Fractal Dimension,Optical Properties,and Radiative Forcing Of Atmospheric Individual Black Carbon Particles

Black carbon（BC）,produced by incomplete combustion of biomass and fossil fuels,is an important component of fine particulate matter(PM_2.5)in the atmosphere.BC can strongly absorb the solar radiation and affect regional and global climate.BC particles undergo complex aging processes after being emitted into the atmosphere.BC particles generally change from loose chain-like structure to compact cluster and their mixing state changes from“bare”to“internally mixed”state.The aerosols mixing with BC particles are called“coating”,which can enhance the absorption of BC particles（The phenomenon is commonly called“lensing effect”）.At present,there are great differences in the absorption enhancement of BC particles among field observations,laboratory experiments,and numerical simulations.The morphology,mixing structure,and optical absorption of BC particles change during atmospheric aging processes,which have not been coupled in climate models.Hence the radiative forcing of BC particles is uncertain.Therefore,it is important to accurately quantify the impact of morphology and mixing structures of atmospheric BC particles on absorption enhancement,which will improve the evaluation of BC climate effect.In this work,we used a transmission electron microscope（TEM）,combined with three-dimensional（3D）modeling,optical numerical simulation,and atmospheric radiative transfer modelling,to obtain the fractal dimension and mixing structures of BC particles in different environments and then further simulated their optical properties and radiative forcing.First of all,the morphological variation of BC particles in the atmospheric aging processes and cloud processing was quantified using the single particle analysis method.Then a novel 3D optical modeling tool（named as“EMBS”）was developed based on the individual particle information from field observation.The EMBS was used to study the evolution mechanism of absorption enhancement of total BC particles population at different sampling sites.Finally,the optical simulation results of individual BC particles were input into an atmospheric radiative transfer model to study the effect of mixing structure on radiative forcing.In the morphological analysis of BC particles at mountain sites,we found that the fractal dimension（D_f）values of cloud interstitial bare（INT-Bare）,cloud interstitial coated（INT-Coated）,and cloud residual（RES）BC particles were 1.77,1.95,and 1.94,respectively.The D_f of INT-Coated BC particles is 10.2%larger than that of INT-Bare,indicating that the atmospheric aging process has an obvious compaction effect on the morphology of BC.The D_f of RES BC particles（1.94）is 4.3%larger than that of cloud interstitial BC particles（1.86）,indicating that the BC compaction effect of cloud processing is relatively weaker.Laboratory experiments suggest that the weaker BC compaction in cloud processing may be due to the low density of cloud droplets.The study on 3D optical modeling of individual BC particles showed that the EMBS method could construct 3D models based on TEM images,which improved the accuracy of optical simulation of BC and provided a more precise and more effective optical simulation tool.The EMBS played an important role in reducing the derivation of optical simulation caused by the simplification of BC morphology and mixing structure.Then the EMBS was applied to calculate the absorption enhancements of the total BC particles population,which were～1.6,～1.6,and～2.0 at the urban,suburban,and mountain sites in North China,respectively.The findings suggest that both embedded fraction（F）and coating thickness（D_p/D_c）affected the absorption enhancement of individual BC particles,but the effect of F was more significant when D_p/D_c>2.0.The embedded fraction of individual BC particles causes that the absorption enhancement of BC particle population ranges between～1 and 2.25.The mixed structures of individual BC particles in particle population are diverse,leading to the variety of absorption enhancement of BC particle population.The direct radiative effect（DRE）at three sites in North China was simulated.The DRE at the top of the atmosphere at Beijing,Xianghe,and Mt.Tai sites was+1.91 W m^-2,+11.25 W m^-2,and-0.17 W m^-2,respectively.The increment of the direct radiative effect（ΔDRE）at the top of the atmosphere due to mixed structures was Mt.Tai(+1.34W m^-2)<Beijing(+3.79 W m^-2)<Xianghe(+5.56 W m^-2).The increment efficiency of the direct radiative effect(ΔDRE/BC_mass)at Mt.Tai site(+1.16 W m^-1μg^-1)was higher than that at Xianghe(+0.82 W m^-1μg^-1)and Beijing(+0.82 W m^-1μg^-1)sites.The BC particles at Mt.Tai site have more complex mixing structures.There were more BC particles with high D_p/D_c and F at Mt.Tai site,so theΔDRE/BC_mass due to the mixing structure was higher for BC particles.Therefore,the results suggest that the mixing structure of aged BC particles can effectively increase the radiative forcing.In this work,the morphology of individual BC particles,the variation of mixing structures during atmospheric aging and cloud processes,and the impact of different mixing structures of individual BC particles on optical properties and radiative forcing were studied.The evolution mechanism of light absorption enhancement of BC particle population was clarified.The study provides theoretical basis for the research of BC climate effect.