Temporal And Spatial Distribution Of Water-soluble Species Of Particulate Matter In Beijing And Its Surrounding Areas

Water-soluble species are essential components of particulate matter (PM). Observations on water-soluble species of PM and meteorology were carried out before and during the 2007 heating period, before, during and after 2008 Olympiad at four sampling sites (an urban site of Beijing, Tsinghua University; a suburban site of Beijing, Miyun Reservoir; two regional background sites, Baiyangdian, Hebei Province and Shangdu, Inner Mongolia). Based on these data and emission information of major primary atmospheric pollutants, this study firstly characterized the spatial distribution of water-soluble species. And then the temporal variation and the influence of meteorology were discussed under different source emission intensity. Furthermore, the spatio-temporal differences of the relative variations for NO₃^-and SO₄^2- in the sawtooth-shaped (rising slowly and falling rapidly) process of PM associated with key causes were interpeted.Mass concentrations of water-soluble species in Tsinghua and Miyun were higher than those in Baiyangdian and Shangdu during sampling periods. Secondary inorganic species (sulfate plus nitrate and ammonium, SNA) were the major components of water-soluble species in PM (the ratio was 70%⁹0%). Compositions of water-soluble species in PM were similar among Tsinghua, Miyun and Baiyangdian, which distingished these three sites from Shangdu. Such spatial distribution could be attributed to the regioncal differences of source emission and meteorology.Massive changes of source emission intensity in Beijing and its surrounding areas were the dominant factor that caused the great variations of water-soluble species in PM. A series of temporary measures of atmospheric pollutants reduction were carried out in Beijing and its surrounding areas before, during, and after 2008 Olympiad. The reductions of these major atmospheric pollutants were observed on account of the attribution of these measures. And all of the water-soluble ions in PM decreased to a minimum. Central heating measures were the major causes that increased the emission of atmospheric pollutants, which caused the increases of SO₄^2- and NH₄⁺ in Tsinghua.Meteorology could strengthen or weaken the influence of source emission on the variations of PM. Backward trajectory analysis indicated that the slow-moving air masses from the south prior to the Olympiad (7-20⁸-7) was the main reason for two PM heavily polluted processes. In contrast, the quick movement of air masses from the north during the Olympiad was a factor in the improved air quality in Beijing. Further, the increase of the ratio for air masses deriving from the clean areas was an essential factor in decline of mass concentrations of most water-soluble species.Similar to the sawtooth-shaped cycles of SNA in PM, the ratio of NO₃^-/SO₄^2- displayed similar cycles, and spatio-temporal differences of such ratio cycles were observed in Beijing and its surrounding areas. Before and during the heating period, with the increase of the mass concentrations of NO₃^-and SO₄^2- in one sawtooth-shaped process, ratio of NO₃^-/SO₄^2- also went up for Tsinghua, Miyun and Shangdou, which implied that the rising speed of NO₃^- mass concentration was greater than SO₄^2-. In contrast, the rising speed of NO₃^- mass concentration was less than SO₄^2- for Baiyangdian. Before, during and after 2008 Olympiad, with the increase of the mass concentrations of NO₃^-and SO₄^2- in one sawtooth-shaped process, the rising speed of NO₃^- mass concentration was less than SO₄^2- for all sampling sites. The key causes of the spatial and seasonal difference for ratio of NO₃^-/SO₄^2- in one sawtooth-shaped process were probably the spatial difference of the neutralization degree of NH₄⁺ to the sum of NO₃^-and SO₄^2-, and seasonal difference of meteorolgy (Temperature) respectively.