Abstract
After global phasing out of leaded gasoline, anthropogenic sources of atmospheric lead (Pb) are dominated by coal combustion emissions, high-temperature metallurgical processes, and vehicle exhausts. Thailand was one of the first countries in Southeast Asia to completely phase out leaded gasoline by the year 1994. This study investigates the sources of atmospheric Pb in Thai aerosols, a quarter-century after phasing out of leaded gasoline using a multiproxy approach, Pb isotopes in conjunction with trace metal composition of PM2.5. Aerosol samples were collected for 1 year from January 2018 to April 2019 from Bangkok and Chonburi to understand the influence of seasonal variation on aerosol chemistry. Bangkok is the only megacity in Thailand and notorious for traffic congestion and Chonburi is a coastal town, 80 km southeast of Bangkok where the inter monsoon winds make landfall. Aerosol Pb concentrations significantly decreased in 25 years from 74 +/- 5 ng/m(3) in 1994-1995 to 14 +/- 13 ng/m(3) measured in Bangkok during Northeast (NE) monsoon winds. Concentrations of all the metals are higher in Chonburi compared to Bangkok in the NE monsoon wind, owing to the addition of local emissions from the megacity over the long-range transported metals before the winds reach the coastal town. The Pb isotope ratios have a wide range (Pb-206/Pb-207 range 1.1343-1.1685 and Pb-208/Pb-207 range 2.4138-2.4450) but the average ratios are not significantly different in Bangkok and Chonburi for all three seasons. However, present day Pb-206/Pb-207 ratios are more radiogenic than those measured 25 years ago in Bangkok. The Pb isotopes from both locations in all seasons have considerable overlap with unleaded gasoline and diesel used in Southeast Asia indicating automobile exhaust as an important source. Additionally, the NE monsoon winds are influenced by mixing of crustal dust (Al/Pb ratios correlates well with Pb-206/Pb-207, r(2) = 0.63) with coal combustion emission (strong inter-correlation of Pb, Zn, As and Cd, r(2) from 0.72 to 0.94) from China and Vietnam. Pb isotopes of the Southwest (SW) monsoon winds are also influenced by crustal dust and falls on the mixing line between coal combustion and ore processing emission from India in a Pb triple isotope plot. Chemical composition of aerosols in the inter-monsoon winds are similar to SW monsoon winds. After a quarter-century of leaded gasoline banning in Thailand, the atmospheric Pb sources have changed from less radiogenic leaded gasoline exhaust and Chinese ore processing to more radiogenic unleaded gasoline and diesel exhaust and coal combustion.
Keywords
Lead (Pb) in Thai aerosol, Pb isotope ratios, PM2.5, Trace metals, Unleaded gasoline