南京2007年12月持续雾霾过程的大气消光特性

2007年南京冬季雾外场综合试验期间，雾、霾交替持续的最长时间达100 h以上。利用大气气溶胶粒子和雾滴数浓度尺度谱分布、能见度、相对湿度等同步观测资料，从Mie散射理论出发，研究了雾、霾不同阶段大气消光特征，重点分析了大气气溶胶粒子和雾滴在雾、霾持续和转化过程中的消光作用。结果表明，雾、霾过程不同阶段平均能见度的大小关系为：雾<湿霾<霾~轻雾。平均而言，雾阶段雾滴和气溶胶粒子的消光作用相当，其中，雾滴消光波动幅度大于气溶胶粒子消光，能见度的变化趋势主要由雾滴的消光决定。湿霾、霾和轻雾阶段的消光主要由气溶胶粒子造成。湿霾阶段的低能见度是由于大量积聚模态的气溶胶粒子在较高相对湿度环境中吸湿增长所致。霾阶段气溶胶粒子数浓度达到最大，核模态粒子占总数浓度的80%左右，是导致该阶段能见度较低的主要原因。轻雾阶段气溶胶粒子的消光系数最小，但雾滴可提供10%~15%的消光贡献，导致能见度与霾阶段相当。

Atmospheric extinction of a persistent fog/haze event in Nanjing during December 2007

Fog and haze occurred alternately for more than 100 hours during the comprehensive field observation conducted in Nanjing during the winter of 2007. Using simultaneous measurements of aerosol particle and fog droplet size distribution, visibility and relative humidity, the extinction of aerosol particles and fog droplets in the process of transformation between fog and haze were investigated based on the Mie theory. The results showed the relationship of visibility in different conditions was fog < wet haze < haze ~ mist. On average, the extinction of fog droplets and aerosol particles was similar in fog condition. Droplets extinction, the dominant factor determining the change of visibility in fog, was highly variable while the extinction of aerosol particles remained relatively constant. The extinction in haze, wet haze and mist was mainly caused by aerosol particles. The low visibility in wet haze was primarily due to the extinction of a large number of wet aerosol particles distributed in the accumulation mode. The number concentration of aerosol particles reached a maximum value in haze, and the nucleation mode particles consisting about 80% of the total number concentration was mainly responsible for the low visibility. In mist, the aerosol extinction was much smaller than those in the other three conditions. As the droplets also contributed 10%~15% to the extinction, the visibility was similar to that in haze.