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 (N_a), the average liquid water content (L_a) and the maximum liquid water content (L_max) 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 PM_2.5 (C_PM2.5) 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 PM_2.5 is more remarkable than PM₁₀. When C_PM2.5 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 C_PM2.5 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.

Using trend coefficient method, vector analysis method, and monitored meteorological data across China, climatic characteristics and spatial pattern of fogs in China were investigated. The results show that most fogs occur in southeastern China. Thin fogs usually occur in fog-rare regions and dense fogs take place in fog-prone regions. The number of annual fog days in most regions of China exhibits a decreasing trend from 1980 to 2010. It also found that the regions with more fog days correspond to the lower concentration degree of fogs, and vice versa. In terms of the national scale, the concentration periods of fogs are mainly in November, December, and January in China. We further classified the occurrence frequencies of fogs into five spatial distribution patterns over a single year according to the spatial distribution characteristics of fogs occurrence frequencies of 36 dekads, namely, a whole year can be correspondingly divided into five phases. Based on this, multi-year average fog-prone regions in the five phases are obtained. Our results also identify the high incidence periods of fogs in different fog-prone regions.