Investigating ammonia emissions in a coastal urban airshed using stable isotope techniques.

Increases in reactive nitrogen (N) production and consumption have caused ammonia (NH3) emissions to the atmosphere to increase up to five times since the industrial revolution, resulting in negative impacts on the environment and human health. Understanding regional NH3 emissions and atmospheric dynamics is essential for creating realistic mitigation strategies. This study utilized stable isotope techniques to quantify and characterize NH3 emissions in a coastal urban air shed (i.e., Corpus Christi, TX, U.S.A.). NH3 was then passively collected at eight sites from September 2016 through August 2017 and analyzed for isotopic composition. The average atmospheric NH3 concentration for the region was 1.24 ± 0.98 μg/m3 with mean and concentration-weighted mean δ15N-NH3 values of -19.1 ± 12.7‰ and -17.1‰ respectively. The mixing model IsoError was used for source attribution, indicating that ambient NH3 was equally influenced by non-agricultural (55 ± 6%) and agricultural (45 ± 6%) emission sources with some seasonal variation. Sites adjacent to large water bodies exhibited low [NH3] coupled with a significant influence from non-agriculture emissions indicating a possible influence from marine-sourced NH3 which was further investigated using a three endmember SIAR bayesian mixing model. This work also examined the effects that varying degrees of kinetic and equilibrium fractionation via PM2.5 formation may have had on the δ15N-NH3(g) values observed in this study. A practical scenario assuming competing equilibrium and kinetic effects was used to calculate the δ15N-NH3(g) value of the original NH3 gas before fractionation occurred and calculated mean and concentration-weighted mean δ15N-NH3 for the region were -21.5 ± 12.4‰ and -22.3‰ respectively with no significant changes in source attribution. This work has shown that agricultural and non-agricultural sources of NH3 can equally contribute to ambient NH3 in urban environments and that marine NH3 must be taken into consideration when performing emission inventories in coastal regions.