A multi-site passive approach to studying the emissions and evolution of smoke from prescribed fires

Abstract

We conducted a 2-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 at a military base in the southeastern USA. The measurement approach and an assessment of the instrument performance are described. Smoke sources, including those within and off the base, are identified, and plume ages are determined to quantify emissions and study the evolution of smoke PM2:5 (particulate matter with aerodynamic diameters 2.5 μm or smaller) 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 PM2:5 mass, BC, BrC, and carbon monoxide (CO), of which 61 were linked to a specific burning area. Smoke transport times were estimated in two ways: using the mean wind speed and the distance between the fire and the measurement site, and from Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back-trajectories. PM2:5 emission ratios based on 1PM2:5 mass =1CO for fresh smoke (age ≤1 h) ranged between 0.04 and 0.18 μgm-3 ppb-1 with a mean of 0.117 μgm-3 ppb-1 (median of 0.121 μgm-3 ppb-1). Both the mean emission ratio and the variability were similar to findings from other prescribed fire studies but were lower than those from wildfires. The mean emission ratios of BC and BrC were 0.014 μgm-3 ppb-1 and 0.442Mm-1 ppb-1, respectively. Ozone enhancements (1O3) were always observed in plumes detected in the afternoon. 1PM2:5 mass =1CO was observed to increase with plume age in all of the ozone-enhanced plumes, suggesting photochemical secondary aerosol formation. In contrast, 1BrC/1CO was not found to vary with plume ages less than 8 h during photochemically active periods.