Absorption Enhancement Effect Of Carbonaceous Aerosol At Traffic Station In Nanjing Winter

Black carbon(BC),also referred as elemental carbon(EC,for optical BC),is one of the important species in the atmospheric aerosols and the second most important human emission in terms of its climate forcing in the present-day atmosphere after carbon dioxide.When fresh BC exposed in the atmosphere and coated with additional materials,the mass absorption cross-section(MAC)of BC would increase,which is named absorption enhancement effect.However,the researches about the enhancement of MAC(EMAC)of BC is still insufficient and difficult to measure by climate models.Hence,it is necessary to correct optical parameter of observations by an effective experimental method.There is a severe air pollution with exceedingly high concentrations of PM2.5 and BC in Nanjing due to the rapid industrialization and urbanization.Particularly during the winter,the low temperature facilitated the coagulation or condensation of aerosols.A total of 75 fine particulate matter(PM2.5)samples were collected from 3 December 2017 till 20 January 2018 in Nanjing at a traffic station.For measuring the absorption enhancement of MAC in Nanjing,an aerosol filter filtration-dissolution(AFD)method was adopted to remove both inorganic and organic coating of black carbon by two-step.The MAC value was determined by OC/EC analyzer at 678nm.High PM2.5(134 ± 64 ?g m-3)and black carbon(3.4 ± 1.7 ?g m-3)concentrations were observed in the northwestern suburb of Nanjing.The OC/EC ratio suggested a secondary pollution and significant dominance of fossil fuel emission(coal and vehicular exhausts)at sampling site.The concentrations of SO42-,NO3-and NH4+accounted for significant fractions of PM2.5.The high concentration of NO3-and high NO3-/SO42-ratio could be ascribed to the predominance of mobile source over stationary source pollutions.The backward trajectory results indicated that the concentration of pollutants increased when the air mass came from local and North China Plain.In this study,all water-soluble salts and?88%organic carbon was dissolved after AFD treatment.Meanwhile,the original and treated concentrations of EC showed a good linear correlation(R2= 0.77).The split point of treated samples would back to setting point of temperature protocol.A significant linear relationship is not only exhibited between the measured ATN and ECs concentration of original samples but also for treated sample(R2=0.80 and 0.83 respectively),which indicated that EC is a principal absorbing component in the aerosols.The MAC average values of original and treated samples were 8.9 ± 2.1 m2 g-1 and 5.6 ± 1.4 m2 g-1 at 678 nm respectively.The latter was suggested as a MAC value of pure carbon without coatings.The average enhancement of MAC(EMAC)was 1.7 ± 0.6 in this study indicated there was a light enhancement in the traffic station.The ratios of SO42-/EC,NO3-/EC,NH4+/EC and OC/EC showed significant positive correlation with MAC and EMAC,which indicated that absorption enhancement is under the control of coating level of second aerosols.Low ambient temperatures and unfavorable meteorological conditions helped in efficient trapping of aerosols with SO42-,NO3-,NH4 and OC,which led to a thick coating of black carbon and resulted in the light absorption enhancement.In this study,an experimental method was adopted to determine the MAC and EMAC directly.It made a contribution to correct optical parameter in climate molding as a reference value.