Formation of Ozone and PM2.5 in Smoke from Prescribed Burning in the Southeastern United States

Abstract

Ground-based measurements of smoke from prescribed fires in the southeastern US during the burning seasons of 2021 through 2024 are used to assess ozone (O₃) and PM_2.5 mass formation and their changes with age in 69 smoke events. O₃ production occurred in nearly all plumes (31 out of 32) measured between 12:00 and 18:00. The O₃ to carbon monoxide ratio (ΔO₃/ΔCO) increased from 5.32 to 143 ppb ppm^–1 as plumes aged from 1 to 480 min, showing a rapid initial increase, doubling in approximately 60 min, followed by a gradual slow down. Residual O₃ from daytime fires was detected during the evening and night, disrupting the typical nighttime O₃ pattern. The ΔPM_2.5 mass/ΔCO ratio ranged from 40.6 to 466 μg m^–3 ppm^–1. Little age-related change was observed in smoke measured at night with ΔPM_2.5 mass/ΔCO levels similar to those observed at the time of emissions (132 μg m^–3 ppm^–1). However, in plumes of observed O₃ enhancement during photochemically active periods (12:00–18:00), ΔPM_2.5 mass/ΔCO followed a similar increasing trend as ΔO₃/ΔCO, and the two were correlated (r² = 0.5), although the O₃ trend was more pronounced. For these data, a stronger correlation between ΔPM_2.5 mass/ΔCO and age was found in plumes of higher PM_2.5 concentration (PM_2.5 mass > 35 μg m^–3). The impact of the prescribed burning season in the southeast was evident in state-operated air quality monitors near burning areas, where PM_2.5 mass concentrations were 25–30% higher than nonburning seasons. In contrast, changes in daily maximum 8 h O₃ concentrations were less pronounced. Our data indicates that the formation of both O₃ and PM_2.5 frequently occurred in smoke from prescribed fires during photochemically active periods in the studied regions. These findings are significant, as exposure to O₃ and PM_2.5 can negatively impact human health.

We investigate the impact of prescribed burning on the production of secondary PM_2.5 mass and ozone (O₃) in a region where extensive prescribed burning is widely used for land management.