Abstract
From November 2016 to January 2017, there were large-scale dense fog processes in Tianjin area on the west coast of Bohai Bay, China, even strong dense fog with visibility less than 50 m occurred. Based on the observation data of fog droplet spectrum monitor, visibility sensor, environmental particle monitoring equipment and meteorological automatic station, the characteristics of fog droplet size distribution and the interaction between the fog droplets and fine particles during dense fog events were analyzed. The results show following characteristics: (1) The average concentration of fog droplets (Na), the average liquid water content (La) and the maximum liquid water content (Lmax) in the strong dense fog process are larger than those in the dense fog. The average spectrum of fog droplet size distribution conforms to Junge distribution, and they are all broad-spectrum fog with a spectrum width of about 45 μm. The average spectrum is similar to the dense fog of heavily industrialized inland in the world. (2) The maximum of fog droplet diameter during the formation stage have a good indication for the outbreak of strong dense fog. (3) The mass concentration of PM2.5 (CPM2.5) is ranged from 121–375 μg/m3, and the interaction between fog droplets and fine particles is analyzed. During the formation, development and maturity stages, fog process can scavenge atmospheric fine particles, and the scavenging efficiency of PM2.5 is more remarkable than PM10. When CPM2.5 does not exceed 350 μg/m3, the increase in the concentration of fine particles is conducive to the rapid growth of fog droplets and the sharp drop of visibility. However, when CPM2.5 exceeds the critical value, the increase has a negative feedback effect on the development of the fog process. More investigations and cases are necessary to fully assess the mechanisms related to the dense fog events in Tianjin area and further analysis will be done.
Highlights
The evolution from fog to dense fog is very rapid and difficult to predict, the sudden low visibility phenomenon is very easy to lead to sudden safety accidents [1]
Comparing the main chemical composition of fog water and the concentrations of the main aerosols with several coastal and inland cities around the world (Table 3), we found that CPM2.5 or CPM1 are about 200 μg/m3 in heavily industrialized inland area (Nanjing [27,28], Baoding [23,39], Kanpur [40,41]), as well as that in Tianjin
Considering the fog-haze harmful effect to the important port city and the largest open coastal city in North China, field observations of fog droplet size distribution and fine particles are set to improve the ability of fog modification at Tianjin atmospheric boundary layer observation station
Summary
The evolution from fog to dense fog is very rapid and difficult to predict, the sudden low visibility phenomenon is very easy to lead to sudden safety accidents [1]. Fog droplet microphysical parameters, such as number concentration (N), liquid water content (L), diameter (D), fog droplet size distribution and atmospheric fine particle concentration directly affect visibility (Vis) in fog [2] and determine the formation and dissipation of fog. Moroccan scholars improved the skill of coastal fogs by adjusting the microphysics scheme of Casablanca area based on the droplet size distribution analysis [9,10]. Atmospheric fine particles have a complex interaction with the growth of dense fog droplets in recent studies. In order to improve the ability of fog modification, Tianjin Weather Modification Office introduced fog monitor equipment and environmental particle monitoring equipment to carry out microphysical observation experiments in the winter dense fog process in Tianjin, China, for the first time. It is hoped that this experiment can clarify the characteristics of fog droplet generation and the interaction between fine particles and droplet growth during the dense fog in Tianjin in winter, and provide a basis for future fog modification
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