Lin'an Atmospheric Background Station, built in 1983, is one of the first three regional background stations built by the China Meteorological Administration. The station is located in the center of the Yangtze Delta Economic Circle, representing the background change of atmospheric composition in the Yangtze Delta Economic Circle. At present, the observation projects include greenhouse gas bottle sampling, atmospheric aerosols, reactive trace gases, precipitation chemistry, solar radiation, and ground meteorological observations, which basically cover the main items and elements of current atmospheric background composition observation. The observational data can well reflect the background change of atmospheric composition in the Yangtze Delta.