Gaseous composition of the atmosphere is an important factor determining the radiation balance on the Earth's surface and the Earth's climate. It is monitored by different means including a ramified ground-based observational network. Observations of the pollutants playing a leading role in climate formation were carried out at the stations Zvenigorod, Kislovodsk, and Issyk-Kul' of the former USSR. Data of long standing observations of the O3, NO2, CO, CH4, CO2, and H2O contents in the atmosphere and atmospheric transparency are presented and analyzed in the paper. Main peculiarities of their long-term variations have been established. Evaluated trends of CO, CH4, and CO2 variations are toward the steady increase of their contents in the atmosphere. The contributions of various factors determining the variation of the atmospheric composition are examined including anthropogenic and natural sources and sinks of pollution.