The climatic and environmental effects of black carbon (BC) aerosol have recently become a hot research topic for the international scientific community. The size distribution and mixing state of BC have an important effect on radiative and other physicochemical properties of BC. However, there have been few measurements of the size distribution and mixing state of BC because of the limitations of analytical technology. Here, using a newly developed single-particle soot photometer (SP2), we studied the BC mass concentration, size distribution, and mixing state in Shenzhen, China, from January to February 2009. During the observation campaign, it was found that the average BC mass concentration was 6.24 μg/m3 and the average mass size distribution had one mode peaking at a BC volume-equivalent diameter of 211 nm. Internal mixing was evidenced by 32.4% of the mass having a volume equivalent diameter in the range of 127–264 nm. The size distribution of the internal mixing ratio of BC was consistent with that of the particle surface area, indicating that internally mixed BC was closely associated with ambient gas-to-aerosol conversion processes. The concentration of externally mixed BC, which was highly correlated with the NOx concentration, appeared to have a diurnal pattern of being lower during the day and higher at night, which is expected to be strongly linked to the variation in the boundary layer height. These phenomena suggest that the concentration of externally mixed BC was closely related to fresh local emissions, such as vehicular emissions. Compared with the concentration of externally mixed BC, the concentration of internally mixed BC did not vary greatly throughout the day, which is characteristic of regional pollutants. Back trajectory analysis showed that the BC internal mixing ratio corresponded to the aging of air masses. This study provides a better understanding of the sources and mechanisms of BC pollution in China and provides basic data for the evaluation of radiative forcing and climate effects of BC.
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