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TROPOSPHERIC CHEMISTRY AND COMPOSITION | Volatile Organic Compounds Overview
Volatile Organic Compounds Overview: Anthropogenic RG Derwent, rdscientific, Newbury, UK Ó 2015 Elsevier Ltd. All rights reserved.
Synopsis The term anthropogenic volatile organic compounds (VOCs) refers to those organic compounds other than methane that arise from human activities and are capable of producing photochemical oxidants such as ozone by atmospheric chemical reactions with nitrogen oxides in the presence of sunlight. VOCs are an important class of air pollutants commonly found in the atmosphere at ground level in all urban and industrial centers. There are many hundreds to thousands of compounds, which come within the category of VOCs. Not all VOCs contribute equally strongly to photochemical oxidant formation. Some VOCs show a high propensity to form oxidants whilst others are largely unreactive.
Definition The term anthropogenic volatile organic compounds (VOCs) refers to those organic compounds other than methane that arise from human activities and are capable of producing photochemical oxidants such as ozone by atmospheric chemical reactions with nitrogen oxides in the presence of sunlight.
Background The role and importance in atmospheric chemistry of VOCs produced by human activities was established about 60 years ago by Haagen-Smit in his pioneering studies of Los Angeles smog (Haagen-Smit et al., 1953). He identified the key role of hydrocarbon (HC) oxidation, in the presence of sunlight and oxides of nitrogen (NOx), as a photochemical source of ozone (O3) and other oxidants. Detailed understanding of the mechanism of photochemical smog formation has developed since then through the combination of smog chamber, laboratory chemical kinetics, field experiment, air quality monitoring, and computer modeling studies. Since these early pioneering studies in Los Angeles, photochemical smog and the elevated ozone levels associated with it, have subsequently been detected in almost all of the world’s major urban and industrial centers, at levels which exceed internationally agreed criteria set to protect human health (World Health Organisation, 2006). Despite the importance given now to VOCs, their routine measurement in the atmosphere has only recently become commonplace. Furthermore, there are few detailed emission inventories for the major urban and industrial centers for which anthropogenic VOC emissions are fully resolved by species.
Properties of VOCs VOCs are an important class of air pollutants commonly found in the atmosphere at ground level in all urban and industrial centers. There are many hundreds to thousands of compounds, which come within the category of VOCs and the situation is yet further complicated by different definitions and nomenclature. Strictly speaking, the term VOC refers to those organic
Encyclopedia of Atmospheric Sciences 2nd Edition, Volume 6
compounds, which are present in the atmosphere as gases but which under normal conditions of temperature and pressure would be liquids or solids. A VOC is by definition, an organic compound whose vapor pressure at say 20 C is less than 760 torr (101.3 kPa) and greater than 1 torr (0.13 kPa). Many common and important organic compounds would be ruled out of consideration if these upper and lower limits were strictly adhered to. Here, this strict definition is not applied and the term VOC is taken to mean any carbon-containing compound found in the atmosphere, excluding elemental carbon, carbon monoxide, and carbon dioxide. This definition is deliberately wide and encompasses both gaseous carbon-containing compounds and those similar compounds adsorbed onto the surface of atmospheric suspended particulate matter. The definition here includes substituted organic compounds so that oxygenated, chlorinated, and sulfur-containing organic compounds would come under the definition of VOCs. Some of the most strongly and widely emitted VOCs are listed in Table 1. Although this top 50 list has been compiled using emissions data for the United Kingdom, the major VOC sources, such as motor vehicle exhausts and solvent usage, are common across much of northwest Europe and so the list has a wider relevance than just to the United Kingdom. Other terms used to represent VOCs are HCs, reactive organic gases (ROGs), and nonmethane VOCs. The use of common names for the organic compounds is preferred here since these are more readily understood by industry and more commonly used in the air pollution literature. IUPAC names are, however, provided in all cases in Table 1 where they differ significantly from the common names. VOCs play a crucial role in ground level photochemical oxidant formation since they control the rate of oxidant production in the presence of sunlight in those areas where NOx levels are sufficient to maintain ozone production. The contribution that organic compounds make to the exceedance of environmental criteria for ozone is now widely recognized. Long-range transboundary transport of ozone is an important feature of the problem. Organic compounds come within the scope of the Geneva Protocol to the United Nations Economic Commission for Europe International Convention on Longrange Transboundary Air Pollution (http://www.unece.org/ env/lrtap/vola_h1.html). Ground level ozone is of concern
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Tropospheric Chemistry and Composition j Volatile Organic Compounds Overview: Anthropogenic
Table 1 Common names, IUPAC names, percentage emission by mass, and POCPs of the 50 most prolifically emitted VOCs in the United Kingdom
Common name
IUPAC name
n-butane ethanol ethane propane toluene n-pentane i-pentane ethylene n-hexane methanol m-xylene trichloroethene 2-methylpropane formaldehyde acetone n-heptane ethylbenzene propylene n-octane benzene methylethylketone o-xylene 1,2,4-trimethylbenzene dichloromethane butyl acetate acetylene p-Xylene 2-propanol 2-methylpropene ethyl acetate n-decane tetrachloroethene 4-methyl-2-pentanone 2-butene 1-butanol n-nonane 2-butoxyethanol 1,3,5-trimethylbenzene acetaldehyde 1,3-butadiene 2-methylpentane methyl acetate undecane 2-pentene 1-propanol 1-methoxy-2-propanol 1,2,3-trimethylbenzene methylethylbenzene 2-methylhexane 3-methylpentane
butane
methylbenzene pentane 2-methylbutane ethene hexane 1,3-dimethylbenzene
methanal propanone heptane propene octane 2-butanone 1,2-dimethylbenzene
ethyne 1,4-dimethylbenzene
decane
nonane
ethanal
Percentage emission by mass, %
POCP
8.9 7.4 5.0 3.9 3.6 3.5 3.2 2.8 2.6 2.0 2.0 2.0 1.9 1.7 1.6 1.4 1.3 1.2 1.2 1.1 1.1 1.0 1.0 1.0 1.0 0.9 0.9 0.8 0.8 0.8 0.8 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2
31 34 8 14 44 40 34 100 40 13 86 29 28 46 6 35 46 117 34 10 32 78 110 3 26 7 72 18 63 19 36 1 52 113 52 34 45 107 55 89 41 7 36 111 48 34 105 73 32 37
Percentage emissions by mass taken from http://naei.defra.gov.uk/. POCPs taken from Derwent, R.G., Jenkin, M.E., Passant, N.R., Pilling, M.J., 2007. Environmental Science Policy 10, 445–453.
not only with respect to human health but also because of its effects on crops, plants, and trees. Elevated ozone concentrations during summertime photochemical pollution episodes may exceed environmental criteria set to protect human health and natural ecosystems. It is these concerns, which led to the formulation of the Geneva Protocol and which underpin the reductions in emissions and control actions, which it stipulates. Not all VOCs contribute equally strongly to photochemical oxidant formation. Some VOCs are highly reactive and show a high propensity to form oxidants. Others are largely unreactive and make little or no contribution to ground level ozone formation. Reactivity scales have been constructed to provide an indication of each VOCs’ propensity to contribute to ground level ozone formation. One such scale is the photochemical ozone creation potential (POCP) scale (Derwent et al., 2007). POCPs are expressed relative to ethylene (¼100) and give the formation potential of each VOC on a mass emitted basis. The POCPs of the top 50 most strongly and widely emitted VOCs are listed in Table 1. POCPs range from 1 for the most unreactive chlorinated solvent, tetrachloroethylene, a widely used dry cleaning agent to 117 for the highly reactive propylene, an important component of motor vehicle exhaust. Representatives can be found in Table 1 of some of the more important classes of organic compounds including alkanes, alkenes, aromatic, and oxygenated organic species.
Sources of VOCs There are a wide range of sources in urban and industrial areas that give rise to VOC emissions. Motor vehicle traffic is an important source of VOCs, which arise from the exhaust and evaporative emissions of petrol-engined road vehicles. Motor spirit has a high vapor pressure and so emissions of VOCs can occur from moving and stationary vehicles, from the petrol stations when vehicle and storage tanks are being filled, and from the petrol distribution chain that supplies petrol station forecourts. VOC emissions arise from oil refineries and industrial facilities that produce bulk organic chemicals. VOCs are often valuable solvents and are widely used in the home and in industry in paints, lacquers, varnishes, inks, and degreasing agents (http://www.ceip.at/). Some of the most widely emitted VOC species are listed in Table 1, together with their fractional emission rates and POCPs. To reduce exposure levels to elevated ozone levels, actions have been taken to control photochemical ozone formation in many countries across the world. Because ozone is not emitted into the atmosphere and all the ozone present at ground level has been formed there by atmospheric chemical reactions, these control actions have to act on the emissions of the ozone precursors: VOCs and NOx. Actions to control VOC emissions have usually involved fitting three-way exhaust gas catalyst and evaporative canister systems to petrol-engined motor vehicles and by tackling emissions from oil refineries and the petrol distribution chain. Substitution of water-based paints, for example, has led to reductions in solvent emissions from homes. In this way, much progress has been achieved to reduce episodic peak ozone levels and hence exceedances of the environmental criteria set to protect human health, crops, and vegetation.
Tropospheric Chemistry and Composition j Volatile Organic Compounds Overview: Anthropogenic
267
References
Further Reading
Derwent, R.G., Jenkin, M.E., Passant, N.R., Pilling, M.J., 2007. Environmental Science Policy 10, 445–453. Haagen-Smit, A.J., Bradley, C.E., Fox, M.M., 1953. Industrial and Engineering Chemistry Research 45, 2086. World Health Organisation, 2006. Air Quality Guidelines. Global Update 2005. WHO Regional Office for Europe, Copenhagen, Denmark.
Finlayson, B.J., Pitts, Jr., J.N., 2000. Chemistry of the Upper and Lower Atmosphere. Academic Press, San Diego, CA. Seinfeld, J.H., Pandis, S.N., 2006. Atmospheric Chemistry and Physics. John Wiley and Sons, Hoboken, NJ.
Synopsis The term anthropogenic volatile organic compounds (VOCs) refers to those organic compounds other than methane that arise from human activities and are capable of producing photochemical oxidants such as ozone by atmospheric chemical reactions with nitrogen oxides in the presence of sunlight. VOCs are an important class of air pollutants commonly found in the atmosphere at ground level in all urban and industrial centers. There are many hundreds to thousands of compounds, which come within the category of VOCs. Not all VOCs contribute equally strongly to photochemical oxidant formation. Some VOCs show a high propensity to form oxidants whilst others are largely unreactive.
Definition The term anthropogenic volatile organic compounds (VOCs) refers to those organic compounds other than methane that arise from human activities and are capable of producing photochemical oxidants such as ozone by atmospheric chemical reactions with nitrogen oxides in the presence of sunlight.
Background The role and importance in atmospheric chemistry of VOCs produced by human activities was established about 60 years ago by Haagen-Smit in his pioneering studies of Los Angeles smog (Haagen-Smit et al., 1953). He identified the key role of hydrocarbon (HC) oxidation, in the presence of sunlight and oxides of nitrogen (NOx), as a photochemical source of ozone (O3) and other oxidants. Detailed understanding of the mechanism of photochemical smog formation has developed since then through the combination of smog chamber, laboratory chemical kinetics, field experiment, air quality monitoring, and computer modeling studies. Since these early pioneering studies in Los Angeles, photochemical smog and the elevated ozone levels associated with it, have subsequently been detected in almost all of the world’s major urban and industrial centers, at levels which exceed internationally agreed criteria set to protect human health (World Health Organisation, 2006). Despite the importance given now to VOCs, their routine measurement in the atmosphere has only recently become commonplace. Furthermore, there are few detailed emission inventories for the major urban and industrial centers for which anthropogenic VOC emissions are fully resolved by species.
Properties of VOCs VOCs are an important class of air pollutants commonly found in the atmosphere at ground level in all urban and industrial centers. There are many hundreds to thousands of compounds, which come within the category of VOCs and the situation is yet further complicated by different definitions and nomenclature. Strictly speaking, the term VOC refers to those organic
Encyclopedia of Atmospheric Sciences 2nd Edition, Volume 6
compounds, which are present in the atmosphere as gases but which under normal conditions of temperature and pressure would be liquids or solids. A VOC is by definition, an organic compound whose vapor pressure at say 20 C is less than 760 torr (101.3 kPa) and greater than 1 torr (0.13 kPa). Many common and important organic compounds would be ruled out of consideration if these upper and lower limits were strictly adhered to. Here, this strict definition is not applied and the term VOC is taken to mean any carbon-containing compound found in the atmosphere, excluding elemental carbon, carbon monoxide, and carbon dioxide. This definition is deliberately wide and encompasses both gaseous carbon-containing compounds and those similar compounds adsorbed onto the surface of atmospheric suspended particulate matter. The definition here includes substituted organic compounds so that oxygenated, chlorinated, and sulfur-containing organic compounds would come under the definition of VOCs. Some of the most strongly and widely emitted VOCs are listed in Table 1. Although this top 50 list has been compiled using emissions data for the United Kingdom, the major VOC sources, such as motor vehicle exhausts and solvent usage, are common across much of northwest Europe and so the list has a wider relevance than just to the United Kingdom. Other terms used to represent VOCs are HCs, reactive organic gases (ROGs), and nonmethane VOCs. The use of common names for the organic compounds is preferred here since these are more readily understood by industry and more commonly used in the air pollution literature. IUPAC names are, however, provided in all cases in Table 1 where they differ significantly from the common names. VOCs play a crucial role in ground level photochemical oxidant formation since they control the rate of oxidant production in the presence of sunlight in those areas where NOx levels are sufficient to maintain ozone production. The contribution that organic compounds make to the exceedance of environmental criteria for ozone is now widely recognized. Long-range transboundary transport of ozone is an important feature of the problem. Organic compounds come within the scope of the Geneva Protocol to the United Nations Economic Commission for Europe International Convention on Longrange Transboundary Air Pollution (http://www.unece.org/ env/lrtap/vola_h1.html). Ground level ozone is of concern
http://dx.doi.org/10.1016/B978-0-12-382225-3.00422-9
265
266
Tropospheric Chemistry and Composition j Volatile Organic Compounds Overview: Anthropogenic
Table 1 Common names, IUPAC names, percentage emission by mass, and POCPs of the 50 most prolifically emitted VOCs in the United Kingdom
Common name
IUPAC name
n-butane ethanol ethane propane toluene n-pentane i-pentane ethylene n-hexane methanol m-xylene trichloroethene 2-methylpropane formaldehyde acetone n-heptane ethylbenzene propylene n-octane benzene methylethylketone o-xylene 1,2,4-trimethylbenzene dichloromethane butyl acetate acetylene p-Xylene 2-propanol 2-methylpropene ethyl acetate n-decane tetrachloroethene 4-methyl-2-pentanone 2-butene 1-butanol n-nonane 2-butoxyethanol 1,3,5-trimethylbenzene acetaldehyde 1,3-butadiene 2-methylpentane methyl acetate undecane 2-pentene 1-propanol 1-methoxy-2-propanol 1,2,3-trimethylbenzene methylethylbenzene 2-methylhexane 3-methylpentane
butane
methylbenzene pentane 2-methylbutane ethene hexane 1,3-dimethylbenzene
methanal propanone heptane propene octane 2-butanone 1,2-dimethylbenzene
ethyne 1,4-dimethylbenzene
decane
nonane
ethanal
Percentage emission by mass, %
POCP
8.9 7.4 5.0 3.9 3.6 3.5 3.2 2.8 2.6 2.0 2.0 2.0 1.9 1.7 1.6 1.4 1.3 1.2 1.2 1.1 1.1 1.0 1.0 1.0 1.0 0.9 0.9 0.8 0.8 0.8 0.8 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2
31 34 8 14 44 40 34 100 40 13 86 29 28 46 6 35 46 117 34 10 32 78 110 3 26 7 72 18 63 19 36 1 52 113 52 34 45 107 55 89 41 7 36 111 48 34 105 73 32 37
Percentage emissions by mass taken from http://naei.defra.gov.uk/. POCPs taken from Derwent, R.G., Jenkin, M.E., Passant, N.R., Pilling, M.J., 2007. Environmental Science Policy 10, 445–453.
not only with respect to human health but also because of its effects on crops, plants, and trees. Elevated ozone concentrations during summertime photochemical pollution episodes may exceed environmental criteria set to protect human health and natural ecosystems. It is these concerns, which led to the formulation of the Geneva Protocol and which underpin the reductions in emissions and control actions, which it stipulates. Not all VOCs contribute equally strongly to photochemical oxidant formation. Some VOCs are highly reactive and show a high propensity to form oxidants. Others are largely unreactive and make little or no contribution to ground level ozone formation. Reactivity scales have been constructed to provide an indication of each VOCs’ propensity to contribute to ground level ozone formation. One such scale is the photochemical ozone creation potential (POCP) scale (Derwent et al., 2007). POCPs are expressed relative to ethylene (¼100) and give the formation potential of each VOC on a mass emitted basis. The POCPs of the top 50 most strongly and widely emitted VOCs are listed in Table 1. POCPs range from 1 for the most unreactive chlorinated solvent, tetrachloroethylene, a widely used dry cleaning agent to 117 for the highly reactive propylene, an important component of motor vehicle exhaust. Representatives can be found in Table 1 of some of the more important classes of organic compounds including alkanes, alkenes, aromatic, and oxygenated organic species.
Sources of VOCs There are a wide range of sources in urban and industrial areas that give rise to VOC emissions. Motor vehicle traffic is an important source of VOCs, which arise from the exhaust and evaporative emissions of petrol-engined road vehicles. Motor spirit has a high vapor pressure and so emissions of VOCs can occur from moving and stationary vehicles, from the petrol stations when vehicle and storage tanks are being filled, and from the petrol distribution chain that supplies petrol station forecourts. VOC emissions arise from oil refineries and industrial facilities that produce bulk organic chemicals. VOCs are often valuable solvents and are widely used in the home and in industry in paints, lacquers, varnishes, inks, and degreasing agents (http://www.ceip.at/). Some of the most widely emitted VOC species are listed in Table 1, together with their fractional emission rates and POCPs. To reduce exposure levels to elevated ozone levels, actions have been taken to control photochemical ozone formation in many countries across the world. Because ozone is not emitted into the atmosphere and all the ozone present at ground level has been formed there by atmospheric chemical reactions, these control actions have to act on the emissions of the ozone precursors: VOCs and NOx. Actions to control VOC emissions have usually involved fitting three-way exhaust gas catalyst and evaporative canister systems to petrol-engined motor vehicles and by tackling emissions from oil refineries and the petrol distribution chain. Substitution of water-based paints, for example, has led to reductions in solvent emissions from homes. In this way, much progress has been achieved to reduce episodic peak ozone levels and hence exceedances of the environmental criteria set to protect human health, crops, and vegetation.
Tropospheric Chemistry and Composition j Volatile Organic Compounds Overview: Anthropogenic
267
References
Further Reading
Derwent, R.G., Jenkin, M.E., Passant, N.R., Pilling, M.J., 2007. Environmental Science Policy 10, 445–453. Haagen-Smit, A.J., Bradley, C.E., Fox, M.M., 1953. Industrial and Engineering Chemistry Research 45, 2086. World Health Organisation, 2006. Air Quality Guidelines. Global Update 2005. WHO Regional Office for Europe, Copenhagen, Denmark.
Finlayson, B.J., Pitts, Jr., J.N., 2000. Chemistry of the Upper and Lower Atmosphere. Academic Press, San Diego, CA. Seinfeld, J.H., Pandis, S.N., 2006. Atmospheric Chemistry and Physics. John Wiley and Sons, Hoboken, NJ.