Concentrations of fine (PM2.5) and coarse (PM10–2.5) particles, whose aerodynamic diameters are less than or equal to 2.5 µm, and greater than 2.5 and less than or equal to 10 µm, respectively, at ambient air monitoring stations in Seoul between 2002 and 2008 were analyzed. Effects of Asian dust are mainly manifested as concentration spikes of PM10–2.5, but were considerable on PM2.5 levels in 2002 when Asian dust storms were the strongest. Excluding the effects of Asian dust, annual average PM2.5 showed a downward trend. Despite a similarity in year-to-year variations, PM10–2.5, mostly affected by fugitive dust emissions, and CO and NO2, primarily affected by motor vehicle emissions, did not show a decrease. PM2.5 along with CO and NO2 had peak concentration during the morning rush hour; the PM10–2.5 peak lagged one hour behind the PM2.5 peak. On high PM2.5 days, PM2.5 peaks occurred two hours later than usual as the effects of secondary formation through photochemical reactions became more important. A test for the spatial variability shows that PM10–2.5, which is known to be greatly influenced by local effects, is lower in its correlation coefficient and higher in its coefficient of divergence (COD, which serves as an indicator for spatial variability) than PM2.5, albeit by only a small difference. The average COD of PM2.5 among monitoring stations was about 0.2 but was lowered to 0.13 when considering high PM2.5 days only, signifying that spatial uniformity increases due to the pervasive influence of photochemical reactions.