Significant Contribution of C3 - Type Forest Plants’ Burning to Airborne PM2.5 Pollutions in Chiang Mai Province, Northern Thailand

This study aimed to identify the contributing sources of PM2.5 in Chiang Mai Province during February to April 2016. We therefore investigated the chemical compositions of PM2.5 at two different sites. An urban site is in Chiang Mai University (CMU) while a rural site is in Mae Chaem (MC) District where frequent intensive biomass burning was reported. Thirty pair samples of 24-h PM2.5 were analyzed for organic carbon (OC), elemental carbon (EC), levoglucosan and stable carbon isotope (δ13C). The mean concentrations (Mean ± SD) of PM2.5, OC and EC at the CMU vs MC sites were not significant different (P >0.05) including 44.5 ± 32.1 vs 40.5 ± 21.2 µg/m3; 14.9 ± 12.5 vs 14.8 ± 10.0 µg/m3; and 1.80 ± 1.60 vs 1.62 ± 0.80 µg/m3, respectively. Levoglucosan concentrations, a tracer of biomass burning from both sites were not significant different (P >0.05) and the mean ± SD concentrations at CMU vs MC sites were 0.46 ± 0.56 µg/m3 vs 0.55 ± 0.67 µg/m3, respectively. Meanwhile, the mean values of δ13C in total carbon (TC) at CMU vs MC sites were -27.9 ± 0.68 vs -27.6 ± 0.60‰, respectively which major data (n = 48, 85.4%) fell within the ranged of C3-type plants and minor data (n = 48, 14.6%) in C3-type plants and motor vehicle sources. This finding corresponds to the vast biomass burning area from satellite data. Forest plants in northern Thailand, Chiang Mai particular are mostly mixed deciduous forest i.e. C3-type plants which falling leaves in dry season and easily causing fire. The results of this study therefore strongly suggest that the burning of C3-type forest plants attribute to airborne PM2.5 pollutants in Chiang Mai Province. Keywords: Air pollution, Biomass burning, C3-type plants, Levoglucosan, Stable carbon isotope