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

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 O3 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 O3 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. O3 pollution was highest in 1997 with the maximum hourly average of 370 ppbv and the total hours exceeding the national hourly O3 standard (100 ppbv) of 314 h, which is most likely related to the strong El Niño and the forest fire in Southeast Asia in this year. Meteorology-unadjusted trend shows a slight increase in O3 from 1998 to 2000. Local emission and photochemistry are mainly responsible for O3 episodes in the BMR. Seasonal fluctuations of O3, however, were found to relate to the regional transport associated with the Asian monsoon. Highest O3 pollution was found in the period from January to April (winter and local summer) and lowest during mid-rainy season, August. The O3 increase isopleth diagram was constructed which shows that O3 production in BMR is effective when the NOx/NMHC ratio is in the range of 0.04–0.15 with optimum ratio of around 0.07. Seasonal variations in NOx/NMHC ratios are consistent with the O3 variations, i.e., optimum in summer (0.07), followed by winter (0.05), and the lowest in rainy season (0.03).