南京冬季持续性强浓雾天气中三级分档雾水的理化特性分析

2013年12月7—9日南京出现持续性强浓雾天气过程。利用观测试验中获取的三级分档雾水和分粒径气溶胶的水溶性离子浓度，气溶胶数浓度谱、雾滴谱以及自动气象站获取的气象要素等资料，通过对比研究浓雾过程中4—16 μm（3级）、16—22 μm（2级）、>22 μm（1级）3个粒径范围雾滴（雾水）与分粒径气溶胶的微物理特征和化学特性，探讨不同粒径气溶胶粒子吸湿增长对雾滴的贡献以及气溶胶离子组分对不同尺度雾滴化学性质的影响。结果表明，在强浓雾天气中，能见度为50 m左右，短波辐射减弱形成稳定逆温层，有助于污染气溶胶的累积并吸湿增长向雾滴转化。雾滴的增加主要集中在小滴范围, 直径在0.5—1 μm的气溶胶粒子对其贡献最大。各粒径段气溶胶中阴离子NO₃-和SO₄2-均表现出较高值，阳离子中Ca2+浓度最高。三档雾水中各阴、阳离子浓度均在4—16 μm小滴雾水中最高，16—22 μm中滴雾水和>22 μm大滴雾水的阴、阳离子浓度没有明显的高低之分。阳离子中Ca2+的浓度在第1级小滴雾水中最高，2级和3级雾水中NH₄+的浓度最高，阴离子中NO₃-和SO₄2-在各级雾水中浓度相差不大。雾水pH值2.7—6.9，呈酸性，小雾滴酸性更强。 

Physical and chemical characteristics of three-stage fog water in deep dense fog during the winter in Nanjing

During the time period of 7-9 December 2013, two deep dense fog events occurred in Nanjing. By using the observational data acquired from a field experiment conducted during this time period, this study investigated the contributions of different suspended aerosols with various sizes to the formation of fog droplets as well as the impacts of the ions in condensation nucleus on the chemical properties of the fog water. The observational data used in the present study consist of the concentrations of the water-soluble ions in three-stage fog droplets, the particle-size aerosols, number distributions of aerosol particles, distributions of the fog droplets, and meteorological factors etc. The micro-physicochemical properties of the three stages fog water (4-16 μm for stage 3, 16-22 μm for stage 2, larger than 22 μm for stage 1) as well as the particle-size aerosols were compared and discussed. The result demonstrates that under the condition of the deep dense fog (visibility≈ 50 m), a stable temperature inversion layer tended to form due to the weakening of the radiation. As a result, the contaminated aerosols would accumulate and transit to fog droplets. Moreover, it was found that the aerosols in 0.5-1 μm contributed the most to the increase in the number of the fog droplets in stage 3. There existed higher concentrations of NO₃- and SO₄2- in the particle-size aerosols and that of Ca2+ was the highest among the cations. In addition, among the fog droplets in these three stages, both of the anions and cations possessed the highest concentrations in the droplets in stage 3. In contrast, the difference in the anion and cation concentrations between stage 1 and stage 2 was negligible. The results also show that the anion and cation in drop size fog water mostly were originated from the suspended aerosols and the chemical reactions. It was also found that the concentration of Ca2+ was the highest among all the cations in stage 1 and NH₄+ was the highest in both stage 2 and stage 3. For the anions, NO₃- and SO₄2- had the highest concentration with little difference. At last, it was observed that the fog was acidic (pH=2.7-6.9), and the fog drops with smaller size had relatively stronger acidity.