Technical note: Contribution of ammonia emitted from livestock to atmospheric fine particulate matter (PM_2.5) in the United States

Ammonia emitted from animal feeding operations is an air pollutant contributing to the formation of fine particulate matter (PM_2.5), considered a major environmental risk to human health. In the United States, farm animals are the greatest contributor to gaseous ammonia emissions. Ammonia reacts with atmospheric nitric and sulfuric acids to form PM_2.5 (nitrate and sulfate), but the proportion of PM_2.5 attributable to ammonia emitted from animal farming operations has not been quantified. Thus, the objective of this analysis was to estimate the contribution of ammonia emitted from farm animals to PM_2.5 in the United States. The following approach was used: (1) the amount of ammonium in sulfate and nitrate PM_2.5 was calculated based on chemically speciated measurements published by the United States Environmental Protection Agency; and (2) the amount of ammonium in sulfate and nitrate PM_2.5 originating from livestock was assumed equal to the fraction of the total ammonia emissions attributable to livestock. Across different regions of the United States and under different weather conditions, PM_2.5 formed from ammonia emitted from livestock operations were estimated to contribute on average from 5 to 11% of the total PM_2.5 concentrations. In certain areas (North Central, for example) and in cool weather, farm animal contribution to atmospheric PM_2.5 concentration may be as much as 20%.