier Science Ltd. All rights reserved.