smoke Plumes: emissions and effects Susan M. O’Neill, Shawn Urbanski, Scott Goodrick, and Narasimhan K. Larkin

moke can manifest itself as a towering plume rising against Smoke can affect public health, Sthe clear blue sky—or as a vast transportation safety, and the health swath of thick haze, with fingers that settle into valleys overnight. It and well-being of firefighters. comes in many forms and colors, from fluffy and white to thick and black. Smoke plumes can rise high 1). Overall, dry air is made up of 78 affect visibility, degrading vistas and into the atmosphere and travel percent nitrogen (N2), 21 percent creating transportation hazards. great distances across oceans and oxygen (O2), and about 1 percent Smoke combined with high humid­ continents. Or smoke can remain trace gases (about 0.9 percent ity can produce whiteout conditions close to the ground and follow fine- argon (Ar) and 0.1 percent other known as superfog (Achtemaier scale topographical features. trace gases). Water vapor in the 2002). air ranges from almost nothing to Along the way, the gases and par­ 5 percent. Yet the trace amount Wildland Fire Emissions ticles in the plumes react physically of smoke in the air can have sig­ If fire were 100-percent efficient, and chemically, creating additional nificant impacts, such as making the only products released would particulate matter and gases such the air smell bad; limiting vis­ be carbon dioxide, water vapor, as ozone (O3). If atmospheric water ibility; and affecting the health of and heat. However, the combus­ content is high, smoke plumes can vegetation and animals, including tion of wildland fuels is never also create “superfog” (Achtemeier humans. Smoke can affect public 100-percent efficient. In addition 2002). Over the past decade, health, transportation safety, and to carbon dioxide and water vapor, researchers have studied the suite the health and well-being of fire­ the process transforms some of of trace gases and aerosols emitted fighters. by wildland fires, along with the the fuel into ash, char, particulate matter, carbon monoxide, and physical and chemical reactions and Due to health and safety concerns, other carbon-containing gases—a transformations that occur within the Clean Air Act regulates aspects rich and complex mixture of gases a plume. of atmospheric smoke. If smoke and particles. Figure 1 shows the concentrations in a region exceed distribution of carbon emitted by Why Do We Care? the national ambient air qual­ Smoke gases and particles con­ ity standards (NAAQSs), then the stitute only a tiny fraction of the region is designated as “nonattain­ air (less than 0.1 percent of the ment.” Nonattainment triggers reg­ atmosphere in any location, even ulatory actions, such as controls on under the worst conditions) (table smoke emissions; fees/charges for emissions; restrictions on industry; Susan O’Neill is an air quality scientist and, in some cases, restrictions for the Forest Service, Pacific Northwest on the use of fire. Currently, the Research Station, Seattle, WA; Shawn NAAQS threshold for particulate Urbanski is a research physical scientist for the Forest Service, Rocky Mountain matter finer than 2.5 micrometers Research Station, Missoula, MT; Scott in diameter (PM2.5) is 35 micro- Goodrick is a research meteorologist for grams per cubic meter (24-hour Figure 1—The distribution of carbon the Forest Service, Southern Research average); for ozone, the threshold Station, Athens, GA; and Sim Larkin is emitted by a typical low-intensity a research scientist and team leader for is 75 parts per billion (8-hour aver­ understory prescribed fire in light fuels the Forest Service AirFire Team, Pacific age). Even at levels below these (based on Urbanski (2014)). CO2 = carbon Northwest Research Station, Seattle, WA. dioxide; CO = carbon monoxide; PM = standards, smoke can significantly particulate matter.

Fire Management Today 10 Table 1—Gases and particles in the atmosphere, including those related to biomass burning. Chemical/particle Symbol Name Description Ar Argon About 0.9 percent of the atmosphere. BC Black carbon Particles from combustion that strongly absorb incoming solar radiation and emit longwave radiation. CO Carbon monoxide Emitted from the incomplete combustion of biomass.

CO2 Carbon dioxide A primary product of biomass burning.

H2O Water A primary product of biomass burning.

N2 Nitrogen About 78 percent of the atmosphere.

NH3 Ammonia A precursor of inorganic particle formation; trace amounts emitted from biomass burning.

NOX Oxides of nitrogen NO and NO2, precursors to ozone and particle formation.

NO Nitrogen oxide Part of NOX; released from biomass burning as a function of the fuel nitrogen content and combustion phase.

NO2 Nitrogen dioxide Part of NOX.

O2 Oxygen About 21 percent of the atmosphere.

O3 Ozone Created by the reaction of NOX and VOCs in the presence of sunlight; regulated under the Clean Air Act. OC Organic carbon Carbon and hydrogen compounds, oxygenated carbon/hydrogen compounds, and other trace elements; a major constituent of particulate matter from biomass burning.

PAN Peroxyacetyl nitrate Can sequester NOX and travel long distances in the atmosphere; mixed back down near the surface, warmer temperatures break it apart, freeing up NOX to generate more O3 far from the fire.

PM2.5 Particulate matter Fine particulate matter; can comprise organic and inorganic (aerodynamic compounds; regulated under the Clean Air Act. diameter < 2.5 μm)

SO2 Sulfur dioxide A precursor of inorganic particle formation; trace amounts emitted from biomass burning. SOA Secondary organic Particles formed by a series of physical and chemical reactions. aerosol

VOC Volatile organic Precursors of O3 and SOA formation; hundreds identified from compound biomass burning, hundreds yet to be identified; examples: methane, ethane, benzene, furan, formaldehydes, methanol, monoterpenes.

Volume 75 • No. 1 • 2017 11 a typical low-intensity understory trace elements. Black carbon is the Secondary organic aerosols are prescribed fire in light fuels. Minor fraction of the particle that strongly particles formed in the atmosphere smoke constituents such as carbon absorbs incoming solar radiation through a series of gas-phase chem­ monoxide, particulate matter, and and emits longwave radiation into ical reactions that lower the volatil­ “other gases” are mainly respon­ the atmosphere. It is a product of ity of VOCs to the point where they sible for smoke impacts on visibility combustion; in its purest form, it can condense into or onto particu­ and air quality. would be graphite. It has been iden­ lates. These organic compounds tified as a short-lived climate forcer, can continue reacting until the car­ Over the past decade, scientists bolstering the greenhouse effect; bon is oxidized to carbon monoxide have made tremendous progress deposition of black carbon on snow or carbon dioxide or until the par­ in identifying the “other gases” accelerates snowmelt. ticles are removed from the atmo­ produced by wildland fires (fig. sphere through deposition. Cooler 1). Multiple research projects temperatures help semivolatile have gotten support from the gases condense into particulates. Forest Service’s Research and Smoke particles are As compounds react and age in Development, the multiagency quite small, making the atmosphere, they tend to more Joint Fire Science Program, the them very efficient at readily attach to water, increasing U.S. Department of Defense scattering light, thereby the amount of water in the particu­ Strategic Environmental Research late. Most semivolatile compounds and Develop Program, and oth­ reducing visibility and formed in the atmosphere remain ers. Researchers have studied generating “haze.” to be identified. Secondary organic the composition of smoke from aerosols are an important area of simulated fires in the large-scale current research. combustion facility operated by the Particles rarely exist solely as Rocky Mountain Research Station’s organic or black carbon. Instead, Most smoke particles are quite Missoula Fire Sciences Laboratory they form a continuum, rang­ small, often less than 1 micrometer and from operational prescribed ing from pure graphite to mostly in diameter. They are very efficient fires in the Southeastern and organic compounds. Smoke par­ at scattering light, reducing visibil­ Southwestern United States. Recent ticles from wildland fires tend to ity and generating “haze.” Smoke advances in instrumentation and have a higher percentage of organ­ plumes lofted high into the atmo­ chemical analytical techniques have ics than do particles from anthro­ sphere can also interact with cloud- allowed scientists to identify nearly pogenic combustion sources, which forming processes. 200 volatile organic compounds tend to have a higher black carbon (VOCs) in fresh smoke. The gases content. Smoke and Ozone include methane, ethane, benzene, Ozone is created in the atmosphere furan, formaldehydes, methanol, New particles form in the atmo­ through the reactions of oxides of monoterpenes, and more. Despite sphere through gas-to-particle nitrogen (nitrogen monoxide and such advances, a large percent­ reactions involving VOCs, oxides nitrogen dioxide) with VOCs in the age of VOCs remain unidentified, of nitrogen (nitrogen monoxide presence of sunlight. Nitrogen diox­ including 30 to 40 percent of them and nitrogen dioxide), ammonia, ide undergoes photolysis (the sepa­ in forest fuels and over 70 percent and sulfur dioxide. Factors such ration of molecules by light) into of them in organic soils and duff. as time, temperature, sunlight, nitrogen monoxide and oxygen, and the proportion of gases in the with the oxygen atoms (O) then Particulate Matter atmosphere influence these reac­ reacting with the abundant oxygen Particulates released from fire are tions. The result is ultrafine par­ in the atmosphere (O ) to gener­ ticles; water vapor and other gas- 2 mostly carbon based and are typi­ ate ozone (O3). In an atmosphere cally referred to as organic carbon phase species can easily attach and without VOCs, the ozone would and black carbon. Organic carbon other reactions can occur, resulting then react with nitrogen monoxide consists of carbon and hydrogen in particle growth. These small (NO) to regenerate nitrogen dioxide particles can also grow through compounds, oxygenated carbon/ (NO2) and oxygen (O2). However, hydrogen compounds, and other particle-to-particle coagulation.

Fire Management Today 12 the abundant VOCs in the atmo­ Observatory during 32 wildfires a wildland fire. The very dense fog sphere provide for the regeneration from 2004 to 2011. The observatory that results is called superfog, of nitrogen dioxide without remov­ is located in central Oregon atop with visibilities of less than 10 feet ing ozone. the mountain at 9,064 feet (2,763 (3 m) and often less than 3 feet m) above sea level. It is a remote (1 m) (Achtemeier 2002). Smoke plumes are rich sources site, with measurements that have of VOCs and also contain oxides identified episodes of Asian air Fog forms when water vapor con­ of nitrogen (NOX). The NOX are a transport and biomass burning denses into tiny liquid water drop­ function of the nitrogen content across North America. Many of the lets suspended in the air. It nor­ of the fuel and the fuel combus­ air masses measured were local to mally occurs at a relative humidity tion phase. Smoke plumes are said Oregon; but wildfire events from of nearly 100 percent. The presence to be “NOX-limited” in that the British Columbia, California, Idaho, of particles in the air can boost the generation of ozone is limited by Montana, and Washington have also condensation process at humidities the amount of NOX available. By affected the site. as low as 80 percent if the particles contrast, many urban areas tend to attract moisture. Such particles, be VOC-limited in that they have For smoke plumes with less than 2 referred to as hygroscopic, serve as an abundance of NOX in the atmo­ days transport time, fine particulate cloud condensation nuclei. sphere. In the simplest case, when matter (PM2.5) increased over the smoke plumes mix with emissions period, indicating the generation The smoldering combustion of from urban areas with lots of NOX, of secondary organic aerosols. For moist fuels such as organic soil, the extra NOX they get can generate older events, however, the gen­ duff, and logs emits considerable more ozone. eration of fine particulate matter amounts of water vapor and hygro­ was low, indicating that aerosol scopic cloud condensation nuclei. However, smoke is smoky: it blocks removal from the air could exceed When the warm, moist smoke solar radiation, hindering photoly­ generation. Only 13 of the 32 mixes with cold air that already has sis. Oxides of nitrogen can also be plumes measured had significant a relative humidity approaching sequestered in peroxyacetyl nitrate, ozone generation. The two plumes 100 percent, the result is a super­ limiting the generation of ozone. that traveled over the Seattle met­ saturated atmosphere and the for­ However, peroxyacetyl nitrate can ropolitan area had greater ozone mation of superfog. The abundance travel long distances in the atmo­ generation but not necessarily the of cloud condensation nuclei from sphere before mixing into the air highest. Transport time did not the fire makes for very small con­ back down near the ground, where necessarily equate to greater ozone densed droplets. Small droplets are temperatures are warmer. Warmer generation. more effective at scattering light temperatures lead to thermal disas­ than larger droplets, leading to sociation, freeing up NOX to gener­ Superfog greater reductions in visibility. ate more ozone long distances away A major component of the emis­ from a wildland fire. Smaller droplets also have less mass sions from the combustion of veg­ and a slower settling velocity, let­ etation is water vapor. Although ting them remain suspended in the Plume Interactions water vapor is not a pollutant, the atmosphere for longer periods of Wigder and others (2013) ana­ emission of water vapor and partic­ time. Achtemeier (2009) estimated lyzed measurements of fine par­ ulate matter under the right envi­ the liquid water content of superfog ticulate matter, carbon monoxide, ronmental conditions can result to be about 23 times greater than in and ozone at Mount Bachelor in extremely low visibilities near regular fogs. He also found that 1 percent of the particulate emissions from a wildland fire formed enough The water vapor and particulate matter cloud condensation nuclei to shift in smoke can result in superfog, with the distribution of droplet sizes in superfog toward smaller droplets, visibilities of less than 10 feet. reducing visibility to as low as 0.3 feet (0.1 m).

Volume 75 • No. 1 • 2017 13 Achtemeier (2013) created a across northern Idaho and western meteorological and smoke modeling Superfog Index. Ranging from 0 Montana in September 2012, before can now simulate patterns of smoke to 100, the index represents the daytime heating dispersed the dispersion, including inversions. probability of superfog formation. smoke. New advances in fine-scale The index increases rapidly at air temperatures of less than 55 °F (13 °C) when the relative humid­ ity is 90 percent or higher. Figure 2 shows Superfog Index curves for two ambient relative humidity sce­ narios.

Fire managers should be aware of the possibility of superfog forma­ tion. Other critical questions to raise in assessing fog-related risk include the following:

• Do you have a smoldering fire that will burn all night? • Is there a transportation route within 3 miles (4.8 km)? • Is the wind direction toward the road? • Do drainages lead from the fire to the road? • Are temperatures predicted to be Figure 2—Superfog Index indicating the probability of superfog formation as a function less than 50 °F (10 °C)? of ambient air temperature for two different relative humidity (RH) scenarios (yellow = 90 percent; blue = 96 percent) (adapted from Achtemeier (2013)). Smoke in Complex Terrain As the sun goes down, the surface of the Earth cools and inversions form within valleys, trapping smoke and causing smoke concentra­ tions to be higher than they would be if the atmosphere were mixing in cleaner, fresher air. Nighttime drainage flows can also carry smoke for tens of miles down valleys, allowing smoke to affect areas at night that had little or no smoke impacts during the day.

The result is that smoke impacts in valleys near or downwind of fires can be significantly higher than in Figure 3—Satellite view of smoke in valleys over northern Idaho and western Montana flat terrain. Figure 3 gives a satel­ on September 13, 2012, at 1200 mountain daylight time, from the Moderate Resolution lite view of smoke pooled in valleys Imaging Spectroradiometer instrument aboard the National Aeronautics and Space Administration Terra satellite. Cities range from Missoula, MT, in the north; to Salmon, ID, in the south; to Butte, MT, in the east.

Fire Management Today 14 The Challenge of Many of the trace gases emitted VA. Boston, MA: American Meteoroidal from wildland fires have yet to be Society: 15–16 Smoke Achtemeier, G.L. 2009. On the formation identified, as do the intermediary and persistence of superfog in woodland Smoke is challenging. It can be products produced in a plume. smoke. Meteorological Applications. 16: lofted high into the atmosphere With the outlook for more wild­ 215–225. to interact with cloud processes. Achtemeier, G.L. 2013. A Superfog Index fires in the future, especially in a based on historical data. International It can smolder near the ground, changing climate—and with tighter Smoke Symposium; 21–24 October; depositing emissions. health standards—understanding The University of Maryland University these processes will become more College, College Park, MD. The combination of aerosols and Urbanski, S.P. 2014. Wildland fire emis­ critical in the years to come. sions, carbon, and climate: Emission trace gases create their own chemi­ factors. Forest Ecology and Management. cal mix, with reactions that are as References 317: 51–60. yet unidentified. Temperature and Wigder, N.L.; Jaffe, D.A.; Saketa, F.A. 2013. Achtemeier, G.L. 2002. “Super-fog”—A Ozone and particulate matter enhance­ atmospheric water content inter­ combination of smoke and water vapor ments from regional wildfires observed act with the smoke plume and fog that produces zero visibility over road­ at Mount Bachelor during 2004–2011. processes. Smoke also blocks the ways. 12th Joint Conference on the Atmospheric Environment. 75: 24–31.  transmission of solar radiation, hin­ Applications of Air Pollution Meteorology with A&WMA; 20–24 May; Norfolk, dering photolysis reactions.

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