The Concentrations And Variations Of Atmospheric Ammonia And Size-resolved Particulate Ammonium In Tianjin

Recently,due to vast energy comsuption and a rapid increase in the number of vehicles,China has faced serious particulate pollution.Particulate pollution is characterized by elevated levels of fine ammonium particulates,which has negative effects on human health and visibility because of their chemical compositions and longer lifetimes in the atmosphere.Coarse particulates are also an important factor for the reduced visibility.Moreover,the deposition of acids and toxic species through coarse particulates causes adverse effects in ecosystems.Recent researches focus on the single type of particulates,such as PM2.5(particulate matters with aerodynamic diameters?2.5 ?m)and TSP(total suspended particulates).The lack of knowledge about different size-resolved particulate impedes us to understand the particulate pollution better.The chemical compositions of size-resolved particulates are important for characterzing the physical and chemical process in the atmosphere.Since early last century,to satisfy the increasing demand of human for food and energy,ammonia(NH3)emission derived from fertilizer application and livestock experiences a sharp increase.Enhanced NH3 emission and deposition caused negative effects on air quality and ecosystems.NH3,as important alkaline gas,plays an important role in forming particulates,particularly fine particulates.However,this pollutant is currently not included in emission control policies in China.It is of significance for environmental regulators to understand the conversion of NH3 to PM-NH4+.The NH3(n=45)and size-resolved particulates(n=261)were simultaneously collected from March to September,2018 in Tianjin,China.These data supported policy makers and scientists in understanding particulate pollution and the conversion of ammonium particulates.Main research results are as follows:(1)For size-resolved particulates,PM average concentrations were orderly(the subscripts of PM mean size bins and the unit is ?m):PM<0.43(6.4 ?g m-3)<PM0.43-0.65(14.3 ?g m-3)<PM2.1-3.3(14.4 ?g m-3)<PM4.7-5.8(14.6 ?g m-3)<PM3.3-4.7(16.4?g m-3)<PM1.1-2.1(17.2 ?g m-3)=PM0.65-1.1(17.2 ?g m-3)<PM5.8-9(17.8 ?g m-3)<PM>9(23 ?g m-3).The PM2.1 comprises 38.8%of total PM and PM1.1 comprises 68.2%of PM2.1 on the average.This means fine particulates is important for particulate pollution and fine particulate pollution is primarily derived from submicron aerosols in Tianjin.(2)The average proportions of water-soluble ions(Na+,NH4+,K+,Mg2+,Ca2+,F-,Cl-,NO2-,SO42-,NO3-,PO43-;WSI)in different size-resolved particulates to corresponding particulate concentration were orderly:PM4.7-5.8-WSI(17.5%)<PM3.3-4.7-WSI(19.5%)<PM5.8-9-WSI(22.2%)<PM>9-WSI(22.5%)<PM2.1-3.3-WSI(24.5%)<PM1.1-2.1-WSI(38.6%)<PM0.43-0.65-WSI(39.7%)<PM0.65-1.1-WSI(44.5%)<PM<0.43-WSI(70.5%).Generally,the average proportions of PM<2.1-WSI are higher than PM>2.1-WSI.There are two probable reason.First,fine particulates are mainly formed by secondary particulates(NH4+,SO42-and NO3-,i.e.SNA)and will grow to the coarse mode.Second,fine partiuclates have larger specific surface areas,which makes them absorb pollutants easily.The average propotions of PM2.1-SNA to PM2.1 concentration is 38%in sampling period,especially 58%in March,which indicates the importance of SNA in fine particulate pollution.The average molecular ratios of NH4+to(No3-+2*SO42-)in PM<0.43,PM0.43-0.65,PM0.65-1.1,PM1.1-2.1,PM2.1-3.3,PM3.3-4.7,PM4.7-5.8,PM5.8-9,PM>9 are 0.20,1.00,0.97,0.74,0.23,0.12,0.09,0.06,and 0.06,respectively.The average molecular ratio in PM0.43-2.1 is larger and close to 1(stoichiometric coefficients),where SNA are more likely to concentrate.This means the reacted precursors of SNA react more fully in these size.(3)The average NH3 concentrations from March to September are 6.97 ?g m-3,3.78 ?g m-3,4.59 ?g m-3,3.88 ?g m-3,4.73 ?g m-3,8.04 ?g m-3,and 9.58 ?g m-3,respectively.The average concentrations of corresponding total PM-NH4+from March to September are 8.91 ?g m-3,4.40 ?g m-3,3.20 ?g m-3,3.76 ?g m-3,3.67 ?g m-3,1.99?g m-3,3.44 ?m--3,respectively.The average concentrations of total PM-NH4+in the colder months are higher than those in the warmer months,the NH3 concentrations are opposite.The average rate of conversion to total PM-NH4+in colder months(41.7%)is higher than that in warmer months(27.1%).The average rate(53.2%)of conversion to total PM-NH4+in the period of particulate pollution(PM2.1>75 ?g m-3)is higher than that(31.1%)in the period of clean air(PM2.1<75 ?g m-3).In conclusion,fine PM is an important part of particulate pollution,and abosorbs more pollutants relatively.The ratios of convertsion to PM-NH4+are higher during the colder and polluted periods.