Abstract
Abstract. We report trace-gas emission factors from three pine-understory prescribed fires in South Carolina, US measured during the fall of 2011. The fires were more intense than many prescribed burns because the fuels included mature pine stands not subjected to prescribed fire in decades that were lit following an extended drought. Emission factors were measured with a fixed open-path Fourier transform infrared (OP-FTIR) system that was deployed on the fire control lines. We compare these emission factors to those measured with a roving, point sampling, land-based FTIR and an airborne FTIR deployed on the same fires. We also compare to emission factors measured by a similar OP-FTIR system deployed on savanna fires in Africa. The data suggest that the method used to sample smoke can strongly influence the relative abundance of the emissions that are observed. The majority of fire emissions were lofted in the convection column and were sampled by the airborne FTIR. The roving, ground-based, point sampling FTIR measured the contribution of individual residual smoldering combustion fuel elements scattered throughout the burn site. The OP-FTIR provided a ~ 30 m path-integrated sample of emissions transported to the fixed path via complex ground-level circulation. The OP-FTIR typically probed two distinct combustion regimes, "flaming-like" (immediately after adjacent ignition and before the adjacent plume achieved significant vertical development) and "smoldering-like." These two regimes are denoted "early" and "late", respectively. The path-integrated sample of the ground-level smoke layer adjacent to the fire from the OP-FTIR provided our best estimate of fire-line exposure to smoke for wildland fire personnel. We provide a table of estimated fire-line exposures for numerous known air toxics based on synthesizing results from several studies. Our data suggest that peak exposures are more likely to challenge permissible exposure limits for wildland fire personnel than shift-average (8 h) exposures.
Highlights
Biomass burning is a significant, global source of trace gases and particles that impact the chemical composition and radiative balance of the atmosphere (Crutzen and Andreae, 1990)
The open-path Fourier transform infrared (OP-FTIR) successfully sampled smoke generated by ignition activities near the measurement path; post-ignition, wind-blown smoke from the wake of the receding, local flame front; occasional smoke from more distant combustion; and any upwind residual smoldering combustion emissions that were directed through the open path
Our expectation before the experiment was that both ground-based FTIRs would observe much lower modified combustion efficiency (MCE) than the Airborne FTIR (AFTIR) and that the OP-FTIR data would help us weight the relative contribution of the different smoldering point sources sampled by the LAnd-based FTIR (LAFTIR) to the overall ground-level smoke layer
Summary
Biomass burning is a significant, global source of trace gases and particles that impact the chemical composition and radiative balance of the atmosphere (Crutzen and Andreae, 1990). Akagi et al.: Field measurements of trace gases emitted by prescribed fires in southeastern US occurs when the majority of the smoke is produced by flaming combustion, lofted via convection, and directed away from major population centers. This requires that fuel conditions, boundary layer depth, wind speed, and wind direction are within specific limits. We combine the OP-FTIR mixing ratio measurements on the fire-line with results from the other DoD studies to generate a preliminary assessment of fire-line exposure to air toxins
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