High O 3 episodes in Thailand's Bangkok Metropolitan Regions: sources, spatio-temporal variations and associations with O 3 precursors and meteorological factors

High ozone levels were often observed from December to April at suburban stations in the Bangkok Metropolitan Region (BMR). Wind direction analyses show significant roles of air masses from outside BMR on elevating the BMR O3 levels. These air masses include biomass burning smoke from the northwestern BMR and sea spray from the Gulf of Thailand mixing with industrial emissions from the southern BMR. Seasonal O3 profiles corresponded with NO2 and CO trends and diurnal O3 corresponded with solar radiation. Diurnal peaks of NO2 and CO were results of traffic emissions during traffic rush hours, whereas nighttime NO2 peaks, for northern BMR, attributed to NO-O3 reactions. Both NO2 and CO at 7:00-9:00 highly correlated with the O3 in the following afternoon. Principal component analysis (PCA) and an analysis of non-methane hydrocarbons-to-NO2 ratio suggest the NO2-limited O3 formation in the BMR suburb. The PCA also suggests a negative response of wind speeds on the afternoon O3 formation, whereas roles of ambient temperature on the O3 were seasonally different.