Characteristics Of Atmospheric Ammonia Pollution And Its Impacts On Fine Particles In Beijing

Ammonia?NH₃?,a predominant alkaline gas in the atmosphere,can easily transfer into the particle phase as ammonium salts through the neutralization reactions with acid gases and cause fine particle pollution.In North China Plain,the NH₃ emission intensity is high and the PM_2.5.5 pollution is severe;consequently,how NH₃ affects PM_2.5 pollution in this region becomes a particularly important scientific question.Focusing on Beijing,this study investigated the pollution characteristics of NH₃ and the possible causes and assessed the impacts of NH₃ on secondary inorganic fine particles and the aerosol aidity of PM_2.5 via field observation and modeling methods.The analysis of the NH₃ concentration data obtained from observations at three sites in Beijing and its surrounding area indicated that the variation ranges of daily NH₃concentrations at urban,suburban and rural site were 5.1-46.0?g/m³?4.9-52.6?g/m³and 5.5-59.3?g/m³,respectively.Compared with other regions in the world,the NH₃concentrations at the three sites were relatively high,reflecting the serious NH₃pollution.Among the three sites,high NH₃ concentrations were more easily occurred in suburban and rural sites,and the highest hourly NH₃ concentration at the rural site reached 257.1?g/m³.During the spring and summer simultaneous observation period,the NH₃ concentrations between the suburban and rural sites showed a certain regional synergistic variation.The seasonal variation of NH₃ concentration in Beijing and its surrounding area was obvious that highest in spring and summer and lowest in winter,wich mainly due to the seasonality of temperature and agricultural activities.The results of potential source contribution function?PSCF?and concentration weighted trajectory?CWT?modeling showed that,although the results varied slightly from season to season and site to site,the potential source regions of NH₃ in Beijing and its surrounding area were mainly distributed in the south of the observation sites,and the regional transport from the central and southern Hebei,northwestern Shandong and northeastern Henan contributed most to the NH₃ pollution in the study area.The impacts of NH₃ on secondary inorganic components in PM_2.5 were not identical in different seasons in Beijing.In each season,the combination of NH₄⁺and SO₄^2–in PM_2.5 was mainly in the form of?NH₄?₂SO₄,and substantial increased summertime?NH₄?₂SO₄ formation was observed.In winter,the sufficient NH₃ in ambient atmosphere not only promoted both homogeneous and heterogeneous formation of NO₃^–,but also enhanced the nighttime NH₄Cl formation;while in spring and summer,on the basis of the analysis of theoretical equilibrium constants of semi-volatile ammonium salts,NH₄NO₃ formation and NH₄Cl formation were thermodynamically favored in spring and spring night,respectively.In typical PM_2.5 heavy pollution episodes in the above three seasons,the substantial increased SNA caused by the intense NH₃ to ammonium salt convertion processes was one of the key reasons for the formamation and develop of PM_2.5 heavy pollution in Beijing.Sensitivity analysis based on ISORROPIA II model showed that the atmosphere of Beijing in spring and summer was ammonia-rich.PM_2.5 acid strength was the strongest in summer,followed by spring,and the weakest in winter,which was related to the difficulty of conversion of NH₃ to semi-volatile ammonium salts and the degree of influence of alkaline mineral components in different seasons.