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Every spring, biomass burning (BB) occurs extensively in the mountain area of northern Indochina. The plume produced from BB is uplifted to the elevated atmosphere and transported by prevailing westerly from Indochina to East Asia. As the BB plume distributes spatially, it will affect solar radiation budget when mixing with cloud layers during transport and thus resulting in an effect on climate change in the region.
In this study, atmospheric aerosol was observed intensively at Mt. Doi AnKhang (DAK) in Chiang Mai, Thailand from March to April 2015. During the observation period, PM2.5 and PM10 mass levels were averaged at 88.7±36.1 and 112.0±39.0 µg m-3, respectively. The fraction of PM2.5 over PM10 was 79%. The most important PM2.5 component was organic carbon (OC) with 37.2% of the collected PM2.5 mass. The mass fraction of OC3 in OC and EC1-OP in elemental carbon (EC) were 34.2% and 93%, respectively. Therefore, OC3 and EC1-OP can be considered as BB tracers. In addition, 68.9% of OC was water-soluble (WSOC). This implies a change of chemical properties in cloud droplets and the subsequent solar radiation extinction when the transported aerosol entering cloud layers. Levoglucosan is predominant in anhydrosugars in the near-source BB region. The ratio of chemical components of char-EC/soot-EC was found better than OC/EC in assessing BB influence and the degree of burning in this study. Moreover, the ratio of levoglucosan/mannosan was 13.3, which suggested the burning material was soft wood at the DAK site.
During the same observation period, PM2.5 and PM10 mass levels were averaged at 21.1±9.6 and 25.1±10.9 µg m-3, respectively, at Mt. Lulin. The mass fraction of PM2.5 in PM10 was 84.2%. Meanwhile, nss-SO42- and OC were the two most important PM2.5 components with the mass fractions of 14.2% and 24.0%, respectively. Moreover, the mass fraction of OC3 in OC and EC1-OP in EC were 36.5% and 77.0%, respectively. During the non-biomass burning (NBB) period, nss-SO42- was the most important component with the mass fraction of 24.8% in PM2.5. The mass fraction of OC in PM2.5 was 22.8%. OC3 was still the highest component in OC. However, EC2 became the highest component in EC. The changes in major components of PM2.5 and EC indicate the modification of contributing sources in PM2.5 during long-range transport (LRT). In addition, the ratios of PM2.5 components helped to infer that vehicle emissions and coal burning were the two major source types at Mt. Lulin during the NBB period. Since nss-K+, NO3-, levoglucosan, and di-acids and their salts were enhanced at the downwind Mt. Lulin site compared between the BB and NBB periods, they can be considered as tracers for transported BB plume.
By inspecting all PM2.5 component fractions, nss-K+ was found a stable component with the mass fraction of 2% in PM2.5 in the upwind and downwind sites. A Modification Factor (MF) was adopted to determine enhancement or degradation during LRT. The results showed that nss-SO42- was enhanced mostly indicating nss-SO42- was either enhanced by joining the existing nss-SO42- aerosol or converting from its precursor gas in the BB plume during LRT. The degradation of levoglucosan was the greatest which implies its disappearance in the atmosphere for staying a long time.
Given the fact that the quartz fiber filters used in collecting PM2.5 carbonaceous components will adsorb volatile organic compounds. OC1 of carbonaceous components was found interfered mostly in this study. The deviations of interference were significant when PM2.5 mass concentration was lower than 77 µg m-3. Even in the transported BB plume, OC1 showed significant deviation when compared two or three filters in series used in aerosol collection with one filter. In water soluble-inorganic ions, Cl- was the one with greatest volatility during collection. The corrected over non-corrected ratios of Cl- in the near-BB sources and transported BB plume were 3 and 2.2 folds, respectively.
Four source types were resolved from near-source BB aerosol using Positive Matrix Factorization (PMF). Seventy two percent of PM2.5 mass concentrations were associated with BB at Mt. DAK. The results are consistent with the classification of backward trajectory analysis. Nonetheless, anthropogenic and sea salt still show their influences even in the near-source BB area. Six source types were resolved from PMF with 77.8% of PM2.5 mass concentrations with BB origin at Mt. Lulin. It confirms the fact of LRT of BB plume. | en_US |