Simplifying the estimation of volatile organic compounds emitted from wildfires

Yokohama, October 26, 2023

Wildfires are known to ravage ecosystems, including forests, grasslands, and shrublands. They leave behind many enduring effects, such as the release of smoky emissions from the burning of biomass. These emissions are known to deteriorate air quality at regional and continental levels, negatively impact human health, and aggravate the climate crisis. It is therefore important to understand the chemical composition and extent of the compounds contained in such wildfire emissions to be able to predict and mitigate their detrimental effects.

Among these compounds, volatile organic compounds (VOCs) are known to be a serious cause of concern. The composition of wildfire VOC emissions mainly depends on the type of the biomass and the underlying combustion processes. Importantly, several complex combustion processes, such as flaming, smoldering, and pyrolysis, could simultaneously occur during a wildfire. Since these processes vary throughout the wildfire, the resultant VOCs can vary in composition and distribution across the region, making it difficult to accurately quantify and characterize them. Moreover, previously used methods for quantifying VOCs are known to exhibit inadequate accuracy, underscoring the need for a framework that allows highly accurate estimation of VOC emissions.

Researchers led by Associate Professor Kanako Sekimoto from the Graduate School of Nanobioscience at Yokohama City University in Japan had previously reported that VOC emissions measured in laboratory simulations of the wildfires in western United States could be described by just two key processes—high- and low-temperature pyrolysis. Building upon this research, Dr. Sekimoto and her team now focused on understanding VOC emissions from real-world wildfires in the western US. Their study was made available online on August 23, 2023 and published in Volume 57, Issue 35 of Environmental Science & Technology on September 5, 2023.

In this study, the researchers analyzed VOC measurements made using the proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) on board the National Aeronautics and Space Administration (NASA) DC-8 aircraft during the 2019 ‘Fire Influence on Regional and Global Environments and Air Quality’ project—a joint campaign by the National Oceanic and Atmospheric Administration (NOAA) and NASA. “We used a statistical method known as constrained positive matrix factorization (PMF) analysis and found that the high- and low-temperature pyrolysis factors accurately describe the variability in VOCs emitted from western US wildfires, irrespective of the type of plants that undergo combustion,” explains Dr. Sekimoto.

Usually, wildfire emissions contain long-lived, short-lived, and secondary VOCs, with each of them behaving differently based on the atmospheric conditions, timespan, and other physical and chemical factors. Hence, the research team employed three distinct PMF configurations to thoroughly characterize the VOC profile and its fluctuations within wildfire plumes. These configurations included all VOCs within fresh smoke plumes, long-lasting VOCs across all the plume types, and short-lived and secondary VOCs within all plumes. While the first two configurations depended only on high- and low-temperature pyrolysis factors, the third one included photochemical aging of VOCs as a third factor.

The researchers found that more than 70% of the variability in emissions could be effectively described by the two temperature factors for 22 long-lived VOCs, in all sampled wildfire plumes. The use of aging factor in the third PMF configuration improved the profiling of short-lived and secondary VOCs to account for the photochemical changes that occur over time as the wildfire plume moves downwind.

These findings were further substantiated by the measurements of fire radiative power (FRP) obtained from geostationary operational environmental satellites at the estimated time of emission. FRP is a measure that quantifies the degree of energy released from wildfires and is often used along with emission factors to estimate total VOC emissions. In this case, the relative contribution of the high-temperature factor for the studied wildfire plumes strongly correlated with the satellite-derived FRP at the time of emission, which also helped determine the emission ratios of VOCs to carbon monoxide (CO) in the smoke plumes.

Based on these results, the researchers suggest that pyrolysis temperatures in wildfires and satellite FRP measurements are enough for accurately determining the VOC profile and variability in emitted wildfire plumes. “Our study provides a new, simpler, and more efficient framework for characterizing wildfire emissions and estimating VOC/CO emission ratios, covering over a hundred VOCs. Moreover, this makes it easier to explain and predict the generation mechanism of air pollutants like tropospheric ozone and secondary organic aerosols, from wildfires,” adds Dr. Sekimoto.

We hope that the results of this study will help us devise interventions for the betterment of human as well as environmental health!