Chemical speciation of PM_2.5 collected during prescribed fires of the coconino national forest near flagstaff, arizona

The use of prescribed fire is expected to increase in an effort to reduce the risk of catastrophic fire, particularly at urban/forest interfaces. Fire is a well-known source of particulate matter (PM) with particle sizes ≤2.5 μm (PM_2.5), small diameter PM known to affect climate, visibility, and human health. In this work, PM_2.5 was collected during seven first-entry burns (flaming and smoldering stages) and one maintenance burn of the Coconino National Forest. Samples were analyzed for organic and elemental carbon, cations (sodium, potassium [K⁺], and ammonium [NH₄⁺]), anions (nitrate [NO₃⁻] and sulfate), and 48 elements (with atomic weights between sodium and lead). The PM_2.5 contained high organic carbon levels (typically >90% by mass), commonly observed ions (K⁺, NH₄⁺, and NO₃⁻) and elements (K⁺, chlorine, sulfur, and silicon), as well as titanium and chromium. Flaming produced higher K⁺ and NH₄⁺ levels than smoldering, and the elemental signature was more complex (20 versus 7 elements). Average organic carbon × 1.4 mass fractions (± standard deviation) were lower during flaming (92 ± 14%) than during smoldering (124 ± 24%). The maintenance (grassland) burn produced lower particle concentrations, lower NH₄⁺ and NO₃⁻ levels, and higher K and chlorine levels than did the first-entry fires.