The air quality is very important component for all terrestrial organisms and therefore it is a very significant theme for nowadays. The goal of the air quality control is registering and following the changes of the air quality, determining the pollution sources, level of the pollution, moving of the pollution during the year, defining protection measurements and as the final purpose protection of the human health. In this publication there has been given the role to the lichen as an bioindicator and a model which characterizes the ecosystem in the area of park surfaces of Valjevo. Using the index of atmospheric purity (IAP) there has been determined the existence of three different air pollution zones in the places examined: "the zone of the lichen desert", "intermediate zone" (the zone of the battle) and "the normal zone".