As one of the most dominant pollutants in the air, the pollution of sulfur dioxide can cause significant damage to the surrounding population and environment. The acid rain caused by sulfur dioxide do much more harm to environment than pollution of sulfur dioxide, which is endangering the most regions of south China. In order to assess the role played by forests in the urban ecosystems, the atmospheric SO₂ concentration was measured continually from Jan., 2004 to Ja
n., 2005 in specifically selected forested and non-forested sites of the campus within Changsha and Zhuzhou cities, Hunan Province, representing two types of area, respectively. The prosaniline formaldehyde spectrophotometric method was employed to determine atmospheric SO₂ content. The results showed that the atmospheric SO₂ concentration in both site categories exhibited a significant seasonal fluctuation (p=0.001) with the highest in winter and the lowest in autumn. The seasonal change in SO₂ concentration was closely related to local geographical condition, climatic characteristic, heating period, and distribution of urban industry district. It was found that coal burning was the dominant pollution source, and precipitation, wind speed and direction, and temperature were the dominant factors for affecting atmospheric SO₂ concentration in the cities. In addition, SO₂ concentration level in the atmosphere had a tight relationship with the urban forest cover. The annual mean atmospheric SO₂ concentration was significantly higher in the non-forested sites ((0.18±0.08) mg/m³) than in the forest sites ((0.09±0.07) mg/m³). Tree physiological activities made a contribution in the decrease of the atmospheric SO₂ concentration in urban forest sites. According to the abating effect amount of atmospheric SO₂ concentration in forested sites, it ranked as summer（55.4%）> winter（54.1%）> autumn（49.3%）> spring（29.6%）. Our results demonstrated that urban forest played an important role in controlling and reducing the atmospheric SO₂ concentration in urbanized areas. Therefore, to sustain urban forest resources and increase urban forest canopy cover will be benefit to improve environmental quality, enhance human health, and enrich urban beauty