δ^(15)N-stable isotope analysis of NH_(x): An overview on analytical measurements, source sampling and its source apportionment

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 2.5µm)concentrations remains uncertain.Stable nitrogen isotopic composition(δ^(15)N)of NH_(3)and NH_(4)+(δ^(15)N(NH_(3))andδ^(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 forδ^(15)N(NH_(3))andδ^(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 fingerprintingδ^(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 refinedδ^(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.

Chemical characteristics and sources of water-soluble organic aerosol in southwest suburb of Beijing

PM2.5 filter sampling and components measurement were conducted in autumn and winter from 2014 to 2015 at a suburban site(referred herein as "LLH site") located in the southwest of Beijing.The offline aerosol mass spectrometry(offline-AMS) analysis and positive matrix factorization(PMF) were applied for measurement and source apportionment of watersoluble organic aerosol(WSOA).Organic aerosol(OA) always dominated PM2.5 during the sampling period,especially in winter.WSOA pollution was serious during the polluted period both in autumn(31.1 μg/m3) and winter(31.9 μg/m3),while WSOA accounted for 54.4%of OA during the polluted period in autumn,much more than that(21.3%) in winter.The oxidation degree of WSOA at LLH site was at a high level(oxygen-to-carbon ratio,O/C=0.91)and secondary organic aerosol(SOA) contributed more mass ratio of WSOA than primary organic aerosol(POA) during the whole observation period.In winter,coal combustion OA(CCOA) was a stable source of OA and on average accounted for 25.1% of WSOA.In autumn,biomass burning OA(BBOA) from household combustion contributed 38.3% of WSOA during polluted period.In addition to oxygenated OA(OOA),aqueous-oxygenated OA(aq-OOA) was identified as an important factor of SOA.During heavy pollution period,the mass proportion of aq-OOA to WSOA increased significantly,implying the significant SOA formation through aqueous-phase process.The result of this study highlights the concentration on controlling the residential coal and biomass burning,as well as the research needs on aqueous chemistry in OA formation.