Photochemical smog pollution in the Bangkok Metropolitan Region of Thailand in relation to O3 precursor concentrations and meteorological conditions

Abstract Analysis of photochemical pollution was done using the available 5-yr monitoring data (1996–2000) from 11 monitoring stations in Bangkok and 5 stations in other surrounding provinces, i.e. the Bangkok Metropolitan Region (BMR). Status and trend of O 3 as well as the monthly and diurnal variations were analyzed in relation to the local meteorological conditions as well as the regional transport of pollutants associated with the monsoon. The O 3 in Bangkok was found to be typical for the polluted urban areas with a lower concentration in the city center, especially at curbside stations, and higher concentration at the downwind locations. O 3 pollution was highest in 1997 with the maximum hourly average of 370 ppbv and the total hours exceeding the national hourly O 3 standard (100 ppbv) of 314 h, which is most likely related to the strong El Nino and the forest fire in Southeast Asia in this year. Meteorology-unadjusted trend shows a slight increase in O 3 from 1998 to 2000. Local emission and photochemistry are mainly responsible for O 3 episodes in the BMR. Seasonal fluctuations of O 3 , however, were found to relate to the regional transport associated with the Asian monsoon. Highest O 3 pollution was found in the period from January to April (winter and local summer) and lowest during mid-rainy season, August. The O 3 increase isopleth diagram was constructed which shows that O 3 production in BMR is effective when the NO x /NMHC ratio is in the range of 0.04–0.15 with optimum ratio of around 0.07. Seasonal variations in NO x /NMHC ratios are consistent with the O 3 variations, i.e., optimum in summer (0.07), followed by winter (0.05), and the lowest in rainy season (0.03).