Study On The Optical Properties,Mixing States And Sources Of Light-Absorbing Carbon In East China

As an important species of aerosols,light-absorption carbon(black carbon,brown carbon)plays a vital role in energy balance of the earth-atmosphere system by scattering and absorbing solar radiation,and may further deteriorate air pollution during extreme haze episodes.Optical properties of black carbon(BC)can be affected by its mixing state and particle size distribution.Currently,long-term quantitative determination of the contribution of brown carbon(BrC)on light absorption is still difficult.As one of the most polluted regions in China,East China is suffering from regional and mixed air pollution,leading to more complicated characteristics of aerosol light absorption.However,only a few studies have reported BC mixing states in East China and investigation of the long-term measurement of BrC absorption is rather limited up to now.Therefore,this thesis firstly conducted long-term observations of aerosol optical properties to get their characteristics in typical regions in East China.Then,single particle black carbon measurement together with optical algorithm were used to study the mixing states and size distribution of BC and evaluate the impact of mixed states of BC on light absorption properties.Finally,based on ovserved mixing state and size distribution of black carbon in each season,this study optimized the optical segregation method for BrC light absorption,making it adequate for long-term observation and retrospection of historical data.By using this method,the long-term variations of BrC absorption was analyzed in the Yangtze River Delta region.Potential sources of brown carbon in different seasons were also discussed.The observation shows a strong scattering intensity and clear seasonal variation of aerosol in eastern China.The wavelength dependence of aerosol light absorption/scattering(Absorption Angstrom exponent,AAE and Scattering Angstrom exponent,SAE)shows differences at two sites.The distribution of AAE and SAE indicates a higher proportion of mixed aerosol in SORPES station.The short-wavelength AAE is always higher than long-wavelength AAE,which is in contrast to the conventional assumptions of black carbon absorption properties,indicating the presence of brown carbon or the impact of black carbon mixing states in these East China regions.The mass size.distribution of the black carbon observed at SORPES site exhibits a bimodal distribution in the three seasons(spring,summer and autumn).The corresponding coating thicknesses are also significantly different,suggesting that this site may be affected by multiple dominant sources of black carbon.In spring,there are higher proportion of internally mixed BC and larger BC size,indicating a higher aging stage due to intense open biomass burning in surrounding regions during wheat harvest season.In winter,BC particles have larger core sizes but thinner coating.The mixing states and size distributions are different in different seasons,resulting in different extent of the absorption enhancement by black carbon.Light absorption enhancement(Eabs)of internally mixed BC is 1.3 to 2.0.Eabs is higher in spring(mean value of 1.7),and are similar in other three seasons,with mean values ranging from 1.55 to 1.65.The case study in autumn shows that the coating thickness of BC exhibits a similar variation pattern with nitrate(NO3-)mass concentration during several pollution periods,while its variation is less similar with sulfate(SO42-),indicating that nitrate is possibly the dominant coating component of black carbon at SORPES site in autumn.Based on the mixing state information of single particle black carbon,brown carbon optical separation method is optimized in this study.Long-term variations of BrC absorption intensity and its contribution to light absorption are quantitatively described.The study finds that brown carbon has a significant light absorption contribution in the Yangtze River Delta region.During open biomass burning season and winter season,the absorption contribution of brown carbon can reach to around 33%.Both babs_BrC and PBrC shows a clear bimodal seasonal variation pattern,with peak value in May-June and December.However,sources of brown carbon in these two periods are largely different.In May-June,BrC is mainly from the primary emission of intensive open biomass burning.In December,the contribution of open biomass burning to BrC is small,but the absorption contribution of BrC is the highest in this month.Residential combustion,including household biomass burning and residential coal burning,are likely to be the main source of brown carbon in December.