Assessing Wildfire Impact on Diffusive Flux of Parent and Alkylated PAHs: A Pilot Study of Soil-Air Chemical Movement before, during, and after Wildfires

Abstract

The global wildfire risk is predicted to rise due to contributing factors of historical fire management strategies and increases in extreme weather conditions. Thus, there is a need to better understand contaminant movement and human exposure to wildfire smoke. Vapor-phase polycyclic aromatic hydrocarbons (PAHs) are elevated during wildfires, but little is known about how these chemicals move during and after wildfire events for exposure risk assessment. Paired air and soil pore air passive samplers were deployed before, during, and after wildfires to determine diffusive flux of vapor-phase parent (p-PAH) and alkylated (a-PAH) PAHs in the Western United States. Naphthalene and 2-methylnaphthalene contributed to most of the volatilization and deposition (6.3-89%) before and after a wildfire. Retene (41%) and phenanthrene (27%) contributed substantially to deposition during a wildfire. During wildfires, the number of PAHs in deposition increased at sites with worse air quality. Most p-PAHs and a-PAHs were either depositing or near equilibrium after a wildfire, except for retene at several locations. A majority (≥50%) of PAHs had a 50% magnitude difference between flux before and after a wildfire. This study increases the understanding of PAH movement and exposure during each stage of the wildfire cycle.

Keywords: community-engaged; disaster research; passive sampling; polycyclic aromatic hydrocarbons; wildfire smoke.

Figure 1

Individual PAH volatilization and deposition contribution of ≥5% of the sum flux for all Wildfire samples. All PAHs contributing to less than 5% are categorized as “Other”. The number of PAHs in the “Other” category is indicated in parentheses. Pre-Wildfire and Post-Wildfire volatilization and deposition are shown in Figure S4.

Figure 2

Individual PAH Wildfire flux for p-PAHs (top row) and a-PAHs (bottom row) for Newport, OR, St. Helena, CA, Prineville, OR, and Corvallis, OR. NOTES: Red box around Corvallis, OR indicates different scales. Naphthalene and alkylated naphthalenes are not included due to larger order of magnitudes and are in Figure S5.

Figure 3

Average AQI at sites during a Wildfire and the corresponding proportion of A) p-PAHs and B) a-PAHs in deposition from air to soil.

Figure 4

Individual p-PAH (top row) and a-PAH (bottom row) Post-Wildfire flux for each site. NOTES: Different scales between p-PAHs and a-PAHs. AQI data was not available for Newport, OR in 2018. Naphthalene and alkylated naphthalenes are in Figure S6.

Figure 5

PAHs with a minimum of 50% difference in flux between Pre-Wildfire and Post-Wildfire for naphthalenes (top row), p-PAHs (middle row), and a-PAHs (bottom row) at each sample site ordered by increasing AQI. Red boxes around the different PAH types represent cases where a majority (≥50%) of the total PAHs in flux (naphthalenes, p-PAHs, or a-PAHs) have a minimum of 50% difference in flux. Sites are ordered from least to greatest AQI of the wildfire in between sampling events. NOTES: AQI data was not available for Newport, OR in 2018. Naphthalene and alkylated naphthalenes are separated due to larger order of magnitudes.