Sources of atmospheric fine and ultrafine particulate matter and novel PAH diagnostic ratios from northern Thailand forest fires

Abstract

Fires from deciduous dipterocarp forests (DDF) and mixed deciduous forests (MDF) in northern Thailand during haze episodes remarkably affect the ambient fine and ultrafine particulate matter (PM), along with associated polycyclic aromatic hydrocarbon (PAH) concentrations. Average mass concentrations of atmospheric fine and ultrafine particles during haze periods were 2.0–4.8 and 1.7–3.7 times higher than those during normal and transition periods, respectively. Dominant PAHs emitted from forest fires were pyrene (Pyr; 4-rings), benzo[b]fluoranthene (BbF; 5-rings), and benzo[g,h,i]perylene (BghiPe; 6-rings). High molecular weight PAHs constituted a large proportion of total PAHs across all particle sizes. Size distribution of 4-ring PAHs from major vegetation fires in DDF and MDF peaked in the accumulation mode (1.75 μm), whereas 5- and 6-ring PAHs peaked at 0.75 μm. Emission factors were 0.01–0.35 g/kg for PM_0.1, 0.16–1.4 g/kg for PM₁, and 0.24–1.55 g/kg for PM_2.5; for PM-bound PAHs, they were 18–276 μg/kg, 165–874 μg/kg, and 199–989 μg/kg, respectively. A new binary PAH diagnostic ratio—BbF/(BbF + indeno[1,2,3-cd]pyrene (IDP)) = 0.45–0.76 vs IDP/(IDP + BghiPe) = 0.23–0.45—was an effective indicator for PM_0.1, PM₁, and PM_2.5 in fires from DDF and MDF forests. This was verified using a chemical mass balance model and backward trajectory analysis. Results obtained improve understanding of fine and ultrafine particle emission sources from forest fires in upper Southeast Asia.