| PM2.5（particulate matter with a diameter less than2.5μm） pollution is stillsevere in some cities of China, and concentrations of PM2.5were significantly higherthan the new ambient air quality standard （GB3095-2012） of annual secondarystandard limit. The type of urban air pollution has been changed from soot pollutionto complex pollution, which resulted in enhancing of atmospheric oxidation,increasing of fine particle concentration, degrading of atmospheric visibility, anddramatically increasing number of haze days. Therefore, studying influencing factorsof haze, establishing haze assessment system, and making source apportionment ofvisibility degradation were very necessary to prevent and control haze pollution.Correlations between the visibility and the concentrations of fine particulatematter （FPM） and relative humidity （RH） were very complicated. There werenon-linear, non-mutual independence as well as the obvious geographical differencesbetween visibility and FPM concentration and RH. Based on reasons mentionedabove, a new model, named as visibility variation sensitivity hybid model, wasestablished to study sensitivity degree of visibility variation depending on theconcentration of FPM and RH, to discern the different control areas of visibilityvariation （such as the control areas of FPM concentration, control areas of RH, FPMand RH together control areas）, to determine threshold values of FPM concentrationand RH, which can provide scientific basis for haze judgment.According to the threshold values of FPM concentration studied above, the hazyweather was defined, at the same time, taking convenience of environmental qualitymonitor, meteorological observation and environmental management into account,the key factors which can characterize the hazy weather were selected, hazy weathergrade standards and haze assessment index system were established.Source apportionment of visibility degradation is very important to prevent andcontrol haze pollution. However, research methods of visibility degradation attributedto the corresponding sources were lacked, in view of this situation, a new coupled model developed from the chemical mass balance （CMB） and Mie model wasintroduced to apportion sources of visibility degradation in hazy weather.Based on the methods mentioned above, the city of Tianjing was selected as acase for study: threshold values of PM2.5concentration and RH which impacted onthe degradtion of visibility were analyzed; hazy weather grade standards and hazeassessment index system were established; and source apportionment of visibilitydegradation was also studied. Results were as follows:1.As visibility<2km, when PM2.5≥350μg/m3, visibility variation was mainlydetermined by concentration of PM2.5, When RH≥85%, visibility variation wasmainly determined by RH; As2km≤visibility<5km, when PM2.5≥220μg/m3,visibility variation was mainly determined by concentration of PM2.5; When RH≥80%,visibility variation was mainly determined by RH; As5km≤visibility<10km, whenPM2.5≥130μg/m3, visibility variation was mainly determined by concentration ofPM2.5, When RH≥85%, visibility variation was mainly determined by RH; As10km≤visibility<15km, when PM2.5≥80μg/m3, visibility variation was mainly determinedby concentration of PM2.5; When RH≥80%, visibility variation was mainlydetermined by RH; As visibility≥15km, PM2.5concentration which impactedvisibility degradation was10μg/m3, and there was no obvious threshold value of RH.2.Hazy weather of Tianjin was when RH<80%, visibility degradation caused byPM2.5concentration and visual range lower than10km. hazy weather grade standardsand haze assessment index system were established as follows: when visibility≥10km, type of hazy weather was non-haze, concentration limit of PM2.5was80μg/m3,haze grade was0; when5km≤visibility<10km, type of hazy weather was light haze,concentration limit of PM2.5was130μg/m3, haze grade was1; when2km≤visibility＜5km, type of hazy weather was moderate haze, concentration limit of PM2.5was220μg/m3, haze grade was2; when visibility<2km, type of hazy weather was severehaze, concentration limit of PM2.5was350μg/m3, haze grade was3.3.Results of source apportionment of visibility degradation in autumn showedthat nitrate was the largest source to visibility degradation, and followed by sulfate,vehicle exhaust, the total contribution of these three sources was above70%,contributions of suspended dust, flying ash from coal combustion to visibility degradation were13.57%,7.05%, respectively, contributions of secondary organiccarbon （SOC） was about6%. Nitrates, sulfate, vehicle exhaust, suspended dust, flyingash from coal combustion and SOC were important sources to visibility degradationin Tianjin autumn. |
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