Chemical speciation of water-soluble ionic components in PM_2.5 derived from peatland fires in Sumatra Island

We conducted a field study to characterize water-soluble ionic species in PM_2.5 from peatland fires using ground-based samplings at fire sources and receptor sites in the Riau Province, Sumatra, Indonesia. We determined the concentrations of PM_2.5 mass, water-soluble ions, and some chemical elements. Through PM_2.5 field samplings at three peatland fire sources, we have shown that the mass fractions of typical peatland fire water-soluble ionic components tend to differ between peatland fire sources. Thus, our results indicate that PM_2.5 source profiles of water-soluble ionic components for peatland fire must be selected with extreme caution if applied to a receptor model. From the viewpoint of ionic composition of each peatland fire sample, Cl⁻ and NH₄ ⁺ were consistently dominant anions and cations, respectively, for all peatland fire samples, i.e., NH₄Cl was a consistently dominant component. Through field samplings of the ambient PM_2.5 in Pekanbaru during peatland fire-induced haze and non-haze periods, we found differences in PM_2.5 mass and total water-soluble ionic component concentrations between haze and non-haze samples. Four components, C₂O₄ ²⁻, NO₃ ⁻, SO₄ ²⁻, and NH₄ ⁺, showed highly elevated levels during haze periods. Since these four ions are recognized as the major secondarily formed aerosol components, the increased total concentrations of water-soluble ionic components during haze periods can mainly be derived from the gas-to-aerosol conversion process. The ionic compositions of haze samples at receptor sites are obviously different from those at peatland fire source samples. In particular, NH₄Cl, which is characteristic of peatland fire PM_2.5 sources, is low at sites during haze periods.