Suspension of Crustal Materials from Wildfire in Indonesia as Revealed by Pb Isotope Analysis

Southeast Asia experiences recurring wildfire events. Aerosol particles from these wildfires include metal species, which are suggested to have critical influences on the environment and climate. However, the mechanisms for emission of these elements from the wildfires in the region remains unclear. We analyzed both the elemental composition and the isotope ratios of lead (Pb), which is an environmentally important element. Our samples were collected in Singapore as the total suspended particulate (TSP) matter during 2013, especially focusing on the wildfire haze period in June. The Pb isotope ratios for wildfire fuels as well as crustal materials in the region were also analyzed. The Pb isotopic ratios during the haze period (²⁰⁸Pb/²⁰⁷Pb = 2.448 ± 0.013 and ²⁰⁶Pb/²⁰⁷Pb = 1.169 ± 0.012) were significantly (p < 0.05) different from those of the non-haze period (²⁰⁸Pb/²⁰⁷Pb = 2.438 ± 0.006, ²⁰⁶Pb/²⁰⁷Pb = 1.159 ± 0.005). This difference demonstrates that the suspension of the crustal material is the dominant emission source of TSP. Comparison of elemental composition (Al/Pb) with previous studies in the region, which have dominantly been sampled as PM_2.5, suggests that the emission mechanism of particulate matter from wildfire in the region is likely size-dependent. We suggest that the suspension of crustal soil material needs to be included in future emission inventory for wildfire emission.