Observational Study Of Black Carbon Aerosol Mass Absorption Efficiency And Its Influencing Factors In Typical Areas

In this manuscript,we selected four representative regions,including Beijing,Huainan,Xianyang and Zhangye.The Single Particle Soot Photometer,Aethalometer and other auxiliary instruments were used to observe and collect the black carbon aerosol（BC）.We have discussed and analyzed absorption characteristics,particle size distribution and mixed state of black carbon in detail.In addition,dust aerosols were collected in Zhangye region,and the contribution of light absorption caused by dust aerosol was analyzed.The following results are obtained:（1）The daily variation of BC mass absorption efficiency（MAE）was a double-dip pattern,which was opposite to the change trend of BC mass concentration.The MAE values of Beijing,Huainan,Xianyang and Zhangye were 7.9,10.4,8.9（880nm）and 8.5 m²/g（637 nm）,respectively.It is mainly related to different regions and emission sources.Besides,the different instruments used have an impact on the measurement.（2）The BC size distributions of Beijing,Huainan and Xianyang present a lognormal distribution as a function of the diameter,and the peak diameter of the primary mode are 211,190 and 192 nm,respectively.The peak diameter of the BC size distribution is only affected by the primary emission source and subsidence.The time and size distribution of BC mass concentration in different regions has diverse peaks,and the concentration peak is mainly concentrated in the particle size range of100-300 nm.Moreover,the daily variation of average particle size and MAE of BC exhibit opposite forms.The smaller particle size is,the larger MAE value produced.（3）The average ratio of internal mixing in Beijing,Huainan and Xianyang were25.4%,20.1%and 19.4%,which is opposite to the BC mass concentration.The freshly discharged BC particles in the morning and evening peaks diminish the number fraction of thickly coated BC.In the afternoon,the internal mixing ratio reached the highest due to the aging of BC.By analyzing the absorption enhancement of BC coatings,the corresponding absorption enhancement factor(E_abs)values were1.4,1.6 and 1.3 in Beijing,Huainan and Xianyang,indicating that the outer coatings increased the absorbance of BC.In summary,when the nuclear size distribution of BC is small and the outer layer covered wrap is thick,the enhancement of BC light absorption is relatively larger.（4）In the non-dust day,the floating dust incidents and the dust storm during Zhangye observation,the proportion of dust absorption in the overall amount presented an increasing trend,16.5%,31.1%and 71.6%,respectively.Especially during the sandstorm,the BC mass concentration decreased to half of the non-dust day,but the absorption coefficient increased to 2.6 times as that of the non-dust day,indicating that the dust also produced a large amount of light absorption.The MAE of the dust aerosol was calculated to be 0.014±0.00028m²/g at 637nm,and the refractive index of dust aerosol was reckoned by Mie scattering theory to be k=1.50-0.0007i.Although the dust absorption efficiency is low,the large number of dust particles in the dust process can still make an important contribution to the total absorption of aerosols,which produce optical radiation absorption as important as BC.