A dramatic shift in CO and CH₄ levels at urban locations in Korea after the implementation of the Natural Gas Vehicle Supply (NGVS) program

Concentrations of carbon monoxide (CO) and methane (CH₄) in air were measured at both urban roadside (U-RS) and urban background (U-BG) stations in Seoul, Korea over an 11 yr period (1996-2006). The overall mean values of CO were 1.16±0.63 (U-RS) and 1.08±0.77 ppm (U-BG), while those of CH₄ were 2.24±0.42 (U-RS) and 2.06±0.31 ppm (U-BG). The diurnal patterns of CO tended to peak near rush hour, while those of CH₄ showed increases at night. An examination of the seasonal data showed that the CO values were consistently higher during winter, while CH₄ values were highly variable across seasons with relatively large spatial variations. Because of the noticeable change in air quality parameters after the year 2000, the mean data for both compounds were examined between 1996-2000 (period I) and 2001-2006 (period II). The analysis of long-term trends revealed that the concentrations of both compounds decreased very rapidly during period I, while changes were not significant during period II. The results of this comparative study confirm that both urban locations have experienced dramatic changes in the major pollutant levels, particularly in CO after the implementation of the Natural Gas Vehicle Supply (NGVS) program.