The quality of atmospheric air in the Lobnya Urban District was assessed using active biomonitoring methods based on pollution with heavy metals of the 1st (Cd, Pb, Zn) and 2nd (Cu, Ni) hazard classes. Special air-permeable bags containing Sphagnum girgensohnii Russow samples collected from a pristine (background) area were exposed for three months at 36 sites situated in different functional zones of the Lobnya Urban District. The content of heavy metals was determined by atomic absorption spectrometry. To assess atmospheric air pollution, the Relative Accumulation Coefficient (RAC) and Concentration Factor (CF) were calculated. Data on the accumulation of heavy metals (Pb, Cd, Zn, Cu, Ni) in the exposed moss samples were obtained. Areas with different levels of atmospheric air pollution were identified, and the ecological situation in the Lobnya Urban District was assessed based on air-borne load of heavy metals of the 1st and 2nd hazard classes. The highest CF and RAC values for the metals were observed in traffic zones (Starodmitrovskoye Highway, intersection of Rogachevskoye and Krasnopolyanskoye Highways, intersectrion of Rogachevskoye and Bukinskoye Highways, Krasnopolyanskoye Highway, and intersection of Krasnopolyanskoye and Lobnenskoye Highways) and industrial zones (an elevator manufacturing plant and a metal fabrication facility). Background levels of heavy metals were exceeded in residential and recreational zones near roads and railway lines. The study demonstrated the high performance of the “moss bag” technique of active biomonitoring. The results can help the city administration make informed urban management decisions.