Seasonal Source Apportionment of Volatile Organic Compounds in Bangkok Ambient Air

Abstract

Volatile organic compounds (VOCs) are emitted from various sources, both anthropogenic and biogenic. They are important precursors in photochemical reactions, and the formation of secondary aerosols. Therefore in controlling ozone concentration, sources of precursors need to be defined. Although ozone precursors are both NOx and VOCs, the study from Pollution Control Department concluded that ozone episodes in Bangkok are VOC-limiting. However, uncertainties in emission inventory are greater for VOCs than the criteria pollutants such as TSP, SO2 and NOx. The diversity of VOCs sources includes vehicles, industry, solvent-containing products and biological processes. An estimation of volatilization is very difficult due to uncertainties in estimating solvent use, temperature and manner of applications. In this study the U.S. EPA receptor model, CMB7 was used to complement the emission inventory by identifying contribution from various sources to ambient VOC concentration. The receptor model methodology requires investigation of concentration of VOCs at receptor (ambient), and composition of VOCs at sources, which are then input to a statistical model. Ambient air samplings took place at four PCD air monitoring stations: (1) Din Daeng, in central of Bangkok with heavy traffic, (2) Rajabhat University at Chandrakasem, a commercial and residential area north of Bangkok, (3) Rajabhat University at Ban Somdej Chao Praya, a commercial and residential area south of Bangkok, (4) Ratburana Post Office, an industrial area south of Bangkok. The air samplings were conducted for eight months during July 2003 to February 2004 covering the two prevailing wind directions in Thailand, the southwest and northeast monsoon seasons. The air samples were collected in the morning between 8:00 am – 12:00 pm every 6 days at each station. These air samples were collected on sorbent tubes, Chromosorp 106, by pump and analyzed by the thermal desorption-gas chromatography/mass spectrometer, which speciated the samples into 18 VOC species. VOC emission source profiles included in the study were exhaust gases from tailpipes of gasoline vehicles, exhaust gases from tailpipes of diesel vehicles, gasoline vapors, flue gas from fuel oil boilers, vapors of solvent-based paints and liquid thinners, smoke from burning biomass, smoke from food barbequing on charcoal stoves, and air samples from municipal waste disposal sites. The results from CMB receptor modeling (R2 = 0.95-1.00) showed that during the southwest monsoon season the average contribution to the ambient VOC concentration for all stations were from the exhaust gas from tailpipes of gasoline vehicles 21%, the exhaust gas from tailpipes of diesel vehicles 5%, the vapor of gasoline 12%, flue gas from fuel oil boilers 22%, the vapor of solvent-based paint and thinner 8%, smoke from biomass burning 19%, smoke from food barbequing 2%, air samples from municipal waste disposal 4% and unexplained sources 7%. During the northeast monsoon season the average contribution to the ambient VOC concentration for all stations were from the exhaust gas from tailpipes of gasoline vehicles 50%, the exhaust gas from tailpipes of diesel vehicles 6%, the vapor of gasoline 12%, flue gas from fuel oil boilers 2%, the vapor of solvent-based paint and thinner 3%, smoke from food barbequing 5%, air samples from municipal waste disposal 12% and unexplained sources 10%.