Pollutant Emission Factors for Smoldering Conifer Forest Duff

Smoke from wildland fires (wildfires and prescribed fires) is a major source of air pollutants (Urbanski et al. 2022). Fine particulate matter (particulate matter with a nominal mean aerodynamic diameter ≤ 2.5 μm; PM2.5) is the primary pollutant of concern in smoke due to its deleterious health effects (Jaffe et al. 2020; Reid et al. 2016; USEPA 2019, 2021a) and impacts on visibility (Ashley et al. 2015; Bartolome et al. 2019; National Wildfire Coordinating Group 2020a). In addition to PM2.5, fresh smoke contains hundreds of gaseous pollutants, including a wide variety of volatile organic compounds (VOC) capable of producing ozone (O3) (Alvarado et al. 2015; Jaffe and Widger 2012; Lindaas et al. 2017), a secondary air pollutant, which can have negative impacts on public health and the environment (Grulke and Heath 2020; USEPA 2020; Wittig et al. 2009). In the United States (U.S.) wildland fire smoke has emerged as a major contributor to poor air quality. Over the past decade, wildfire smoke from both the U.S. and Canada, has been responsible for hundreds of days of unhealthy air quality that has impacted tens of millions of people across the western and central U.S. (Buchholz et al. 2022; Childs et al. 2022; David et al. 2021; O’Dell et al. 2019). Increases in wildfire activity have offset reductions in anthropogenic air pollutant emissions resulting in a long-term trend of worsening air quality in regions prone to wildfire smoke impacts (McClure and Jaffe 2018; O’Dell et al. 2019). In the eastern U.S., large wildfires and major smoke intrusions from distant wildfires (e.g., in the western U.S. or Canada) are infrequent compared with the West (the 11 contiguous western States) (Brey et al. 2018). However, in the central and southeastern U.S., prescribed fire use is widespread (Jaffe et al. 2020; Melvin 2018). According to the 2017 National Emission Inventory (USEPA 2021b), prescribed fires accounted for 31 percent of total PM2.5 emissions in the Southeast and were the dominant major emission sector being 80 percent greater than dust and 133 percent greater than fuel combustion, the second and third largest sources. A prescribed fire is a planned fire conducted to meet land management objectives. Prescribed fires, which are conducted under a set of weather and ecosystem conditions (a prescription), maximize the likelihood the land management objectives will be safely achieved. A primary use of prescribed fire is the mitigation of wildfire hazard through the reduction of fuels (combustible live and dead vegetation), and the restoration and maintenance of ecosystem health (Agee and Skinner 2005). The increased occurrence of large wildfires ( 200 ha) over recent decades (Holden et al. 2018; Westerling 2016), the growth of the wildland-urban interface, and the prospect of more frequent large, high severity wildfires in the future (Abatzoglou and Williams 2016; Kitzberger et al. 2007; Littell et al. 2009; USDA 2014; Westerling et al. 2016) has led to increased interest in prescribed fire as a forest management tool (Ager et al. 2014; Stephens et al. 2021). The U.S. Department of Agriculture, Forest Service, Wildfire Crisis Strategy (WCS) Implementation Plan (USDA 2022) calls for an unprecedented increase in fuels treatments—prescribed fire and mechanical treatments such as thinning and mastication of understory vegetation (Agee and Skinner 2005; Mitchell and Smidt 2019)—across all lands with a focus on the western U.S.

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