Concentrations of carbon monoxide (CO) in urban and rural air were analyzed from 16 urban roadside locations in the 7 major cities along with 5 background areas in Korea during an 11-year period (1998-2008). Because of noticeable changes in CO levels after 2000, temporal evaluation of its roadside data was carried out by grouping them into period I (1998-2000) and II (2001-2008). The mean CO values for all 16 roadside stations between the two study periods I and II were significantly different from each other (1.67 +/- 0.31 ppm (I) vs. 0.95 +/- 0.17 ppm (II)). This interperiod reduction in CO levels fell, if compared between different stations, in the range of 8.62-59.94% (mean = 39.8 +/- 14.7%). The statistical analysis confirms that CO concentrations decreased very rapidly with the annual reduction rate of 0.093 ppm year(-1) (9.8% year(-1)). In contrast, in background areas such distinctions are no longer valid between the two periods. A line of evidence collected in this study thus suggests that the implementation of legal and technical support (e.g., upgrading of fuel quality and the natural gas vehicle supply program) should have been the effective driving forces leading to the gradual reduction in CO levels in roadside locations (10 out of 16 stations) on the peninsula. (C) 2011 Elsevier B.V. All rights reserved.