Atmospheric transport and wet deposition of ammonium in North Carolina

Wet deposition and transport analysis has been performed for ammonium (NH4+) in North Carolina, USA. Multiple regression analysis is employed to model the temporal trend and seasonality in monthly volume-weighted mean NH4+ concentrations in precipitation from 1983 to 1996 at six National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites. A significant (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 (NH3) 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 NH4+ concentrations in precipitation in conjunction with boundary layer air mass back trajectories, is developed to statistically test for the influence of a particular NH3 source region on NH4+ concentrations at surrounding NADP/NTN sites for the years 1995-1996. NH3 emissions from this source region, primarily evolving from swine and poultry operations, are found to increase NH4+ concentration in precipitation at sites up to approximate to 80 km away. At the Scotland County (NC36) and Wake County (NC41) sites, mean NH4+ concentrations show increases of at least 44% for weeks during which 25% or more back trajectories are influenced by this source region. (C) 2000 Elsevier Science Ltd. All rights reserved.