Background, aim, and scope Investigations on the temporal variabilities of metals generally show that the concentration levels of many harmful metals in air have been continuously decreasing around the world including North America and Western Europe. However, an excursion from such trends has been expected and demonstrated from E. Asia due to the major source contribution from China. To help understand long-term patterns of airborne toxic metals in East Asia, the particle-bound concentrations of Pb, Mn, Ni, Cr, and Cd were analyzed in the two largest cities of Korea, Seoul and Busan, over a 14-year period (1991 through 2004). The results of this study will provide a comprehensive overview on long-term trends of important metals in major urban areas of E. Asia. Materials and methods This study was conducted to investigate the environmental behavior and long-term trends of toxic metals in Seoul and Busan, the two most populated cities of Korea, from 1991 to 2004. To this end, the concentrations of five toxic metals (including Pb, Cd, Cr, Mn, and Ni) in TSP (total suspended particle) fractions were measured by atomic absorption spectrometry. The analysis of long-term patterns of metal concentration data was conducted using two different temporal grouping criteria for both the full study period (1991-2004) and the second half (1998-2004). The statistical significance of such variabilities was assessed through a comparison with relevant reference data from other areas of the world. The annual mean concentration of the metals from the two cities was generally characterized by the lowest Cd (1.7-9.1 ng m-3) and largest Pb (51-341 ng m-3). The relative ordering of metal concentration levels from both cities was consistently found as: Pb > Mn > Ni > Cr > Cd. An evaluation of the absolute concentration levels showed that the values derived for both cities generally fell in an intermediate range between highly polluted (e.g., major Chinese cities) and moderately clean urban environments (Japanese or US sites). Discussion To help describe the long-term patterns of each metal, the metal concentration data between the two cities were compared in a number of respects. The results show strong compatibility with respect to relative trends between Seoul and Busan. When the long-term trends are compared for the entire study period (1991-2004), most metals tend to experience reductions in their concentrations through the years (with an exception of Cr), regardless of the city. The patterns were so distinctive that the statistical significance of this decline, if compared by the magnitude of probability in the regression analysis, generally increased in the order: Mn < Cd < Ni < Pb. A more detailed analysis of the temporal patterns (i.e., the second half of the study period (period II): 1998-2004), however, indicates that some metals reversed their trend to an increasing direction. For instance, Ni and Cr (both cities) and Mn (Seoul) showed an annual rate of change (ARC) in the range of 5.57 to 11.5%. However, others consistently maintained a declining trend with ARC values of -1.55 (Cd at Seoul) to -12.2% (Pb at Busan). Conclusions According to the analysis of the long-term trend of trace metals, the efficiency of emission control appears to be highly complicated, as its effect is visualized to a certain extent. The patterns investigated in the two largest cities in Korea over a decadal period show that the reduction in concentration levels has been the most prominent and consistent for Pb but the least effective for Cr. Comparison among different studies and locations points out that changes in metal concentration levels, whether being an increase or decrease, should be tightly associated with the status of environmental pollution in the targeted area. Recommendations and perspectives Evaluation of long-term monitoring data for toxic metals is helpful to judge various factors involved in regulation efforts and the response of those target pollutants. Future efforts are desirable to develop methods that allow a comparison of measurement data between the particle-filtrated samples and time-integrated environmental archives (bioaccumulation). Such efforts can provide the descriptive basis for explaining the environmental behavior of toxic metals at various time scales.
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