The combination of a warmer, drier climate with fire-control practices applied over the last century has produced a situation in which we can expect larger and more frequent fires in the U.S. and Canada. The 20th century saw fire suppression become the standard response to wildfires, especially in western North America; this has led to a buildup of fuels in forested areas, a breakdown in the natural ecology of forests, and risks to life and property associated with the development of the urban-wildland interface. Prescribing fires and allowing some naturally-occurring fires to burn are some of the management practices that can address the above problem.
Fire is important for many ecosystems, but it also poses costly risks to human health and property. These risks have increased in recent decades due in part to population growth in the wildland urban interface. Extreme fire seasons attract mounting attention due to the increasing number of costly extreme wildfires that include: the 2016 fires that burned across 8 states in the southeast (48,158 ha); the 2016 Anderson Creek prairie fire that was the largest in Kansas history (161,874 ha); the 2016 Fort McMurray fire, which is the costliest fire in Canadian history ($2.7 B, 589,552 ha, 2400 structures destroyed); the 2004 Alaskan fire season (2.74 M ha), the largest in almost 80 years of Alaskan fire history and the extreme 2015 unusually-early-season Alaskan fires (2.07 M ha). Since 1960, total burned area in a single year has exceeded 3.6 M ha in the United States only 4 times – all of which occurred in last decade. Coupled with the direct threats to life and property, wildland fires have demonstrable detrimental air-quality related health impacts including aggravated asthma, chronic bronchitis, decreased lung function, congestive heart failure, and premature death.
Background and State of the Science
Fire impacts occur over wide time and distance scales, from local to global, via many complex, interdependent, and poorly understood processes. For example, primary fire emissions are affected by a wide variety of factors including fuel conditions (type, structure, quantity, and moisture content), fire intensity, and fire weather (cumulative temperature, relative humidity, wind speed and precipitation), which in turn can be rapidly and heterogeneously modified by fires as they burn. Over the life cycle of a fire, combinations of flaming and smoldering combustion lead to different emissions at different times and at different locations within a fire. These variables also influence plume rise and the subsequent transport and chemical evolution of fire emissions, which determine the secondary products (e.g. evolved gases and aerosol species). Wildfire initiation can be natural (by lightning) or human caused and prescribed fires are becoming a more frequent tool for land management (e.g., land clearing and agriculture). Fire growth is driven by weather conditions and is subject to the limitations of weather-based prediction. Fire activity can be predicted on a broad seasonal scale, but climatologies are inadequate to provide the detailed information needed to understand and predict fire impacts. This is especially true for impacts related to air quality, which depend on the intersection of fire emissions with populations and are sensitive to chemical transformations that can result when emissions from fires and anthropogenic sources combine.
The ubiquity of fire emissions is evident from previous airborne field studies. Some of the more recent missions observing these atmospheric impacts of fires are introduced here, followed by a more detailed explanation of state of the science organized by general subtopic. The international ICARTT study (2004) found strong biomass burning (BB) influence from Canadian and Alaskan fires in the northeast U.S., and NOAA's TEXAQS (2006) identified systematic differences in particle morphology between urban and biomass burning sources. The international POLARCAT study (2008) focused on Arctic measurements, observed strong fire contributions in spring by Asian fires to arctic haze over Alaska and sampled local Canadian fire emissions during summer as well as unexpected fires in California, providing a broad cross section of fire emissions and impacts. NOAA's SENEX (2013) acquired data on the relative contribution of BB to organic aerosols and gases in the southeast U.S. and provided the first airborne measurements of nighttime smoke, while the NASA/NSF DC3 (2012) campaign had the good fortune to encounter a smoke plume interacting with a deep convective tower and evidence for the broad influence of convection on the ventilation of fire emissions . Fire sampling during NASA's SEAC4RS (2013) study enabled evaluation of the plume from the Rim Fire, a large wildfire in California and emissions from a collection of 15 small agricultural fires in the Mississippi River Valley.
Despite this wealth of fire sampling over the years, field studies dedicated specifically to the sampling and characterization of fires and their impacts from the point of emission have been lacking. This need is being met through recent efforts such as the DOE BBOP (2013) mission which was focused on smoke optical properties, but also identified, via morphological analysis, evidence for evolving brown carbon materials in fire plumes. NOAA's FIREX Fire Lab 2016 study and the University of Montana led FLAME-4 study produced some fuel specific emission factors (EFs) and smoke aging simulations.