深圳市冬季黑碳气溶胶的粒径分布和混合态特征

Characteristics of the size distribution and mixing state of black carbon aerosol in Shenzhen in winter

黑碳（Bc）气溶胶的气候效应和环境效应是当今科学界的研究热点．BC气溶胶的粒径分布及混合态对其光吸收和其他理化性质有很大影响，但是受仪器分析技术的限制，目前国内外均鲜有对单个BC粒子大小及混合态连续观测的研究．本研究利用新型的单颗粒黑碳光度计（SP2），对2009年1～2月深圳市BC气溶胶的质量浓度、粒径分布及单颗粒混合态进行连续在线观测．结果表明：观测期间BC浓度均值为6．24μg／m^3；其质量粒径分布呈单峰型，峰值位于211nm；内混态BC（127—264nm粒径段内）质量比例为32．4％．内混态BC比例随粒径变化趋势与表面积浓度的粒径分布趋势相似，说明内混态BC形成与大气中的气-固转化过程密切相关．外混态BC浓度与NOx高度相关，并随大气边界层高度变化而呈现白天低、夜间高，这些特征都说明外混态BC与本地机动车等燃烧源的新鲜排放密切相关；内混态BC浓度的日变化相对平缓，指示出其来自区域传输的特征反向轨迹分析也表明，内混态BC比例与气团老化程度有显著的对应关系．本研究有助于深入认识我国大气BC气溶胶污染的本质及来源特征，并为准确评估BC气溶胶在辐射强迫和气候变化中的作用提供关键的支撑数据．

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 6.24μg/m^3 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.