Some results of experimental investigation of photochemical processes in air over industrial centers of the Far East of the USSR are presented. It is shown that almost permanent presence of two temperature inversions (near–ground and elevated ones) in the atmosphere over this region results in formation of two layers of polluttant over the cities. The first layer accumulates atmospheric emissions from low–altitude sources and the second –– from high–altitude sources. These layers do not mix. The photochemical change of the primary emissions into the secondary products takes place in the upper layer within the 200–400 m altitudes where enhanced content of photochemically active gases takes place. There are no photochemical processes in the near–ground layer because the elevated layer of polluttants screens the UV radiation. However, the secondary products have been detected near the ground. They come here from the upper level due to sedimentation at night. It was noted that over the cities the secondary products of the near–ground layer are rapidly destructed, while outside the cities they live for a longer time. This is caused by the destruction of the secondary products by the atmospheric aerosol especially by its photochemical fraction. It is also shown that after the change of air masses the photochemical equilibrium is reached in a day.