Factors regulating the distribution of O3 and NOx at two mountainous sites in Seoul, Korea

The concentrations of ozone (O3) and nitrogen oxides (NOx) were measured at two mountainous sites in Buk-Han (BH) and Gwan-Ak (GA) and at a reference site in Gwang-Jin (GJ), all in Seoul, South Korea, over a four-year period (2010-2013). The mean concentration (ppb) of NOx was comparable at the mountainous sites (22.9 ± 20.7 (BH) and 20.9 ± 16.9 (GA)) but about half the concentration at the reference site in GJ (47.8 ± 36.4). In contrast, the mean value (ppb) for O3 at the three sites exhibited a reversed pattern such as 40.2 ± 19.7 at GA, 28.6 ± 15.4 at BH, and 21.8 ± 18.3 at GJ. The concentration of NOx was higher in winter/fall relative to summer/spring at all three sites. Like the clear distinctions observed from the spatial distribution patterns, the seasonal trends conrasted sharply between NOx and O3. Results of a correlation analysis suggested that the distribution of O3 was significantly affected by meteorological parameters (e.g., TEMP, UV, and solar radiation). According to a back trajectory analysis, the highest O3 levels at both mountainous sites were caused by air masses passing over marine areas, while the effects of ozone precursors on ozone concentration were attributed to air masses originating from China and/or the mainland (S. Korea).