Determination of the inter-annual and spatial characteristics of the contribution of long-range transport to SO₂ levels in Seoul between 2001 and 2010 based on conditional potential source contribution function (CPSCF)

In this paper, we introduce a new method to estimate the change in mean mixing ratio of a target species at a receptor site due to the contribution of the long-range transport (CLRT). We applied our method to determine inter-annual and inter-seasonal variations in the CLRT of SO₂ in Seoul, a major megacity in northeast Asia, during the period from 2001 to 2010. The major potential source areas of SO₂ for the 2001-2010 period were located in East China according to the potential source contribution function (PSCF) maps. The CLRT of SO₂ in Seoul was estimated to range from 0.40 to 1.03 ppb, which accounted for 8-21% of the ambient mean SO₂ mixing ratio in Seoul. The inter-annual variations of estimated CLRT of SO₂ was well correlated with those of the total emissions in China during the period of 2001-2008 (R = 0.85). We found that both local emissions from around Seoul and long-range transport from East China, especially the Shandong peninsula, affected the SO₂ mixing ratio in Seoul throughout the decade of study. The CLRT of SO₂ in Seoul increased after 2007 even though the total emissions of SO₂ by China have been decreasing since 2006. The CLRT of SO₂ in Seoul was high in spring and winter, which can be attributed to enhanced SO₂ emissions in East China during these seasons and a dominant westerly wind. The CLRTs of SO₂ accounted for 15, 11, 4, and 12% of the seasonal mean SO₂ mixing ratio in spring, summer, fall, and winter, respectively. The uncertainty ranged from 24 to 62% of the estimated CLRT values.