[Source Apportionment of Atmospheric Ammonia: Sensitivity Test Based on Stable Isotope Analysis in R Language]

Ammonia (NH₃) is an important precursor of fine particles and nitrogen deposition. It is critical to identify and quantify the sources of NH₃ before the implementation of a mitigation strategy. Stable isotope analysis in R (SIAR) has potential with regard to the source apportionment of NH₃, but its reliability is closely related to the signatures (δ¹⁵N-NH₃) of emission sources. Based on SIAR, we found that the agricultural contribution varied significantly with mean δ¹⁵N-NH₃ values of endmember input. In contrast, both the contributions of fossil fuel and NH₃ slip showed low sensitivity against the change of endmember input. Moreover, the agricultural contribution changed by about 20% due to the variations in agricultural endmember mean values. Such a change is five times that due to the variations in endmember standard deviation values. Notably, regardless of the number of input sources tested, "non-agricultural source" was the dominant source of NH₃ during hazy days in January 2013 in Beijing. Since various agricultural sources showed large variations in δ¹⁵N-NH₃, future studies should focus on the endmember signatures of agricultural sources to further reduce the uncertainty in SIAR-based NH₃ source apportionment.