海雾过程中大气气溶胶谱变化及消光作用

Relationship of Aerosol Size Distribution and Visibility in a Sea Fog

海雾是沿海地区的常见天气,大雾致灾的现象时有发生,然而对海雾微物理过程的研究相对匮乏。本文以北方的1次海雾过程为例,分析在雾的生成、维持和加强阶段气溶胶谱的变化、消光作用和对能见度的影响。发现暖湿气流影响下,空气比湿持续增加是1μm以上粗粒子数浓度增加的主要原因,比湿持续增加的速度越快,粒子数浓增长的幅度也越大,对能见度的影响也越大。反之,如果比湿持续下降,12.5μm粒子数浓度就会减小,能见度增加。粒子直径的增长速率在10-5（μm/s）量级以内。低能见度出现的时间与15μm粒子浓度相关性最好,低能见度发生的时间基本上与15μm粒子浓度开始增加的时间相一致。在雾的维持阶段小粒子通过增长被消耗,而较大粒子被雾滴清除。在雾的加强阶段,大粒子的增长速度超过了雾滴的清除。雾过程中以大于0.5μm粒子的消光作用为主,消光系数变化与能见度变化有很好的反相关关系,并且粒径越大,反相关关系越好。

Sea fog occurs frequently in China coastal area and causes disaster ever and agah.However research on microphysics of sea fog is still few.A summer advection sea fog was studied to investigate the variation of aerosol size distribution,its induced extinction and influence on visibility during the onset,maintaince and strengthen stage of the sea fog.It was found that ithe continuous increase of specific humidity may cause the increase of number concentration of coarse mode particles larger than 1μm in diameter and in the boundary layer with steady warm and wet onshore wind.The faster the specific humidity increases continuously,the higher the number concentrations increase and the stronger the visibility is impacted.On the contrary,if the specific humidity decreased continuously,the number concentrations of the particles in 12.5 μm would reduce accordingly,and the visibility would become better.The growth rate of particle radius is within 10-5（μm/s）magnitude.The visibility variation is very sensitive to the change of the number concentration of particles with aerodynamic diameter ranged in 15 μm;especially,an inverse correlation was identified between the visibility and the number concentration of the particles in 12.5 μm.In the maintaince stage of the fog,smaller aerosol particles were depleted through growth,and larger particles were deposited by fog droplet.In the strengthen stage of the fog,the growth of larger particles exceeded the remove of fog droplet.The extinction was mainly contributed by particles larger than 0.5μm in the fog process.There is a good reverse relationship between the extinction coefficient and visibility.Larger is the particle size,better is the reverse relationship.