Long-Term Trend of Gaseous Ammonia Over the United States: Modeling and Comparison With Observations

The concentrations of atmospheric ammonia ([NH₃]) have been observed to be increasing over the United States in the last decade, especially in Eastern United States. It is important to understand this temporal trend and variation due to the role of NH₃ in particle formation and its ecological effects. Here the long-term trend of [NH₃] over the United States is investigated using GEOS-Chem, a global 3-D tropospheric chemistry model, and is corroborated with empirical evidence from the Ammonia Monitoring Network. Model simulations, consistent with observations, show increase in [NH₃] over the United States from 2001 to 2016, with magnitude largest in the East (~5% to 12%/year) and smallest in the West (~0% to 5%/year). Reasons for this are examined, and evidence for the role of decreasing SO₂ and NO_x emissions in increasing [NH₃] is provided. The contributions of meteorology and NH₃ emission changes to the [NH₃] increase appear to be small during the period. Our sensitivity study suggests that decreasing SO₂ and NO_x emissions over the United States owing to stringent regulations explain about 2/3 and 1/3 of the increase in [NH₃], respectively. This effect is different for various NH₃ and SO₂ and NO_x regimes. Given the continued reduction of SO₂ and NO_x emissions due to U.S. regulations mainly aimed at PM_2.5 reduction, the present results are important towards better assessing the environmental impact of emission controlling policies.