Sources, Distributions, and Fates of VOCs in the Atmosphere

RICHARD G. DERWENT

1 Introduction

Historical Background

The role and importance in atmospheric chemistry of organic compounds produced by human activity was established about fifty years ago by Haagen-Smit in his pioneering studies of Los Angeles smog. He identified the key importance of hydrocarbon oxidation, in the presence of sunlight and oxides of nitrogen, as a photochemical source of ozone 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 modelling studies.

An understanding of the importance of the organic compounds emitted from the natural biosphere developed somewhat later with the recognition of the importance of the isoprene and terpene emissions from plants and trees. The oxidation of these organic compounds leads to the production of carbon monoxide and aerosol particles, the latter being responsible for the haze associated with forested regions.

Since these early pioneering studies, photochemical smog has subsequently been detected in almost all of the world's major urban and industrial centres, at levels which exceed internationally agreed criteria values set to protect human health. Chlorinated organic compounds from human activities now reach the stratosphere, where processing by solar radiation yields active odd-chlorine species which are potent depleting agents of the stratospheric ozone layer.

Despite the importance given now to organic compounds, their routine measurement in the atmosphere has only recently become commonplace. Furthermore, there are few detailed emission inventories for the major urban and industrial centres for which man-made emissions are fully resolved by species. There is much research to be completed into the sources, distributions, and fates of organic compounds before photochemical smog control programmes can deliver the required air quality standards and before the role of organic compounds in the greenhouse effect is fully quantified.

Definitions

Volatile organic compounds, or VOCs, are an important class of air pollutants, commonly found in the atmosphere at ground level in all urban and industrial centres. There are many hundreds 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 volatile organic compounds refers to those organic compounds which are present in the atmosphere as gases, but which under normal conditions of temperature and pressure would be liquids or solids. A volatile organic compound is by definition an organic compound whose vapour pressure at say 20 °C is less than 760 torr (101.3 k Pa) and greater than 1 torr (0.13 k Pa). Many common and important organic compounds would be ruled out of consideration in this review if the upper and lower limits were adhered to rigidly.

In this chapter, 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. These latter compounds are strictly semi-volatile organic compounds. The definition used here includes substituted organic compounds, so that oxygenated, chlorinated, and sulfur-containing organic compounds would come under the present definition of VOC.

Other terms used to represent VOCs are hydrocarbons (HCs), reactive organic gases (ROGs), and non-methane volatile organic compounds (NMVOCs). The use of common names for the organic compounds is preferred in this review 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 where they differ significantly from the common names.

Sources

Organic compounds are present in the atmosphere as a result of human activities, arising mainly from motor vehicle exhausts, evaporation of petrol vapours from motor cars, solvent usage, industrial processes, oil refining, petrol storage and distribution, landfilled wastes, food manufacture, and agriculture. Natural biogenic processes also give rise to substantial ambient concentrations of organic compounds and include the emissions from plants, trees, wild animals, natural forest fires, and anaerobic processes in bogs and marshes.

Concerns

Because of the very large number of individual air pollutants that come within the above definition, their importance as a class of ambient air pollutants has only recently become recognized. Progress has been slow because intensive air monitoring to confirm their occurrence in the ambient atmosphere has only recently been started and because of the lack of basic information with which to target research activities. The situation has improved dramatically over the last few years and the important role played by organic compounds in a range of environmental problems of concern can now be identified.

These important roles are in:

• stratospheric ozone depletion

• ground level photochemical ozone formation

• toxic or carcinogenic human health effects

• enhancing the global greenhouse effect

• accumulation and persistence in the environment

These phenomena are briefly reviewed in the paragraphs below and some are discussed in more detail in the sections which follow.

Stratospheric Ozone Depletion. Many organic compounds are stable enough to persist in the atmosphere, to survive tropospheric removal processes, and to reach the stratosphere. If they contain chlorine or bromine substituents, the processes of stratospheric photolysis and hydroxyl radical destruction may lead to the release of active ozone-destroying chain carriers and to further stimulation of stratospheric ozone layer depletion and Antarctic 'ozone hole' formation. 5 Many chlorinated solvents and refrigerants, and bromine-containing fire retardants and fire extinguishers have been identified as belonging to the category of organic compounds which may lead to stratospheric ozone layer depletion. Such compounds come within the scope and control of the Montreal Protocol.

Ground Level Ozone Formation. Organic compounds play a crucial role in ground level photochemical oxidant formation since they control the rate of oxidant production in those areas where NOx levels are sufficient to maintain ozone production. The term 'hydrocarbons' is widely used in this context to refer to those organic compounds which take part in photochemical ozone production. The contribution that organic compounds make to the exceedence of environmental criteria for ozone across Europe is now widely recognized. Long-range transboundary transport of ozone and action to control its precursors is an important feature of the problem. Organic compounds, which as a class produce photochemical ozone in the troposphere, come within the scope of the Geneva Protocol to the UN ECE International Convention on Long Range Transboundary Air Pollution.

Ground level ozone is of concern 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 both human health and natural ecosystems. It is these concerns which led to the formulation of the Geneva Protocol 10 and which underpin the reductions in emissions and control actions which it stipulates.

Toxic and Carcinogenic Health Effects. Organic compounds may have important impacts on human health through direct mechanisms in addition to their indirect impacts through photochemical ozone formation. Some organic compounds affect the human senses through their odour, some others exert a narcotic effect, and certain species are toxic. Concern is particularly expressed about those organic compounds which could induce cancer in the human population : the human genotoxic carcinogens. The term 'air toxics' is usually given to those organic compounds that are present in the ambient atmosphere and have or are suspected to have the potential to induce cancer in the human population.

The control of air toxics is currently both a national and an international activity, involving a wide range of international forums. A wide range of chemicals are also coming under scrutiny in this context. The most important organic compounds which belong to the air toxic category, and are widely distributed in the ambient atmosphere, include:

• benzene and 1,3-bu tadiene (buta-1,3-diene), as potential leukaemia-inducing agents

• formaldehyde (methanal), as a potential nasal carcinogen

• polynuclear aromatic hydrocarbons, as potential lung cancer inducing agents

• polychlorinated biphenyl compounds (PCBs) and polychlorinated terphenyl compounds (PCTs)

• dioxins and furans

Global Greenhouse Effect. Almost all of the organic compounds emitted as a result of human activities are emitted into the atmospheric boundary layer, the shallow region of the troposphere next to the earth's surface whose depth is typically a few hundred metres in winter to perhaps 2 km in mid-summer. Many of the reactive organic compounds are quickly oxidized in the atmospheric boundary layer. However, some survive and are transported into the free troposphere above the boundary layer during particular meteorological events such as the passage of fronts, convection, and in the passage of air masses over mountains.

Some of the longer-lived organic compounds are accumulating in the troposphere, or may have the potential to do so. If any of these compounds can absorb solar or terrestrial infrared radiation, then they may contribute to the enhanced greenhouse effect. Such com pounds would be classed as radiatively active gases and their relative effectiveness compared with carbon dioxide can be expressed through their Global Warming Potentials (GWPs – see page 105).

Many organic compounds are not themselves radiatively active gases, but they do have the property of potentially being able to perturb the global distributions of other radiatively active gases. If they exhibit this property, then they can be classes as secondary greenhouse gases and indirect GWPs may be defined for them. Organic compounds can behave as secondary greenhouse gases by :

• reacting to produce ozone in the troposphere

• increasing or decreasing the tropospheric •OH distribution and hence perturbing the distribution of methane

Once in the free troposphere, long-lived organic compounds can stimulate ozone production there. Ozone levels in this region are believed to be rising steadily and this is of some concern because ozone is an important global greenhouse gas. However, the importance of the emissions of organic compounds from human activities in the global tropospheric ozone increase is still under evaluation.

Accumulation and Persistence. Some of the higher molecular mass organic compounds are persistent enough to survive oxidation and removal processes in the boundary layer and may be transported over large distances before being removed in rain. There is an important class of organic compounds, the semi-volatile VOCs which, because of their molecular size and complexity' tend to become adsorbed onto the surface of suspended particulate matter. In this form they undergo long-range transport and may be removed in rain remote from their point of original emission. Once deposited in rain, they may re-evaporate back into the atmosphere and begin the cycle all over again. Ultimately this material may be recycled through the atmosphere before reaching its more permanent sink in the colder aquatic environments in polar regions. Biological accumulation in these sensitive environments can lead to toxic levels in human foodstuffs in areas exceedingly remote from the point of original emission.

The identification of those organic compounds which are likely to persist in the environment, to bio-accumulate, and hence to find a pathway back to man, is still in its early days. Already some classes of organic compounds can be identified including the PCBs, PCTs, and phthalic acid and its derivatives. International action has yet to begin to tackle the problems of the long-range transboundary transport of compounds which may persist and accumulate in polar environments.

2 Sources of VOCs

Emission Inventories for European Countries

Emission inventories are now becoming available for the low molecular organic compounds for most European countries and emission estimates are shown in Table 1 for 1989. The countries with the largest emissions appear to be USSR, Italy, and the Federal Republic of Germany. The major source categories identified include mobile sources through all modes of transport, stationary sources including evaporation, solvent usage, the industrial processes of oil refining and chemicals manufacture, oil and gas production, and agriculture.

Altogether, European emissions of low molecular mass volatile organic compounds from human activities amounted to about 23.8 million tonnes yr-1 in 1989. This total is comparable with that of sulfur dioxide (as S) and nitrogen oxides (as NO2), with each of the order of 20 million tonnes yr-1 for Europe as a whole.

Estimated emissions from natural sources are also included in Table 1. The latter are largely thought to be isoprene emissions from deciduous trees. Natural emissions of isoprene appear to be somewhat lower, 4.8 million tonnes yr-1, in total compared with that from man-made sources over Europe as a whole. Emissions from human activities appear to overwhelm natural sources in most countries. However, in some countries the reverse is true, e.g. in Bulgaria and Turkey, natural sources of isoprene predominate. In the United Kingdom, emissions of volatile organic compounds from human activities are about 80 times higher than those of isoprene from natural sources. Atmospheric VOCs from natural sources are discussed in more detail in Nicholas Hewitt's article on page 17.

Methane Emissions in the UK

In the United Kingdom, emissions of methane are subject to large uncertainties and have shown a slight downwards trend over the period 1970 to 1992. An overall decrease in emissions from coal mines over the period has been largely offset by increases from gas leakage, landfill, and offshore oil and gas operations. In 1992, the total UK emissions of methane have been estimated as 4.7 million tonnes yr-1. Animals account for about 30% of total emissions, with the largest contribution being from cattle.

Emissions of Other Organic Compounds in the UK

Emissions of organic compounds (methane excluded) in the UK for 1992 have been reported as 2.6 million tonnes yr-1. A slight (10%) upwards trend in these emissions over the period since the 1970s has been documented. Although there have been improvements in the accuracy of such emission estimates, there still remain substantial (30% ) uncertainties. In 1990, road transport accounted for 41% of the total, with chemical processes and solvents accounting for 50%.

By combining figures for the total emissions by source category with the profile of the mass emissions of individual organic compounds, it is possible to derive national emission estimates for over 90 individual organic compounds. These are shown in Table 2 for the UK summed over all source categories and presented as percentages of the total emissions.

On this basis, the speciated emissions of over 90 individual organic compounds have been identified in UK source categories. n-Butane (butane) appears to account for the greatest percentage, about 7% of the total. Of all the classes of VOC species, the alkanes appear to account for the greatest percentage of UK national emissions.

3 Ambient Concentrations of Organic Compounds

A summary is provided in Table 3 of the measured concentrations of organic compounds at six representative sites along a pollution gradient across Europe. The concentrations steadily decrease through three decades, from the urban kerbside, to urban background, to rural and to remote maritime background sites. Since the concentration ratios do not stay constant over the six sites, it is clear that the air at the remote sites is not merely diluted urban air. Many different sources, as well as depletion by chemical conversion, contribute to the observed spatial patterns and the differences between the mean concentrations at the different sites.

The species distribution of the organic compounds at the remote maritime sites are dominated by two paraffins (alkanes), ethane and propane, presumably reflecting the importance of natural, marine sources. By comparison, the species distribution observed at the urban kerbside site is heavily dominated by ethylene (ethene), n-butane (butane), and acetylene (ethyne), the major components of motor vehicle exhaust.