δ15N-stable isotope analysis of NHx: An overview on analytical measurements, source sampling and its source apportionment

Abstract

Agricultural sources and non-agricultural emissions contribute to gaseous ammonia (NH 3 ) that plays a vital role in severe haze formation. Qualitative and quantitative contributions of these sources to ambient PM 2.5 (particulate matter with an aerodynamic equivalent diameter below 2.5 µm) concentrations remains uncertain. Stable nitrogen isotopic composition (δ 15 N) of NH 3 and NH 4 + (δ 15 N(NH 3 ) and δ 15 N(NH 4 + ), respectively) can yield valuable information about its sources and associated processes. This review provides an overview of the recent progress in analytical techniques for δ 15 N(NH 3 ) and δ 15 N(NH 4 + ) measurement, sampling of atmospheric NH 3 and NH 4 + in the ambient air and their sources signature (e.g., agricultural vs. fossil fuel), and isotope-based source apportionment of NH 3 in urban atmosphere. This study highlights that collecting sample that are fully representative of emission sources remains a challenge in fingerprinting δ 15 N(NH 3 ) values of NH 3 emission sources. Furthermore, isotopic fractionation during NH 3 gas-to-particle conversion under varying ambient field conditions (e.g., relative humidity, particle pH, temperature) remains unclear, which indicates more field and laboratory studies to validate theoretically predicted isotopic fractionation are required. Thus, this study concludes that lack of refined δ 15 N(NH 3 ) fingerprints and full understanding of isotopic fractionation during aerosol formation in a laboratory and field conditions is a limitation for isotope-based source apportionment of NH 3 . More experimental work (in chamber studies) and theoretical estimations in combinations of field verification are necessary in characterizing isotopic fractionation under various environmental and atmospheric neutralization conditions, which would help to better interpret isotopic data and our understanding on NH x (NH 3 + NH 4 + ) dynamics in the atmosphere.