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...