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 + NO2) 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.