The article studies the problem of assessing the quality of atmospheric air in the green areas of the metropolis. The aim of the work is to compare the data obtained by the biomonitoring method with information from unified state environmental monitoring system (USEMS). The sample object of the study is a Udelny park located in the Primorsky district of the city of St. Petersburg. The objectives of the study include the analysis of atmospheric pollution of the Udelny Park by the method of bioindication, comparison of the data obtained with the general telephone indicators, as well as with information from the nearest USEMS posts. When studying the object, the method of V.M. Zakharov was used to study the fluctuating asymmetry of the leaf plates of the hanging birch (Betula pendula Roth.). The site between Udelny Ave., the Vyborg railway line, Testers Ave. and Kolomyazhsky Ave. with an approximate area of 18 hectares was chosen as a model site. The assessment was carried out on 12 pickets, at each 100 samples of leaf plates were taken in total. For comparison, data were obtained from a stationary observation post for the state of atmospheric air pollution, information on the content of pollutants in the air of the Primorsky district. The study found that the quality of atmospheric air in the territory is low. In part of the pickets, the value of the fluctuating asymmetry index was more than 0.054, which corresponds to the V score (critical pollution). These data are generally correlated with information about air pollution in St. Petersburg. Nevertheless, the state of the environment is assessed by the chosen method more critically. This is due to the fact that the biomonitoring method studies pollution from the point of view of direct impact on the ecosystem, taking into account, among other things, the cumulative effect. The result makes us to conclude that it is more correct to use an integrated approach, combining both methods, to assess the stability of urban ecosystems and the quality of the environment in them. It is important to separate information about the quality of the atmosphere in green areas from general data on residential areas.