Water in the atmosphere exists as gas, liquid and solid. Neither liquid nor solid water are free of impurities. The droplets/ice particles suspended in the gas represent a multi-component system with dissolved (captured) impurities present in molecular, ionic or radical forms. Ice particles can incorporate the gas as a clathrate—solid hydrates of H2SO4, HNO3, HCl. All water-based particles in the atmosphere can be considered as mini-chemical reactors (surface and bulk). These are interchanging with surrounding gas, being involved in the processes of absorption or adsorption of stable and reactive constituents. Thermodynamic and kinetic approaches are needed therefore to describe in detail the basic properties and behavior of atmospheric water caused by emission of man-made pollutants. Despite low water volume content L = Vliq/Vgas, the liquid represents an ideal reaction medium for occurrence of chemical transformations because of the large specific surface (per unit of volume of the gas). The L varies over a wide range, from 10-13 in the stratosphere to approximately 10-6 in the troposphere. Junge was the first to declare, in the early 1960s, the potential importance of aqueous phase oxidation of SO2 within droplets in atmospheric clouds or fogs. The known dramatic drop in stratospheric ozone over the Antarctic in springtime (the ‘ozone hole’) has been attributed to the chemical reactions involving chlorine and bromine reservoirs gases such as HCl, ClNO3, HBr etc which proceed in water-based aerosols (ice particles made up from water or trihydrate of nitric acid, ternary solutions H2SO4/HNO3/ H2O, etc). These reactions are a source of molecular halogens (chlorine, bromine), whose dissociation in sunlight starts the catalytic depletion of stratospheric ozone (halogen activation). Today there are grounds to suspect that the background sulfate aerosol layer (The Junge Layer) is responsible for global ozone depletion also via halogen activation of low stratosphere. After carefully accounting for all of the known natural variations, a net decrease of about 3% per decade for the period 1978 to 1991 was found. This is a global average over latitudes from 66oS to 66oN (i.e. the arctic and Antarctic are excluded in calculating the average). Today there are no doubts that tropospheric clouds also facilitate conversion of a large number of inorganic and organic substances emitted to the atmosphere. For instance, sulfur dioxide and probably nitrogen oxides behave similarly; from 50 to 80% of SO2 in the troposphere is converted by aqueous-phase chemical reactions. The dissolved sulfur dioxide (SO2(aq)) is not a single component in the droplet phase. It is easily hydrolyzed in aqueous media, thereby giving a group of three species (S(IV) species family), namely HSO3 - and SO3 2-. The HSO3 - ions are easily oxidized by so-called strong oxidants such as O3(aq) and H2O2(aq). In process of acidification of the droplets (pushing the equilibrium SO2(aq) ⇔ HSO3(aq) - + H+ to the left) caused by S(IV) oxidation by these oxidants, the former reaction is slowed down (self-inhibited) because the reactivity of HSO3 - with respect to O3(aq) exceeds that for SO2(aq). Conversely the reaction of S(IV) with hydrogen peroxide related to so-called autocatalytic reactions, accelerates on acidification of the droplets. The HSO3 - ions are also oxidized by dissolved molecular oxygen, as an oxidant. The process is initiated by uptake of OH radicals from the gas. These captured hydroxyl radicals (OHaq) with diffusion-controlled rate react with HSO3 - yielding to SO3-5(aq) family radicals. The reaction mechanism is a chain oxidation of S(IV) of short chain length because: 1) relatively low reactivity of SO5(aq) - towards HSO3(aq) -, and 2) low concentration of HSO3(aq) - in the droplet phase..