Metal Concentrations and Source Apportionment of PM2.5 in Chiang Rai and Bangkok, Thailand during a Biomass Burning Season

One of the persistent environmental problems in the provinces of northern Thailand is severe air pollution during the dry season because of open vegetation burning by farmers for land clearance purpose. Aerosol optical depth and Angstrom exponent data from MODIS-Terra satellite indicated that from mid-March to April, 2019, entire Thailand was covered with a high concentration of fine-sized aerosols. Trace metal concentrations of PM2.5 collected from Chiang Rai in northern Thailand and Bangkok in southern Thailand between January and April 2019 were analyzed. Average concentrations of crustal metals such as Al, Ca, and Fe are higher in Chiang Rai compared to that in Bangkok. The Fe/Al ratio in Chiang Rai decreases from 1.65 during the onset of haze to 0.87 during the peak haze approaching a crustal ratio of 0.48. In contrast, Bangkok has higher Na, Mg, and Zn with an average Na/Mg ratio of 6.07 indicative of a sea spray (Na/Mg similar to 8) origin. Principal component analysis identifies three possible sources in Chiang Rai: (1) crustal dust and biomass burning, (2) industrial source, and (3) refuse incineration mixed with road dust; and for Bangkok (1) natural background, industrial emissions, and coal combustion, (2) traffic emission, and (3) sea spray. The ranges of Pb isotope ratios in the bulk fraction of PM2.5 in Chiang Rai (Pb-206/Pb-207 = 1.1445-1.1657 and Pb-208/Pb-207 = 2.4244-2.4468) and Bangkok (Pb-206/Pb-207 = 1.1343-1.1685 and Pb-208/Pb-207 = 2.4138-2.4450) are not significantly different. However, in a time series plot, Pb-206/Pb-207 ratios in Chiang Rai follow PM2.5 during the peak burning season and correlate well with the Al/Pb (r(2) = 0.61) ratios, indicating that at least part of the Pb is derived from crustal dust during peak fire. Using a binary mixing model, the most radiogenic Pb isotopes in Chiang Rai during the peak haze can be explained by similar to 5 to 30% mixing of crustal dust with similar to 35-40% biomass burning generated aerosols with the background. From the trace metal systematics and Pb isotope ratios, it is evident that (1) during the biomass burning season, trace metals from Chiang Rai are not transported down south to Bangkok and (2) in addition to metals released from biomass burning, the raging fire remobilizes crustal dust that forms an important source of Pb and other trace metals in the Chiang Rai aerosol.