Estimating air emissions from animal production in the United States using statistical models: Ammonia emissions from swine grow-finish barns

Animal production has the potential to emit various atmospheric pollutants including ammonia (NH₃), which can impact human health, atmospheric visibility and ecosystem health through gaseous NH₃ and associated <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow> <mml:mrow><mml:mn>4</mml:mn></mml:mrow> <mml:mrow><mml:mo>+</mml:mo></mml:mrow> </mml:msubsup> </mml:math> particulate matter deposition. Emission estimating methodologies were developed using statistical models to estimate daily NH₃ emissions from swine grow-finish barns based on National Air Emissions Monitoring Study (NAEMS) data. Models were developed with variables that represented production, manure management and environmental conditions. Model performance was evaluated for predicting NAEMS and non-NAEMS emissions, consistency of model coefficients and sensitivity to different model input values. Accounting for ease of variable measurement, the best performing models for predicting NAEMS emissions were models 1b and 16a, both of which accounted for the influence of temperature, swine inventory and weight, but used different predictor and response variables. In predicting NAEMS emissions, model 1b had mean error (ME) and mean bias (MB) values of 1.6 kg day^-1 (normalized mean error (NME) = 25.9 %) and 0.1 kg day^-1 (normalized mean bias (NMB) = 1.2 %), respectively, which were slightly lower than the corresponding values for model 16a (ME/NME = 1.8 kg day^-1/25.9 % and MB/NMB = 0.4 kg day^-1/5.8 %). Model 1b performed better in predicting non-NAEMS emissions, but model 16a had more reasonable sensitivity when barn live animal weight was >215,000 kg. Models using nitrogen feed intake as a predictor variable also performed well in predicting emissions and although these models have greater uncertainty due to limited NAEMS measurements, they could potentially account for changes in feed practices.