Characteristic And Mechanism Of Formation Of Triplet State Brown Carbon

Brown carbon(BrC),which is similar as chromophore with charcreristics of light absorption and photochemical reaction,has been found in the organic carbon(OC)of aerosols in recent years.The light absorption intensity of BrC shows a trend of exponential attenuation from ultraviolet to visible band.BrC not only affects the global climate beacause of the ability of light absorption,but also participates in atmospheric photochemical reaction to affect the quality of atmospheric environment,such as driving the aerosols aging and.In this study,the characteristics of optical and the particle size distribution of BrC were clarfied.The characteristics of photochemical reaction,photolysis and the contribution to atmospheric photochemistry of BrC were clarified,too.The main research contents are as follows:(1)The pollution characteristics and main sources of carbonaceous component were studied by thermal-optical analysis.The carbonaceous components,such as OC,element carbon(EC),water-insoluble organic carbon(WISOC)and water-soluble organic carbon(WSOC),show the characteristics of "high at both ends and low in the middle",which means serious pollution in winter and significant decreasing in summer.The proportion of organic aerosols in PM2.5 shows seasonal characteristics,while the proportion of EC in PM2.5 remains stable in different seasons.Secondly organic carbon(SOC)generate significantly in winter,which is related to the higher OC emissions and the climate characteristics in winter in northern China.The sources analysis of carbonaceous aerosols shows that the main sources are coal and vehicle emissions in Xi'an.(2)The PM samples with different particle sizes were collected in Linfen,a typical coal-fired city in China.The samples were analyzed by EEM-PARAFAC(Excitation-emmission matrices-parallel factor analysis).The particle size is in the range of 0-10 ?m.The absorbance,total fluorescence volume,mass absorption coefficient and normalized fluorescence volume of BrC increase with the decreasing of particle size,which indicates that fine particles have more significant optical components.HULIS,which is significant correlated with SOA,widely exists in particles with smaller particle size.Tryptophan and tyrosine may be generated from biological aerosols,and their relative content increases with the increasing of particle size.We studied the contribution of 7 PAHs(Polycyclic aromatic hydrocarbon)to Abs365 and FV(Fluorescence volume)of MSOM,it is found that PAHs had a positive correlation with absorbance and fluorescence volume,but the average contribution to absorbance and fluorescence volume was very small(<3%).(3)BrC were identified through EEM and the photochemical reaction platform made by the laboratory.We quantified the ability of BrC forming triplet state by using chemical probe and established the structure-activity relationship between BrC and triplet state.We quantified the driving effect of triplet state on reactive oxygen species(ROS)through EPRs.The results show that the formation of triplet state basically conform with the first-order reaction kinetics.Tryptophan and N-containing BrC driven by Maillard reaction contribution significantly to triplet state;the triplet state formation ability of biomass combustion samples is the strongest and the emission of motor vehicles is the weakest in POA.The triplet state formation ability is similar in SOA with different oxidation degree.The triplet state formation ability of ambient PM in winter is about twice that in summer.It is found that the BrC further drives about 30%1O2 and·OH in aerosol through triplet state.(4)The photolysis characteristics of BrC were studied through simulated illumination,optical component analysis and carbon component analysis.BrC photolysis under illumination condition.Compared with urban PM,POA photolysis significantly,which indicates that POA have stronger photolysis ability.Before and after photolysis,TFV of water-soluble/water-insoluble BrC in POA decreases by 74.1%/63.2%on average,while 23.4%/79.1%in ambient PM.Driving effect on reactive oxygen species decreased significantly after BrC photolysis,which proved that photolysis had an inhibitory effect on BrC driving ROS.It was also found that the BrC in ambient PM conform the mechanism of triplet state driving reactive oxygen species,but POA do not conform the mechanism.