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Home , Combustion

CHAPTER 4 . EMISSIONS

Armistead (Ted) Russell 1

Many of the toxic air pollutants, or air toxics, combustion, including diesel PM, 1,3-butadiene, to which we are exposed are combustion-related benzene, and carbonyls (e.g. aldehydes, primarily (Kinney et al., 2002; Lim and Turpin, 2002; ). Combustion-related sources, SCAQMD, 2000; Manchester-Neesvig et al., including automobiles and indoor heating and 2003). The International Agency for Research , are widespread and tend to be associ- on Cancer (IARC) list of Group 1 carcinogens ated with more populated areas, leading to high includes a large number of these agents, including potential exposures and health risks. benzene, diesel exhaust, benzo[a] (B[a]P, a Combustion is the reaction between a polycyclic aromatic [PAH]), indoor and oxidant accompanied by the release of : emissions from combustion, and 1,3-buta- Fuel + Oxidant → Products + Heat diene. exhaust is a Group 2B carcinogen, although it contains benzene, Typically, the fuel is carbonaceous, such as B[a]P, and 1,3-butadiene. ( is also gasoline, , or coal, and the oxidant is the a combustion product with similarities to other in air, although there are non-carbo- combustion emissions, but is not covered here.) naceous , notably . While such The World Health Organization, in the Global processes are rare, combustion can take place Burden of Disease study, found that emissions without using air to provide the oxidant. The from indoor fuel combustion and urban partic- heat generated by combustion is typically used ulate matter (PM) (much of which is combus- for cooking, heating, or producing power. The tion-related) are leading causes of premature main products of hydrocarbon fuel combustion mortality from environmental exposures (Ezzati are dioxide (CO2) and . However, et al., 2006). The Multiple Air Toxics Exposure combustion can lead to emissions of other Study II (MATES-II; SCAQMD, 2000) of air compounds due to impurities in the fuel, the pres- toxics in the Los Angeles, California, region ence of in air, or incomplete combus- found that of the air toxics studied, the four tion. Specific sources of potentially carcinogenic compounds that had the greatest potential risk, air pollutant emissions that involve combustion combining both the estimated potential expo- include internal combustion (ICEs) (e.g. sure and carcinogenicity, were primarily from diesel, gasoline, turbine), external combustion

1 Armistead Russell is an associate editor of Environmental Science & Technology, a journal of the American Chemical Society. He received significant funding for research at the Georgia Institute of Technology from Phillips 66, a company with business in refineries, and from Southern Company, an company serving the Southeast of the USA.

37 IARC SCIENTIFIC PUBLICATION – 161 (as used for generation), cement kilns, biomass burning (for cooking, heating, Table 4 1. Examples of combustion-derived air toxics land management, and unplanned ), combustion, and more (Smith, 2002; Lim, 2004; Gas phase Particulate Semivolatilea Zielinska et al., 2004a, 2004b; Lemieux et al., matter 2004). Formaldehyde Diesel Polycyclic aromatic Here, we address combustion-related emis- sions, beginning with a brief discussion of Polychlorinated biphenyls combustion-derived pollutants, the combus- Acrolein Furans tion process, and toxic pollutant formation. Benzene Combustion sources are identified and the asso- Toluene ciated types of emissions discussed. Automotive o-, m-, p-Xylenes emissions and biomass burning are major 1,3-Butadiene contributors to potentially harmful exposures, a May be found in either gas phase or primarily condensed phase as and are addressed in more detail in other chap- particulate matter. ters. When source emissions are discussed, as gases or as part of the PM, including many of potential controls are also identified. the PAHs (Simoneit et al., 2004; Zielinska et al., 2004a, 2004b). Combustion-derived air toxics Inorganic emissions of concern include acids, such as sulfuric and hydrochloric acid, sulfur A large fraction, but not all, of the air toxics and nitrogen (NOx), and minerals. These emitted and/or formed during combustion are are usually derived from contaminants in the organic molecules and carbonaceous struc- fuel, although NOx are formed, in part, from the tures. Some are relatively simple molecules, nitrogen that makes up the bulk of air. Sulfur, such as formaldehyde (HCHO), increasing in which is present in many fossil fuels, is oxidized complexity to compounds like 1,3-butadiene, during combustion to both (SO2) aromatics, PAHs, and dioxins. Some sources and . Sulfur trioxide condenses emit , which is condensed organic material, with water to form . reacts part of which may approach elemental carbon. with hydrogen during combustion to form (Elemental carbon [EC] is currently operationally hydrochloric acid. Coal and oil can contain a defined, i.e. it is dependent on the technique used variety of minerals, including iron and silicon for quantification. Black carbon [BC] is similar, oxides. These minerals typically are emitted as although not identical, to EC, and concentra- small particles. tions of BC and EC tend to be highly correlated.) Soot is not composed of a single type of - cule but is made up of a variety of lower-volatility Combustion process compounds, including PAHs, possibly on a core Combustion is a complex phenomenon of a structure that resembles EC (although it can involving chemical reactions and heat and mass contain a variety of impurities). Table 4.1 lists transfer occurring on scales from atomic to several combustion-derived organic air toxics, potentially centimetres (e.g. a engine), metres some of which, such as benzene, 1,3-butadiene, (e.g. a coal ), or kilometres (forest and diesel exhaust, have been classified by IARC fires). Unless the fuel and oxidant are both simple as known human carcinogens. Several these molecules (e.g. hydrogen and oxygen, leading to species are semivolatile and may be found either

38 and cancer water), the products of combustion can be, and such as , the chemistry becomes compli- often are, as complex and varied as the combus- cated. This sequence shows only a fraction of the tion processes, forming as many compounds as possible reactions; Held and Dryer (1998) show originally present, if not more. Some of these 89 reactions for oxidation, excluding species, such as 1,3-butadiene and PAHs, are any reactions involving NOx formation. If one known human carcinogens. Even a molecule considers combustion in fuel-rich conditions, as simple as methane burning in air can lead to the combustion mechanism becomes many-fold larger molecules and soot. larger. For a two-carbon molecule, the complexity The heterogeneity of real-world combustion of the mechanism increases dramatically; more leads to the complexity of the process and the so for more complex molecules. Second, formal- wide range of compounds that are formed. In dehyde, an air toxic, is produced. It may also and around the combustion zone, the concen- be destroyed later, but it is formed and can be trations of species (e.g. the fuel, oxidant, and emitted if the time for combustion is short (e.g. in combustion products) vary by orders of magni- an ICE). Larger molecules will lead to the forma- tude over molecular scales. also can tion of many other, more complex intermediates, change rapidly over a few millimetres (if not less). including higher aldehydes (e.g. acetaldehyde In addition, fronts can move quickly; thus, from ) and other air toxics (e.g. 1,3-buta- the fuel can heat rapidly, start to combust, and diene). Third, CO2 is the final end product, but then cool, quenching further reaction. Similarly, only after several intermediate reactions. CO part of the flame can have excess oxygen while oxidation is relatively slow, so if conditions do another part has excess fuel. not permit more complete combustion (i.e. low During combustion, the fate of organic fuel temperature, limited availability of oxygen, and molecules is largely determined by the local short residence times), large quantities of CO can conditions (e.g. temperature, abundance of be emitted (e.g. in an automobile engine where oxygen, and time). If ample oxygen is present residence time is limited and the car may be locally, i.e. directly where the combustion is forced into an oxygen-limited condition during occurring, the fuel will tend to oxidize and break higher loads). Again, for larger molecules more down to smaller organic molecules until ulti- reactions are required to form those ultimate mately forming (CO) and CO2. products. For example, consider a mechanism sequence for The above reaction mechanism assumed methane oxidation: ample oxygen and sufficient time to react, and did not consider the possibility of organic CH + OH• → CH •+ H O 4 3 2 intermediates reacting with each other to form • • CH3 + O2 → CH3O2 larger organic molecules, which can be important • • CH3O2 → HCHO + HO to emissions of toxics. If, instead of reacting with oxygen, the methyl radical (CH •) reacts with • → • 3 HCHO + OH H2O + HCO another methyl radical, can be formed, • • HCO + O2 → CO + HO2 along with even larger molecules:

• • • • • • CO + OH → CO2 + H CH3 + CH3 → C2H6 … → C2H5 + CH3 → C3H8 … where the • indicates a very reactive radical Larger molecules can then continue to react, intermediate. ultimately forming various air toxics, including A few characteristics of this process are PAHs and soot, as discussed below. important. First, even for a simple molecule

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In general, the emissions of air toxics from oxygen are present, the 1,3-butadienyl radical combustion can be reduced by raising the will instead oxidize to aldehydes and other temperature of combustion, increasing oxygen oxygenated organics with fewer carbon atoms. availability, and allowing a longer reaction time. For example, such conditions are in place for Soot formation coal combustion in power plants, where rela- tively small (but non-zero) amounts of air toxics Current evidence indicates that PM has are emitted. However, ICEs have a more limited somewhat greater health impact than other air residence time in the combustion region, cooler pollutants. Results from Laden et al. (2000) and reaction zones (particularly near the walls), and others (Metzger et al., 2004) suggest that mobile areas of reduced oxygen. Thus, one finds higher source-derived and/or carbonaceous PM may levels of products of incomplete combustion, have greater impacts than other components. such as CO, aldehydes, 1,3-butadiene, PAHs, and The MATES-II study found that diesel PM is the soot, emitted from such systems. major air toxic of concern in the Los Angeles, California, region (SCAQMD, 2000) (Fig. 4.1). Polycyclic aromatic hydrocarbon These results raise the question as to the forma- emissions tion of carbonaceous PM emissions, for example soot during combustion. PAHs can be formed during combustion Soot formation is related to PAH formation, when carbonaceous (organic) fuels are used. The and aromatic fuels tend to produce more soot formation of PAHs is not so surprising when the than others (Flagan and Seinfeld, 1988; Haynes, fuel (or the engine lubricant) already includes 1991). In one mechanism, PAHs continue to aromatics, but even those engines using so-called grow as discussed above until they can condense clean fuels, involving smaller organic molecules, and ultimately form solid particles. Again, even such as methane, can produce PAHs. smaller organics can form soot; one mechanism Aromatics can grow to PAHs by addition of is the formation of PAHs as discussed above, non-aromatic molecules to an already existing another is via the formation of polyacetylenes aromatic structure, or by reacting directly with and continued reduction of the H:C ratio (soot other aromatic radicals. After a two-ring PAH is has a very low H:C ratio, approaching that of EC). formed, by further reaction (again either by addi- For example, tion of non-aromatic radical intermediates or by • • C2H2 + C2H → C4H2 + H ; C4H2 + C2H2 → C6H2 + H2 …→ C8H2 reacting with aromatic radicals), three-ring and higher chains are formed. Non-aromatic molecules (e.g. and This process can continue until much larger, olefins) also form PAHs, but at much lower effi- non-volatile structures (i.e. soot) are formed. Soot ciencies. While a variety of mechanisms exist, for can also be produced by the removal of hydrogen example consider the reaction involving acety- from liquid carbonaceous fuels. lene (C H ) and the 1,3-butadienyl radical (C H ): Critical to soot formation and growth is 2 2 4 5 particle inception, where the first identifiable • C2H2 + C4H5 → C6H6 (benzene) solid particles are formed. These particles are on the order of a nanometre and are composed of The benzene can then grow by addition of sheets of on the order of 100 carbon atoms. After more organic chain radicals with and without inception, these particles can grow more rapidly aromatic structures. If significant levels of by further reaction with organics on their surfaces

40 Air pollution and cancer

Fig. 4.1 Cancer risks at the MATES-II fixed sites

Other

100% Carbonyls (e.g. aldehydes) 80% Benzene

60% 1,3 Butadiene 40% Diesel Particulate Matter 20% 0% Risk (in one million)

Risks are shown for all sources, including diesel (top). The “other” portion is primarily non-combustion sources, although it includes PAHs not associated with diesel particulate matter. Compiled from SCAQMD (2000). and by condensation of non-volatile species as simultaneously in appreciable quantities in both the environment cools. They can also coagulate the gas and particle phases. with other soot particles, leading to long-chain, fractal aggregates composed of hundreds of Coke and formation smaller spheres. The particles can grow by many orders of magnitude, often to diameters of 0.1 µm Coal and fuel oil combustion can lead to or more, before being emitted. the formation of char and coke. These are the Once formed, soot out more slowly. carbonaceous residue particles that remain if the These kinetics partially explain why diesel vehi- original solid or does not have time cles emit soot even though they are operated fuel- to fully combust. Char is formed as the volatile lean (oxygen-rich overall). Near the fuel droplets components in the coal escape due to the high and on cylinder surfaces, the combustion can be , leaving the solid, nearly EC struc- taking place in a fuel-rich environment, leading ture behind. Coke is formed from the liquid- to formation of PAHs and soot. There is not phase of fuel oil. Ample reaction time ample time to oxidize the soot particles when can allow oxidation of char and coke. they reach an oxygen-rich zone. Soot from virtually any source (e.g. diesel engines, biomass burning, cooking of meat) is formation composed of a large number of different organic Coal and, to a lesser extent, heavy fuel molecules, from very large, very low-volatility contain non-combustible materials such as compounds to semivolatile species that are found minerals, including silicon, nickel, aluminium, and calcium, and trace quantities of other metals

41 IARC SCIENTIFIC PUBLICATION – 161 like selenium, cadmium, and so on as inclusions oil (Sheesley et al., 2009; Goldstein, 2012). There is in the fuel. As the fuel burns, these inclusions evidence that with time the lubricating oil in the become molten and agglomerate with each other. crankcase is enriched in PAHs by leakage from A small fraction of the mineral material can also the pistons, further increasing such emissions. vapourize and then condense as the temperature ICEs also emit NOx from fixation of the nitrogen cools. The particles formed are initially quite in air. Automotive emissions have evolved over small (< 0.01 μm) but grow due to the molten time as various controls, such as catalysts, have material agglomeration, condensation, and coag- been implemented. These controls have dramat- ulation, leaving a large fraction of the produced ically reduced the amounts of emissions (by an particles > 1 μm. Condensing species include order of magnitude for some compounds) but vapourized minerals, sulfuric acid, and organics. have altered the composition as well, and some Chowdhury (2004) reported on the analysis of pollutants, such as and hydrogen coal fly ash and found elevated levels of a variety cyanide, can be formed due to the controls (Baum of PAHs, including pycene. et al., 2007).

Combustion sources and systems Alternative automotive fuels ICEs, both CI and SI, can be operated on There are a variety of combustion systems non-traditional fuels. This is often done to lower that lead to potentially toxic emissions. In more emissions, although other reasons exist (e.g. as a developed countries, emissions from traditional renewable energy source). For example, SI engines diesel engines (also referred to as compression sometimes operate on alcohols (methanol and ignition [CI] engines) and gasoline-fuelled ethanol), (primarily methane), or ignition (SI) engines contribute significantly liquefied gas (largely ). Diesel to human exposure. They are discussed briefly engines can be operated on a range of fuels as below and in more detail in other chapters. well, including natural gas and . Biomass fuel combustion is similarly dominant SI engines using alcohols and natural gas in rural parts of developing countries and is also tend to have simpler, although not necessarily discussed in more detail in other chapters. Other less toxic, emissions compared with those using major sources of concern are discussed below. gasoline. Alcohols used are largely one- and two-carbon molecules and naturally have oxygen Internal combustion engines present, which can lead to lower CO emissions ICEs, primarily diesel (or CI) and gasoline and much lower emissions of air toxics such as (or SI) engines, are typified by having limited 1,3-butadiene, benzene, and PAHs. While often time for combustion in the cylinder. This short viewed as a clean-burning fuel, alcohols tend to time leads to a relative abundance of products form greater quantities of aldehydes with the same of incomplete combustion. When traditional carbon number as the parent fuel. For example, hydrocarbon-based fuels are burned, such prod- ethanol use leads to increased emissions of acet- ucts include: CO and NOx; unburnt organics (e.g. aldehyde, and methanol use leads to larger quan- benzene that was present in the fuel); partially tities of formaldehyde being produced (NRC, oxidized organics, such as aldehydes; products 1996). Like conventionally fuelled vehicles, part of pyrolysis, such as PAHs; and soot (Haynes, of the emissions are due to lubricating oil. 1991). Part of the emissions from ICEs are due to incomplete combustion of fuel and lubricating

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Combustion of municipal or medical solid and mercury). Combustion can be assisted by waste burning a higher quality fuel like natural gas to raise the temperature and obtain more complete Combustion is a widely used practice to MSW destruction. Post-combustion controls, deal with municipal solid waste (MSW) (trash), such as and fabric filters, have substan- competing with landfilling. While it has the tially lowered emissions. advantage of greatly reducing the volume of waste to be disposed of, it can lead to emissions of compounds viewed as potentially toxic. Three classes of compounds stand out: chlorinated Coal is the dominant used for gener- organics (e.g. polychlorinated biphenyls [PCBs], ating electricity and is the closest to being pure dioxins, and furans), PAHs, and mercury. carbon. However, it still contains hydrogen and MSW combustion is typically conducted at a range of other elements, including sulfur and relatively lower temperatures and in less than nitrogen (possibly in relatively large amounts), optimal conditions with an inferior, more heter- chlorine, and metals such as iron, selenium, and ogeneous fuel than in utility boilers and auto- mercury. The chlorine can react to form hydro- mobiles. Even in controlled combustion with a chloric acid, the sulfur to form SO2 and sulfuric relatively homogenous fuel, combustion is subject acid, and the nitrogen to form NOx, which is to widely varying conditions, leading to forma- also formed from oxidation of the nitrogen in tion of undesirable products. The heterogeneity air. Coal-fired utilities remain one of the largest of MSW exacerbates problems, including having anthropogenic sources of both SO2 and NOx, areas of lower oxygen and temperatures, leading both of which lead to the formation of acids in to incomplete combustion. MSW also contains a the . Further, NOx plays a critical much wider range of compounds than virtually role in driving the photochemical production of any other combustion fuel, basically because it and secondary particulates, which include comes from whatever may be thrown away. This nitrated PAHs and organic and inorganic acids. includes items like batteries, cans, , news- Coal combustion is a major anthropogenic print, biomass, used oil, paint, and so on. Many source of mercury to the atmosphere, along of these contain toxic metals such as mercury, with MSW combustion (Seigneur et al., 2004). lead, and chromium. During the combustion is toxic and bioaccumulates; process, these metals can be released, either as levels in fish have become dangerously high. a gas or as PM, while a good fraction can be Much of the mercury emitted from power plants, removed as ash. MSW combustion was a domi- however, is in the elemental, gaseous form, which nant source of mercury, although controls are is not assimilated directly by plants and animals. effective at removing this pollutant. A sizable However, elemental mercury does slowly oxidize fraction of MSW can contain chlorine and other in the atmosphere and then deposits to the halogens, which can add to the organic molecules ground and surface , where it can enter to produce dioxins and dibenzofurans. the food chain. In addition, coal-fired utilities MSW combustion emissions can be emit fly ash containing a variety of minerals and controlled in a variety of ways. First, the fuel can metals. be prepared to improve the combustion charac- Controls on coal-fired utility boilers include teristics, for example pelletized. MSW that has filters (baghouses) (for controlling PM), scrub- potentially harmful compounds can be removed bers (SO2, PM, mercury), electrostatic precipi-

(e.g. waste with harmful metals such as lead tators (PM), selective catalytic reduction (NOx),

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burner modification (e.g. low NOx burners), and Another fuel used by cement kilns is old tyres fuel modification, including the use of cleaner (or other waste with heating value), which can , washing, and using alternative fossil fuels lead to somewhat higher levels of soot and PAHs, such as natural gas. Such controls have effectively although the long residence time tends to mini- reduced emissions by > 80% for NOx and SO2 and mize such emissions. by > 99% for PM. Use of industrial process boilers to provide Because of environmental and economic heat and steam is also widespread. Emissions concerns, natural gas has become increasingly from such sources are fuel-specific, with coal, popular for producing electricity, while the use natural gas, and oil being the dominant fuels of fuel oil is declining. Natural gas combustion used. Emissions from industrial boilers will be is typically cleaner than coal combustion for similar to those from utility boilers using the a variety of reasons. The gas phase allows for same fuel, recognizing that the controls may not more homogeneous combustion conditions, be as extensive since the source is smaller. and the smaller molecules being burned with While combustion often is cited for increasing ample residence time and excess oxygen lead to emissions, flaring has been used to reduce emis- almost complete combustion to CO2 and water. sions of volatile organic compounds (VOCs), Trace species (e.g. small amounts of sulfur, but including toxic compounds such as benzene and virtually no minerals and other contaminants) 1,3-butadiene. While this approach is effective and thermal fixation of nitrogen do lead to small at removing the parent VOC, it can lead to the amounts of air pollutant emissions. When used production of lesser amounts of organic toxics, as the primary fuel, natural gas is usually applied for example PAHs. Catalytic destruction is also in turbines. Combustion is typically conducted used to help remove unwanted VOCs before at high temperatures with a reasonably long resi- emission from industrial facilities. dence time, leading to more complete combus- tion and lower air toxics emissions. Residential coal combustion

Industrial process combustion While not widely used in residential appli- cations in developed countries, coal is still used Several industries use combustion for for heating and cooking in developing countries purposes such as producing heat and destroying where it is a plentiful resource. In this case, the undesirable compounds. An example that may conditions are not as favourable to complete do both is using cement kilns to destroy toxic combustion as in typical industrial or utility waste such as PCBs. Cement kilns are used in facilities, and post-combustion controls will be the process of calcining cement, which requires minimal or non-existent. In such cases, emis- high temperatures. The residence time during sions can be quite large and will include soot, combustion is relatively long, so it was proposed char, ash, PAHs, and CO. The enclosed environ- to toxic organics, which have a heating ments further lead to potentially very high expo- value, along with traditional fuels, to destroy the sures (Smith, 2002). undesirable compounds. While this does destroy almost all of the original organic material, there Biomass combustion can be some slippage (typically < 0.01%) in addi- tion to the products of incomplete combustion. Burning modern fuel (usually biomass) occurs Much of the time coal is used, leading to emis- both intentionally (e.g. for heating, cooking, or sions similar to those of coal-fired power plants. land management) and unintentionally (forest

44 Air pollution and cancer fires). On a worldwide scale this process leads Summary to more PM emissions (and likely more toxics) than does the combustion of fossil fuels, which Combustion is a ubiquitous process leading is typically done under more controlled condi- to environmental air toxics exposures and emis- tions. In developed countries, however, biomass sions of a wide range of species. Combustion- combustion usually occurs in less densely popu- generated compounds can be either organic (e.g. lated areas and the resulting exposures are not aldehydes and PAHs) or inorganic (e.g. acids as severe (unless one includes , and metals such as mercury). In more developed the major exposure to air toxics of all sources). countries, motor vehicle emissions (both on- and However, biomass combustion is still a major off-) play an important role in exposure to air contributor to PM in many urban areas (Schauer pollution, and key pollutants include PM (diesel et al., 1996; Zheng et al., 2000, 2002, 2005) and is and SI), benzene, 1,3-butadiene, aldehydes, and discussed in more detail in other chapters. other organics, although levels are decreasing due to enhanced controls. Use of alternative Anthropogenic fugitive combustion fuels does not eliminate emissions of air toxics, although they will generate mixtures that differ Building fires can lead to the emissions of from those of conventional fuels. In developing PCBs, for example from the formation of PAHs countries, indoor combustion of biomass or coal and soot during the low-temperature smoul- is a continuing source of concern, along with dering and burning of plastics and electrical increasing exposures to automobile emissions as wiring insulation. This occurred after the attack vehicle fleets grow. on the World Trade Center, where the smoul- Electricity generation tends to produce lower dering wreckage led to exposure to air toxics emissions of organic compounds as combus- (Pleil et al., 2004). Fires in scrap tyre stockpiles tion is more complete. Power plants do emit and areas with concentrated refuse catching PM (the amount largely depending on how well can lead to emissions of toxics (including organ- controlled the facility is), sulfur oxides, NOx, ic-laden PM) (Christian et al., 2010; Lemieux and, of particular concern recently, mercury, et al., 2004). although various controls have proven effective at reducing emissions. Historically, MSW combus- Meat cooking tion has also been one of the major sources of mercury and leads to the formation of halogen- While meat cooking is not often considered ated organics such as dioxins and furans. a major source of PM, Cass and co-workers Control of air toxics emissions from combus- (Kleeman et al., 1999; Zheng et al., 2002) have tion sources generally relies on improving found surprisingly large quantities of carbo- combustion conditions and post-combustion naceous PM in urban areas to be due to meat controls, including scrubbers, filters, electro- cooking. In this case, the fat from the cooking static precipitators, flaring (which can form other process can be volatilized and/or partially pollutants), and catalytic destruction. combusted to form less-volatile compounds. The volatilized material then condenses back onto pre-existing particles. Compounds formed from meat cooking include PAHs.

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