This study aims to explore trends of NOx (NO + NO2) pollution over ten years (2004–2013) at an urban monitoring station at Yongsan in Seoul, Korea. The mean concentrations (in nmol/mol) of NO, NO2, and O3 measured over the entire study period were 25.3 ± 7.30, 36.9 ± 1.76, and 17.5 ± 1.31, respectively. The decadal trend of these pollutants exhibited statistically significant, but contrasting, results with downward NO and NOx trends and upward O3 trends throughout the study period. Correlation studies and principal component analysis (PCA) explained association of NO and NO2 with traffic related pollutants (CO, PM, and SO) at a statistically significant level while O3 exhibited patterns correlated with meteorological parameters. The overall results of this study indicate that the decadal trend of NO and NO2 was highly dependent on automotive related pollution, while O3 concentration was influenced by both the availability of NO and meteorological conditions. •From the view point of air quality, NO2 is more important than NO due to its greater human health effects.•Further, O3 is an important greenhouse gas making significant contributions to the climate change.•A study has been undertaken to explain the long-term changes in NOx/O3 levels in urban air.•Korea has been actively pursuing policies to reduce pollutant emissions from traffic-related sources.•The decadal trend of NOx was dependent on traffic pollution, while that of O3 on NO and meteorology.