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 PM_2.5 formation may have had on the δ¹⁵N-NH_3(g) values observed in this study. A practical scenario assuming competing equilibrium and kinetic effects was used to calculate the δ¹⁵N-NH_3(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.