A Multi-site Passive Approach for Studying the Emissions and Evolution of Smoke from Prescribed Fires

Abstract. We conducted a two-year study utilizing a network of fixed sites with sampling throughout an extended prescribed burning period to characterize the emissions and evolution of smoke from silvicultural prescribed burning in the southeastern US. The measurement approach and an assessment of instrument performance is described. Smoke sources are identified, and plume ages are determined to quantify emissions and study the evolution of smoke PM_2.5 mass, black carbon (BC), and brown carbon (BrC). Over the 2021 and 2022 prescribed burning seasons (nominally January to May,) we identified 64 smoke events based on high levels of PM_2.5 mass, BC, BrC, and carbon monoxide (CO), of which 61 were linked to a specific burning area. Smoke transport times were estimated using the mean wind speed along with distance between fire and measurement site, and with HYSPLIT back trajectories. PM_2.5 emission ratios based on ΔPM_2.5 mass/ΔCO for fresh smoke (age ≤ 1 hour) ranged between 0.04 and 0.18 µg m^-3 ppb^-1 with a mean of 0.117 µg m^-3 ppb^-1 (median of 0.121 µg m^-3 ppb^-1). Both the mean emission ratio and variability were similar to findings from other prescribed fire studies, but lower than wildfires. Mean emission ratios of BC and BrC were 0.014 µg m^-3 ppb^-1 and 0.442 Mm^-1 ppb^-1 respectively. Ozone enhancements (ΔO₃) were always observed in plumes detected in the afternoon. ΔPM_2.5 mass/ΔCO was observed to increase with plume age in all ozone enhanced plumes suggesting photochemical secondary aerosol formation. In contrast, ΔBrC/ΔCO was not found to vary with plume ages less than 8 hours during photochemically active periods.