Trends in Ammonium Concentration in Precipitation and Atmospheric Ammonia Emissions at a Coastal Plain Site in North Carolina, U.S.A.

The temporal characteristics of annual volume-weighted average ammonium (NH4+) ion concentration in precipitation and local ammonia (NH3) emissions are investigated over the period 1982−1997 at National Atmospheric Deposition Program/National Trends Network site NC35, located in Sampson County, NC. Multiple regression analysis of annual volume-weighted average values of NH4+ concentration in precipitation identifies a statistically significant (p < 0.01) 4-year cycle and increasing trend during the period. The cycle is likely a function of mean annual ambient surface temperature, which is shown to be a significant (p < 0.01) predictor variable for annual NH4+ concentrations in precipitation. Loess fitting suggests that NH4+ concentration in precipitation began to increase more rapidly between 1989 and 1990. An analysis of estimated population-based annual NH3 emissions from individual sources in an intensively managed agricultural region surrounding NC35 shows that emissions from swine (p < 0.01), fertilizer (p < 0.10), turkeys (p < 0.05), and broilers (p < 0.05) are significantly greater during the period 1990−1997 than the period 1982−1989. Emissions from non-broiler chickens are significantly (p < 0.01) lower during the period 1990−1997. Cattle emissions are not significantly different (10% level) during the two periods. The increase in average annual swine emissions between periods accounts for ≈84% of the increase in average annual emissions from all sources between periods. Variability in local ammonia emissions from swine and mean ambient surface temperature explain ap proximately 90% of the variation in annual volume-weighted average NH4+ concentrations in precipitation at NC35 during the period 1982−1997.