FIREX Science Overview
Motivation and Objectives
A combination of a warmer, drier climate with fire-control practices over the last century have produced a situation in which we can expect more frequent fires and fires of larger magnitude in the Western U.S. and Canada. Present-day forest ecosystems have evolved with wildfire as part of their natural environment. Forests benefit from periodic fire to promote seed germination and healthy ecosystem succession. The 20th century saw fire suppression become the standard response to wildfires, especially in western North America. This fire suppression has led to a buildup of fuels in forested areas, a breakdown in the natural ecology of forests, and an underestimate of the risks 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 [U.S. Department of Interior, 2014]. In addition to the direct risks of fire, other risks include atmospheric impacts on air quality, climate, and health.
Climate change will sharpen the problems involving wildfires in the western U.S. The average area burned per year by wildfire in parts of the western U.S. is projected to increase by "two to four times per degree of warming" [Warming world: Impacts by degree, National Research Council, 2011]. Wildfire activity increases in the Western US in the past several decades have been linked to higher temperatures, earlier snowmelt, changing precipitation patterns, and drought impacts on moisture levels in vegetation and soils, which appear at least as important as fire management practices to determining fire frequency in the West [Westerling et al., 2006; Dennison et al., 2014; Sherriff et al., 2014]. There are urgent needs to better understand the impacts of wildfire and biomass burning (BB) on the atmosphere and climate system, and for policy-relevant science to aid in the process of managing fires.
NOAA's Office of Oceanic and Atmospheric Research (OAR) has as part of its core mission "(to) advance understanding and prediction of the Earth System to enhance society's ability to make effective decisions." [NOAA/OAR, 2014]. The research program proposed here outlines a comprehensive research effort to understand and predict the impact of North American fires on the atmosphere and ultimately support better land management.
State of Science and Scientific Background
Fire impacts occur over various time and distance scales from local to global via many complex, interdependent, and poorly understood, processes. For example, fire dynamics and meteorology determine the vertical profile of smoke injection which then governs the photochemical environment and transport of the fire plumes: e.g. how they are processed and therefore their chemistry and how far fire emissions are transported regionally and globally. Major critical research areas are summarized briefly below including priority areas that require more research.
Numerous recent studies, listed in Table 1, have contributed to the characterization of the complex nature of fire emissions and processing. Studies led by NOAA include ICARTT, which found strong BB influence from Canadian and Alaskan fires in the northeast U.S.; ARCPAC, which found a strong contribution by Asian fires to arctic haze over Alaska; and SENEX, which acquired data on the relative contribution of BB to organic aerosols and gases in the southeast U.S. and the first-ever nighttime smoke sampling. Recent airborne studies led by other agencies include the NASA ARCTAS and DC3/SEAC4RS missions, and the DOE BBOP mission. ARCTAS produced new information on emissions and processing of boreal fires. DC3, SEAC4RS, BBOP produced new information on emissions and smoke evolution regarding western U.S. wildfires, cloud processing of smoke, and agricultural burning in southeast U.S. Recent laboratory studies like the University of Montana led FLAME-4 study produced some fuel specific emission factors (EFs) and smoke aging simulations.
Although scientific analysis of the more recent missions is still ongoing, future missions aimed primarily at BB are still needed to address inadequacies in the experimental datasets, with foci including observation of night-time plume evolution and air quality impacts, better gas- and aerosol-phase tracking of chemical processes, and brown/black carbon chemistry and absorption attribution. An aircraft such as the NOAA WP-3D, equipped with state-of-the-art instrumentation, would provide the most complete airborne platform ever to be dedicated to BB research.
Table 1: Airborne field studies relevant to biomass burning research
|ICARTT||International Consortium for Atmospheric Research on Transport and Transformation||2004||Northeast US||NOAA|
|ARCPAC||Aerosol, Radiation, and Cloud Processes affecting Arctic Climate||2008||Alaska||NOAA|
|SENEX||Southeast Nexus||2013||Southeast US||NOAA|
|ARCTAS||Arctic Research of the Composition of the Troposphere from Aircraft and Satellites||2008||Canada||NASA|
|DC3||Deep Convection Clouds & Chemistry Experiment||2012||Southeast US||NASA|
|SEAC4RS||Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys||2013||Western US||NASA|
|BBOP||Biomass Burn Observation Project||2013||Washington, Tennessee||DOE|
|FLAME-4||Fire Lab at Missoula Experiment-4*|
* laboratory study
|2012||Montana||NOAA, NASA, NSF, DOE|
Fuels and Emission Factors
Fuel-type-specific emission factors are a fundamental need for prediction and assessment of wildfire impacts on the atmosphere. In practice, there are a number of factors that complicate emission estimates. The materials emitted, and their emission intensity, depend on combustion processes: e.g. flaming and smoldering, but real fires are always a combination of these regimes. For this reason, emission ratios are usually measured and reported as a ratio to one of the two main carbon species emitted, carbon monoxide (CO, from smoldering) and carbon dioxide (CO2, from flaming). Emission factors are frequently modeled as a function of a parameter called modified combustion efficiency (MCE), defined as:
MCE = ΔCO2/(ΔCO2 + ΔCO) (1)
MCE accounts for much of the variability in emission factors and is dependent only on ΔCO and ΔCO2, which are easily measured in the atmosphere and in laboratory fires. Emissions of some nitrogen-containing species also follow this pattern (e.g. NOx from flaming). It should be noted that in contrast to other combustion sources, e.g. fossil-fuel power plants or vehicles, the nitrogen emitted from normal convective biomass fires (i.e. wildfires) comes entirely from the fuel and not from atmospheric nitrogen.
The literature on wildfire emission factors is extensive, as exemplified by several recent reviews [Akagi et al., 2011; Yokelson et al., 2013]. There has been significant progress in defining and quantifying the detailed chemistry of BB emissions as analysis methods applied to these measurements continue to evolve in sophistication. Examples of recent new findings include measurements of isocyanic acid (HNCO) emissions and the observation of nitrous acid (HONO) as a consistent product of fires in both the laboratory and field at levels of 5-30% of NOx [Burling et al., 2010; Roberts et al., 2010]. There has also been increased effort to obtain detailed information on the semi-volatile organic compounds (SVOCs) that are an abundant class of secondary organic aerosol (SOA) precursors in fires. Attempts to reconcile the VOC emissions from fires with the observed SOA formation have shown that unidentified SVOCs, while perhaps only 20-50% of the VOC emitted [Hatch et al., 2014; Stockwell et al., 2014], have a disproportionately large effect on the amount of SOA estimated by models [Jathar et al., 2014; McMeeking et al., 2014].
Even with the application of new analytical techniques to fire emissions measurements, a significant fraction of VOCs and SVOCS remain unidentified. A better process-level understanding is needed on gas and particle emissions from North American wildfires. More emission factors for the gas and particle phase are needed for various fuels and conditions (e.g. moisture and wind) from typical North American wildfires. A systematic effort is needed for a comprehensive budget of carbon- and nitrogen-containing materials. This information can then be integrated with land-use and ecosystem data to provide the data products needed for assessment and modeling of wildfire impacts.
Smoke is one of the most prominent and visible aspects of BB. Smoke is primarily comprised of gaseous and aerosol constituents including BC, BrC, OC, and mineral dust, all of which have critical climate and health impacts. The aerosol, controlling most of the optical properties, evolves due to dilution, coagulation, and chemical processing on time scales of seconds to days [Vakkari et al., 2014]. BB smoke has many impacts on the atmosphere; depending on the relative amounts of OC and BC/BrC and surface albedo, it can either heat or cool the atmosphere; it can provide ice and water active aerosols; affect visibility and air quality; be transported over global scales. BB is the largest source of black carbon to the atmosphere [Bond et al., 2013] and a singularly important source of BrC. There is evidence that fires produce BC particles coated with particulate organic matter in a manner that enhances some of their optical properties, specifically short wavelength absorption by "lensing" [Lack et al., 2012]. However, the net effect of all aerosol species generated by BB is currently believed to be slightly cooling.
Satellite Emissions Estimates
The aggregation and systematic accounting of fire emissions on regional to global scales relies on satellite-based detection of visible and infrared irradiance, and some broadly applicable fire indicators such as carbon monoxide or aerosol optical depth. Models have been constructed to use these detected parameters, along with land-use, and emission factor data to produce inventories for fire emissions. These inventories are essential to the inclusion of fire emissions in global chemistry and climate models. It is widely accepted that current satellites undercount wildfire and domestic BB due to limited spatial resolution, clouds, and orbital gaps. In addition, there are limitations in the application of retrieval algorithms to fire situations.
Higher resolution fire products will be developed by other agencies using the next generation of satellites, including not just fire detection, but also chemical species such as carbon monoxide and aerosol optical properties. These enhanced products need to be integrated with the best available fire dynamics and emissions data from laboratory and field measurements. Within NOAA, data from intensive field projects involving wildfires, along with regional chemical-transport modeling, will be used to test these satellite products and provide ground-truth for their continued improvement.
Transport, Transformation and Plume Chemistry
The impact of wildfires on regional- to global-scale atmospheric chemistry depends on the physical and chemical transformations that take place as fire emissions are transported, diluted, and exposed to chemical oxidants. Ozone and other oxidants can be formed along the way, and particle mass-loadings can grow or shrink [Akagi et al., 2012]. In addition, toxic gas and particle materials that have health impacts can be both formed and destroyed. Not all the factors that govern these processes are well understood, and individual fire plumes can have very different behaviors. Fire emissions contain a number of unusual compounds, some of which may have specific health effects [Roberts et al., 2011], and more compounds are being discovered as more sophisticated analytical techniques have been applied [Stockwell et al., 2014]. Fundamental atmospheric chemical behavior of some of these compounds is often unknown.
Photooxidation of the NOx and VOCs emitted by fire plumes shows complex behavior, sometimes leading to production of ozone and sometimes not [Jaffe and Wigder, 2012]. The reasons for this complexity are not understood and may have to do with how fast the plume was lofted and cooled, how efficiently NOx was converted to products such as peroxyacetyl nitrate (PAN), or whether the fire had substantial amounts of radical precursors such as HONO or carbonyls. What is clear is that fire emissions often have broad-scale impacts on ozone formation [Pfister et al., 2006; Wotawa and Trainer, 2000], especially when mixed with urban emissions [Singh et al., 2012], and can be decisive factors in triggering air quality exceedances. Policy mechanisms exist for jurisdictions to apply for waivers due to wildfire impacts. A much better understanding of the photooxidation in fire plumes is needed to predict impacts of wildfires and prescribed or permitted fires. In addition, nighttime processing of fire plumes is likely to be very important considering what we know of NO3 and N2O5 chemistry in NOx-containing plumes, but has received almost no attention thus far.
The physical and chemical processes governing the transformation of particles in BB plumes is quite complex, with evidence of both loss and gain of particle mass, and rapid atmospheric oxidation [Vakkari et al., 2014]. Mass can be lost as smoke is diluted due to the physical equilibration of semi-volatile compounds [Robinson et al., 2007]. Oxidation of both gas and particle phase compounds due to reaction with HOx radicals (daytime) and NO3 radicals, N2O5, and ClNO2 (nighttime) can lead to mass loss or gain, and changes in the optical properties. The chemistry and surface coating properties of particles emitted by fires evolve as these physical and chemical changes take place and affect the optical, cloud nucleation, and toxicological properties of the particles. A proper description is essential for analyzing and predicting the impacts of BB.
Regional Atmospheric Chemistry and Impacts
Wildfires can have profound impacts on regional air quality due to promotion of photochemical ozone production, particle pollution of both primary and secondary origin, and emission of toxic materials. These aspects of wildfires have the most immediate effect on populations and ecosystems, and are a high priority when making decisions on fire management, i.e., when to plan prescribed burns and when to allow fires that started naturally to burn. The research needed to understand and manage these impacts is one of the more challenging aspects of wildfire research, as it involves understanding all of the smaller-scale processes detailed above: fire weather, emissions, and transport and transformation. This information must then be incorporated into models that can be used to make policy decisions on all timescales, from the immediate: e.g. fire management and health advisories, mid-term: air quality waiver, to the long-term: ecosystem and urban-wildland interface management. As National Ambient Air Quality Standards (NAAQS) for ozone and other criteria pollutants are reduced in the future, meeting these stricter standards will require better understanding and prediction of fire impacts (along with long-range transport of pollution from other parts of the world).
Fire Dynamics and Fire Weather
The physical development and dynamics of wildfires on local to regional scales govern how fires impact their immediate environment, including the safety of first responders on the scene, and the air quality of local communities. The development and collapse of strong vertical transport caused by fires can control the rate of vertical transport of heat and chemical emissions, and even create intense pyrocumulus systems. The starting conditions and processes that govern vertical transport are still quite uncertain, making prediction difficult. The duration and extent of these convective systems are crucial features that determine the initial transport of emissions and need to be better understood. Finally, pyroconvection can also cause super-cooled air masses leading to intense downdrafts, with winds that radiate unpredictably upon impacting the ground, thereby accelerating fire propagation.
Global Distributions and Impacts
The impacts of wildfires are mostly associated with short-term climate forcers; ozone and aerosols including BC, BrC and OC. Global climate impacts of BB result from its truly massive contributions to aerosol optical depth (AOD) over large areas. Regions such as the Arctic and the cloud decks of South America are uniquely sensitive to BB emissions and to secondary processes, such as cloud and ice nucleation that can magnify the radiative impact of the emissions. The research needed to advance our understanding of these impacts is broad on both spatial and temporal scales, and relates to a wide spectrum including BB inventories, satellite fire detection, chemical evolution of gaseous species, aerosol microphysical and optical properties, proper integration into models, interactions with warm, mixed phase, and ice clouds, and effects on the vertical structure of the thermal profile. For example, current fire emissions inventories undercount small fires and domestic BB because they are based on satellite fire detection schemes that have limited resolution and therefore regional airborne measurements of fire products are invaluable in assessing inventories. Improving the integration of wildfires and BB into global models requires more detailed emission estimates at finer spatial scales and better understanding of how to represent these emissions in relatively coarse-resolution treatment after significant chemical processing and dilution. These efforts will require combining new chemical details from emission measurements, new insights about chemical and physical transformation of smoke, supported by observations from the next generation of satellites.
The new science and capabilities FIREX will generate
The FIREX research effort will target critical unknowns about BB that can be realistically addressed in the next five years with existing or new technologies, laboratory and field studies and interpretive efforts. The following describes several aspects that will add new science and capabilities to the results from previous laboratory and field studies that were described above.
- New instrumentation has become available, or will be developed, that has not been extensively used in previous field campaigns. For FIREX we will use these new technologies throughout campaign.
- Measurements of previously unidentified compounds using new mass spectrometry techniques (H3O+CIMS, I-CIMS, nitrate-CIMS, 2D-GC-MS, TAG-MS ...)
- Broadband extinction measurements of BrC augmented with polar scattering measurements
- Studies of lensing of black carbon using a thermally desorbed photoacoustic instrument
- Electrospray mass spectrometry for aerosol-phase compounds
- Fire Science Laboratory experiments to characterize emissions before and after the large-scale field campaign
- Atmospheric simulation chamber experiments on chemical transformation
- Investigations of aged fire emissions from a ground site (Storm Peak, Colorado)
- Testing and validation of new instrumentation
- NOAA WP-3D aircraft to do a major emissions and chemical transformation study for multiple fires in coordination with other platforms
- Mobile laboratory to investigate smoldering emissions that are not lofted and stay at ground level particularly during nighttime. These emissions are likely different in composition, and undergo processing that is different than plumes aloft, but are often the ones that affect local air quality the most.
- Twin Otter, other small aircraft or unmanned aerial systems (UAS) to do near field emission study to understand the temporal evolution of emissions
For further information, download the FIREX White Paper