The scientific community that includes meteorologists, physical scientists, engineers, medical doctors, biologists, and environmentalists has shown interest in a better understanding of fog for years because of its effects on, directly or indirectly, the daily life of human beings. The total economic losses associated with the impact of the presence of fog on aviation, marine and land transportation can be comparable to those of tornadoes or, in some cases, winter storms and hurricanes. The number of articles including the word ''fog'' in Journals of American Meteorological Society alone was found to be about 4700, indicating that there is substantial interest in this subject. In spite of this extensive body of work, our ability to accurately forecast/nowcast fog remains limited due to our incomplete understanding of the fog processes over various time and space scales. Fog processes involve droplet microphysics, aerosol chemistry, radiation, turbulence, large/small-scale dynamics, and surface conditions (e.g., partaining to the presence of ice, snow, liquid, plants, and various types of soil). This review paper summarizes past achievements related to the understanding of fog formation, development and decay, and in this respect, the analysis of observations and the development of forecasting models and remote sensing methods are discussed in detail. Finally, future perspectives for fog-related research are highlighted.

[1] A numerical one-dimensional model of the marine boundary layer (MBL) is presented. It includes chemical reactions in the gas phase and aerosol particles, focusing on the reaction cycles of halogen compounds. Results of earlier box model studies were confirmed. They showed the acid-catalyzed activation of bromine from sea salt aerosol, and the role of halogen radicals in the destruction of O 3 . A distinct diurnal variation in BrO mixing ratios with maxima at sunrise and sunset was found which might be the cause of the recently published ''sunrise ozone destruction.'' Maxima of BrO and sea salt acidity are predicted at the top of the MBL and not close to the sea surface where sea salt spray is produced. The presence of sulfate aerosol was found to be important for the recycling of less reactive to photolyzable bromine species. Day/night and seasonal differences in halogen chemistry are shown.

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[1] A companion paper presented a numerical one-dimensional model of the marine boundary layer (MBL) including chemical reactions in the gas and aqueous phase, focusing on the reaction cycles of halogen compounds. In this paper we study interactions between halogen and sulfur chemistry. HOCl and HOBr were found to be generally more important than H 2 O 2 or O 3 in the oxidation of S(IV) in sea salt aerosols in the cloud-free MBL. The inclusion of halogen chemistry lead to an increase in the oxidation of DMS of roughly 63%. This additional oxidation is caused by BrO. The model was also expanded for the study of the cloudy MBL. We found that the effects of stratiform clouds on the evolution and diurnal cycle of halogen species are widespread; they are not restricted to cloud layers. The diurnal variation of gas and aqueous phase bromine was the opposite of that in cloud-free runs. Oxidation of S(IV) by HOBr and HOCl was important for cloud droplets, too. However, the relative importance of these oxidants changed compared to the cloud-free runs.

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