Aerosol and Gas Emissions from Prescribed Fires in the Southeastern U.S.: Evolution and Impacts on Air Quality
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El Asmar, Rime
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Abstract
Prescribed burning is increasingly used in the U.S. as a land management tool and particularly in the southeast. Although prescribed burns typically produce lower emissions than wildfires, their higher frequency and proximity to populated areas can significantly impact local and regional air quality. There are few ground-based studies dedicated to capturing smoke from prescribed burns and accurately assessing their emissions and impacts at ground level. Most research relies on airborne measurements, which, while valuable, may not fully capture the localized air quality effects experienced by nearby communities. Ground-level monitoring is crucial for understanding pollutant concentrations, exposure risks, and the formation of secondary pollutants like ozone (O3) and fine particulate matter (PM2.5). This dissertation presents a unique dataset from ground-based field studies conducted between 2021 and 2024 at various sites across the southeastern U.S., utilizing multiple approaches. By equipping trailers with instruments and sampling inlets and deploying them in burn regions, 88 smoke events at two military bases in Georgia and an Air Force base in Florida during the active burning seasons of 2021–2024 were identified and analyzed.
During the first two years of the study (2021-2022), data collected through continuous monitoring over long period at Fort Moore (also known as Fort Benning), GA, captured emissions from numerous burns occurring both on and off the base. From this dataset, 64 smoke events were identified based on elevated levels of PM2.5 mass, black carbon (BC), brown carbon (BrC), and carbon monoxide (CO), with 61 events successfully linked to specific burn areas. Smoke transport times were estimated in two ways: using the mean wind vector and the distance between the fire and the measurement site, and from Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back-trajectories. Emission estimates of PM2.5, BC, and BrC from these events were determined when CO data was also available at the same measurement site and were compared to emissions from other prescribed burns and wildfires. The emission estimates were determined by calculating the normalized excess mixing ratios (NEMRs), referred to as emission ratios (ER) in fresh smoke which did not undergo significant chemical and/or physical change. ERs from this study were in the range of those reported for other prescribed fires studies but less than those reported for wildfires. However, the range of NEMRs for PM2.5 mass exhibited a broad range and high variability. While factors such as on-base vs. off-base fires, relative humidity, temperature, and fuel moisture content were considered, it became evident that plume evolution played the most significant role in increasing NEMRs, particularly during the daytime when O3 enhancements were also observed.
To further study evolution, and the formation of secondary species, particularly O3 and secondary aerosols, I analyzed field studies data from 2021 through 2024, identifying all events with 20-minutes averaged PM2.5 mass concentration exceeding 25 µg m-3 where CO measurements were available. Out of the 69 identified events, 62 had a known estimated smoke age and showed no clear trend in PM2.5 mass NEMRs with respect to age. However, among the 32 events measured in the afternoon (12:00-18:00), PM2.5 mass NEMRs showed an increasing trend with age. In these cases, a stronger correlation between PM2.5 mass NEMRs and age was seen in plumes with higher PM2.5 concentration (PM2.5 mass > 35 μg m−3).
O3 enhancement was observed in nearly all afternoon plumes (31 out of 32), occurring within minutes after emission with O3 NEMRs increasing significantly with age of smoke, up to 8 hours. The impact of the prescribed burning season was evident in state-operated air quality monitors near one of the study sites, where PM2.5 mass concentrations were 25−30% higher during the burning season compared to the nonburning season in 2022. Increase in daily maximum 8-hour O3 concentrations were also observed but were less pronounced.
To evaluate whether observations at state monitoring sites can be used to effectively assess the contribution of prescribed burning on PM2.5 and O3 levels in Georgia, the data from six state sites were analyzed for the years 2013-2023. The analysis indicates that PM2.5 concentrations are generally higher during the extensive burning season (January–April) at sites closer to fire-permitted areas, with a higher frequency of elevated PM2.5 levels during this period, as shown in the frequency distribution analysis. Afternoon O3 levels on three of the sites were also analyzed and showed similar trends, suggesting that fire-emitted precursors, including NOx and VOCs, contribute to enhanced photochemical O3 production at sites in closer proximity to burning activities.
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Date
2025-04-24
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Text
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Dissertation