FAQ on Wildfire Impact on Polycyclic Aromatic Hydrocarbons (PAHs)

What are PAHs and why are they a concern in relation to wildfires?

PAHs, or Polycyclic Aromatic Hydrocarbons, are a class of organic compounds formed from the incomplete combustion of organic materials, such as during wildfires. They include both parent PAHs (p-PAHs) and alkylated PAHs (a-PAHs). These compounds are concerning because exposure to them has been linked to various health issues, including reproductive effects, respiratory problems, cardiotoxicity, immunotoxicity, neurotoxicity, and different types of cancer. Alkylated PAHs, in particular, are often more abundant and sometimes more toxic than their parent compounds. Importantly, PAHs can be present in both vapor (gas) phase and bound to particles, and vapor-phase PAHs have been shown to significantly contribute to the carcinogenic risk of wildfire smoke through inhalation.

How does wildfire smoke impact the movement of PAHs between soil and air?

Wildfires can drastically alter the typical exchange of PAHs between soil and air. Usually, this exchange occurs as PAHs move to reach an equilibrium between the soil and the air. However, wildfires can cause a significant increase in both the concentration and deposition of PAHs from the air to the soil. During a wildfire, there's an increase in the number of PAHs depositing into the soil, especially at sites with worse air quality. After a wildfire, most PAHs tend to return to an equilibrium state and, in some cases, volatilize from the soil into the air, which may contribute to continued exposure even after visible smoke has dissipated. There can also be long-term changes in PAH movement after a wildfire compared to pre-wildfire conditions.

What types of PAHs are most affected by wildfires, and what are their specific behaviors?

Lower molecular weight (LMW) PAHs, such as naphthalene and its alkylated forms (like 1-methylnaphthalene and 2-methylnaphthalene), tend to be more abundant in the vapor phase and thus are more readily influenced by wildfires. During wildfires, naphthalene and 2-methylnaphthalene are dominant in terms of volatilization, moving from the soil into the air, while retene and phenanthrene significantly contribute to deposition, moving from air to soil. After a wildfire, most PAHs, including both parent and alkylated, tend towards equilibrium or deposition, except for retene which sometimes continues to volatilize from the soil.

How do Air Quality Index (AQI) levels correlate with PAH deposition during wildfires?

There is a positive trend between higher AQI values during wildfires and the deposition of PAHs from air to soil. This indicates that as air quality worsens during a wildfire event, there is more deposition of PAHs to the soil. This trend was observed with both parent and alkylated PAHs, though some individual PAHs and locations may not follow this general rule. For example, while most PAHs deposited during the wildfire, naphthalenes tended to volatilize regardless of the air quality.

How does the soil type impact PAH movement?

Soil characteristics play a role in PAH movement, specifically sandy soils with less carbon content tend to have PAHs volatilize more readily than other soil types. This is because PAHs have less tendency to adhere to the soil and will thus be released into the air more quickly, while soil with higher carbon content will retain more of these pollutants. This was one explanation for why one study site with sandy soil exhibited different PAH movement patterns compared to other sites.

What are the long-term impacts of wildfires on the soil-air exchange of PAHs?

The research indicates that wildfires can lead to lasting changes in the flux (movement) of PAHs between soil and air. There was found to be a significant difference in diffusive flux of a majority of PAHs, (both p-PAHs and a-PAHs) when comparing before and after a wildfire. Notably, a number of PAHs, such as naphthalene, phenanthrene, acenaphthene, and retene, were found to volatilize at a higher magnitude a month after the fire, compared to pre-fire conditions. This suggests the potential for longer-term exposures to these chemicals even after the immediate smoke has cleared.

How does PAH movement during wildfires differ from other environments (urban, industrial, etc.)?

PAH movement during wildfires often exhibits opposite trends compared to other settings. Typically, in urban and industrial environments, PAHs tend to volatilize from soil to air, whereas during wildfires, deposition of PAHs from air to soil is more common. The study found much higher levels of PAH deposition during wildfires in the Western US than in semiurban areas in Turkey or urban areas of Nepal and China. The only exception to this was found at a location in Turkey near a highway, indicating that heavy traffic could contribute to greater deposition of PAHs that surpasses wildfire rates. This demonstrates the unique impact of wildfire events on soil-air PAH dynamics.

What are the limitations and strengths of using passive sampling methods to study PAH movement during wildfires?

Passive sampling devices (PSDs) offer several advantages, such as their low maintenance and ease of use which allow for geographic expansion and the capture of wildfire events throughout the deployment. However, they continuously sample which can make it difficult to pinpoint wildfire-specific periods if it no longer occurs for all of the deployment time. The timing of sampling can be also impacted by participant safety during the wildfire event. Moreover, data used to define wildfire smoke events like AQI stations, might be located far from the actual sampling sites, and it is important to note that the study only focused on vapor-phase PAHs and not other compounds that can be found in wildfire smoke. Despite these limitations, PSDs were a useful tool in this research and provide a good foundation for the study of how wildfires impact PAH movement.