LAUSR

Abstract. The analysis of gaseous criteria pollutants in the Bangkok Metropolitan Region (BMR), Thailand, from 2010 to 2014 reveals that while the hourly concentrations of CO, SO2 and NO2 were mostly within the National Ambient Air Quality Standards (NAAQs) of Thailand, the hourly concentrations of O3 frequently exceeded the standard. The results reveal that the problem of high O3 concentration continuously persisted in this area. The O3 photolytic rate constant ( j1 ) for BMR calculated based on assuming a photostationary state ranged from 0.008 to 0.013 s −1 , which is similar to the calculated j1 using the NCAR TUV model ( 0.021±0.0024  s −1 ). Interconversion between O3 , NO and NO2 indicates that crossover points between the species occur when the concentration of NOx ( = NO + NO 2 ) is ∼60  ppb. Under a low- NOx regime ([ NOx ] < 60 ppb), O3 is the dominant species, while, under a high- NOx regime ([ NOx ] > 60 ppb), NO dominates. Linear regression analysis between the concentrations of Ox ( = O3 + NO2 ) and NOx provides the role of local and regional contributions to Ox . During O3 episodes ([ O3 ] hourly > 100 ppb), the values of the local and regional contributions were nearly double of those during non-episodes. Ratio analysis suggests that the major contributors of primary pollutants over BMR are mobile sources. The air quality index (AQI) for BMR was predominantly good to moderate; however, unhealthy O3 categories were observed during episode conditions in the region.