High O<SUB align="right">3 episodes in Thailand's Bangkok Metropolitan Regions: sources, spatio-temporal variations and associations with O<SUB align="right">3 precursors and meteorological factors

Abstract

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 O 3 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 O 3 profiles corresponded with NO 2 and CO trends and diurnal O 3 corresponded with solar radiation. Diurnal peaks of NO 2 and CO were results of traffic emissions during traffic rush hours, whereas nighttime NO 2 peaks, for northern BMR, attributed to NO-O 3 reactions. Both NO 2 and CO at 7:00–9:00 highly correlated with the O 3 in the following afternoon. Principal component analysis (PCA) and an analysis of non-methane hydrocarbons-to-NO 2 ratio suggest the NO 2 -limited O 3 formation in the BMR suburb. The PCA also suggests a negative response of wind speeds on the afternoon O 3 formation, whereas roles of ambient temperature on the O 3 were seasonally different.