1.1.2. State and self-purification capacity of the atmosphere Through the natural history the biosphere has run as a „treatment plant” which proved to be quite efficient. During the last 50-60 years, however, the self-purification capacity of the atmosphere has been upset by the overwhelming pollution load and, as a result, the air quality is getting worse. Nowadays the conservation and improvement of air quality needs special measures to be taken including end-of-pipe but also a more preventative approach, through cleaner technologies etc. The self-purification capacity is based on a few physical and chemical processes. In the ionosphere, by the influence of solar radiation, the ionization and dissociation of gas molecules and particles take place. Hydrocarbons are oxidized rapidly in the ozone layer. The ozone acts as a powerful oxidizer of many anthropic substances. A number of chemical transformations take place in the lower layers of the atmosphere. CO combines with Cl2 at the influence of UV radiations and in the presence of active carbon, forming the phosgene, which reacts with NH3 to give, finally, NH4Cl. The self-purification process is also accelerated by the interaction of atmosphere and hydrosphere. Atmosferic gases dissolve in water. Carbon dioxide, for instance, undergo chemical transformations to form insoluble carbonates; by this process, the ocean absorbs 50% of the technogenic CO2. In Moldova, the Index of Atmospheric Pollution (IAP) is being used to characterize the quality of the atmospheric air. The following pollution classes are in use depending on the IAP values: low (<2.4), moderate (2.4-2.7), medium (2.7-3.0), high (3.0-3.3) and very high (>3.3) pollution level. The main cities in the Republic of Moldova have a medium air pollution level (IAP=2.8-2.9). However, taking into account the empiric data on the air quality in rural areas and values estimated from atmospheric deposition data, the general air pollution level is rather moderate.