[Size Distribution and Mixing State of Black Carbon Aerosol in Nanjing During a Heavy Winter Pollution Event]

In December 2013,a lasting haze pollution event occurred in the Central and East regions of China.Here in this paper,using a single-particle soot photometer (SP2),we studied mass concentrations,mixing states,and size distributions of black carbon (BC) in this event.The results showed that BC mass concentration in Nanjing during the observation period was in the range of 1.01-14.05 μg·m^-3 with an average of 4.39 μg·m^-3,showing the characteristics of double peaks for lighter pollution,andhigh in the nighttime and low in the daytime during heavier pollution.Using the relative coating thickness (D_p/D_c) to define mixing state of black carbon,diurnal variations were higher in early morning and noon when slightly polluted,and lower during rush hour,indicating the degree of BC ageing was deeper in the morning and afternoon,with fresher particles near the ground source emissions during rush hour.The relative coating thickness did not vary greatly throughout the day when heavily polluted,which was expected to be linked to regional pollutants and it also showed Gas-solid transformation under high relative humidity.The average mass and number size distribution had single peaking with the same BC volume-equivalent diameter in different degrees of pollution,with the peak diameter at 100 nm for the number distribution when heavily polluted,at 91 nm when slightly polluted,and at 210 nm for the mass distribution regardless of the degree of pollution.We could find regional differences of BC sources by comparing the observed research worldwide.This study has very important reference value for the in-depth understanding of BC pollution in Yangtze River Delta.