CHAPTER 1
Introduction
ROY M. HARRISON
1 THE ENVIRONMENT AL SCIENCES
It may surprise the student of today to learn that 'the environment' has not always been topical and indeed that environmental issues have become a matter of widespread public concern only over the past twenty years or so. Nonetheless, basic environmental science has existed as a facet of human scientific endeavour since the earliest days of scientific investigation. In the physical sciences, disciplines such as geology, geophysics, meteorology, oceanography, and hydrology, and in the life sciences, ecology, have a long and proud scientific tradition. These fundamental environmental sciences underpin our understanding of the natural world, and its current-day counterpart perturbed by human activity, in which we all live.
The environmental physical sciences have traditionally been concerned with individual environmental compartments. Thus, geology is centred primarily on the solid earth, meteorology on the atmosphere, oceanography upon the salt water basins, and hydrology upon the behaviour of freshwaters. In general (but not exclusively) it has been the physical behaviour of these media which has been traditionally perceived as important. Accordingly, dynamic meteorology is concerned primarily with the physical processes responsible for atmospheric motion, and climatology with temporal and spatial patterns in physical properties of the atmosphere (temperature, rainfall, etc.). It is only more recently that chemical behaviour has been perceived as being important in many of these areas. Thus, while atmospheric chemical processes are at least as important as physical processes in many environmental problems such as stratospheric ozone depletion, the lack of chemical knowledge has been extremely acute as atmospheric chemistry (beyond major component ratios) only became a matter of serious scientific study in the 1950s.
There are two major reasons why environmental chemistry has flourished as a discipline only rather recently. Firstly, it was not previously perceived as important. If environmental chemical composition is relatively invariant in time, as it was believed to be, there is little obvious relevance to continuing research. Once, however, it is perceived that composition is changing (e.g. CO2 in the atmosphere; 137Cs in the Irish Sea) and that such changes may have consequences for humankind, the relevance becomes obvious. The idea that using an aerosol spray in your home might damage the stratosphere, although obvious to us today, would stretch the credulity of someone unaccustomed to the concept. Secondly, the rate of advance has in many instances been limited by the available technology. Thus, for example, it was only in the 1960s that sensitive reliable instrumentation became widely available for measurement of trace concentrations of metals in the environment. This led to a massive expansion in research in this field and a substantial downward revision of agreed typical concentration levels due to improved methodology in analysis. It was only as a result of James Lovelock's invention of the electron capture detector that CFCs were recognized as minor atmospheric constituents and it became possible to monitor increases in their concentrations (see Table 1). The table exemplifies the sensitivity of analysis required since concentrations are at the ppt level (1 ppt is one part in 10l2 by volume in the atmosphere) as well as the substantial increasing trends in atmospheric halocarbon concentrations, as measured up to 1990. The implementation of the Montreal Protocol, which requires controls on production of CFCs and some other halocarbons, has led to a slowing and even a reversal of annual concentration trends since 1992 (see Table 1).
2 THE CHEMICALS OF INTEREST
A very wide range of chemical substances are considered in this book. They fall into three main categories:
(a) Chemicals of concern because of their human toxicity. Some metals such as lead, cadmium and mercury are well known for their adverse effects on human health at high levels of exposure. These metals have no known essential role in the human body and therefore exposures can be divided into two categories (see Figure 1). For these non-essential elements, at very low exposures the metals are tolerated with little, if any, adverse effect, but at higher exposures their toxicity is exerted and health consequences are seen. In the case of the so-called essential trace elements (see also Figure 1) the human body requires a certain level of the element, and if intakes are too low then deficiency syndrome diseases will result. These can have consequences as severe as the ones which result from excessive intakes. In between, there is an acceptable range of exposures within which the body is able to regulate an optimum level of the element.
Environmental exposure to chemical carcinogens is very topical despite the minuscule risks associated with many such exposures at typical environmental concentrations. Examples are benzene (largely from vehicle emissions) and polynuclear aromatic hydrocarbons (generated by combustion of fossil fuels). Figure 2 shows the structures of benzene, benzo(a)pyrene (the best known of the carcinogenic polycyclic aromatic hydrocarbons), and 2,3,7,8-tetrachlorodibenzodioxin (the most toxic of the chlorinated dioxin group of compounds). Despite great public concern over emission of the last compound, the evidence for carcinogenicity in humans is quite limited.
(b) Chemicals which cause damage to non-human biota but are not believed to harm humans at current levels of exposure. Many elements and compounds come into this category. For example, copper and zinc are essential trace elements for humans and environmental exposures very rarely present a risk to health. These elements are, however, toxic to growing plants and there are regulations limiting their addition to soil in materials such as sewage sludge which is disposed of to the land. Another category of substance for which there is ample evidence of harm to biota, but as yet little, if any, hard evidence of impacts on human populations, are the endocrine-disrupting chemicals (also termed oestrogenics). These synthetic chemicals mimic natural hormones and can disrupt the reproduction and growth of wildlife species. Thus, for example, bis-tributyl tin oxide (TBTO) interferes with the sexual development of oysters and its use as an anti-fouling paint for inshore vessels is now banned in most parts of the world. A wide range of other chemicals including polychlorinated biphenyls (PCBs), dioxins, and many chlorinated pesticides are also believed to have oestrogenic potential, although the level of evidence for adverse effects is variable.
(c) Chemicals not directly toxic to humans or other biota at current environmental concentrations, but capable of causing environmental damage. The prime example is the CFCs which found widespread use precisely because of their stability and low toxicity to humans, but which at parts per trillion levels of concentration are capable of causing major disruption to the chemistry of the stratosphere.
3 UNITS OF CONCENTRATION
The concentration units used in environmental chemistry are often confusing to the newcomer. Concentrations of pollutants in soils are most usually expressed in mass per unit mass, for example, milligrams of lead per kilogram of soil. Similarly, concentrations in vegetation are also expressed in mg kg-1 or µg kg-1. In the case of vegetation and soils, it is important to distinguish between wet weight and dry weight concentrations, in other words, whether the kilogram of vegetation or soil is determined before or after drying. Since the moisture content of vegetation can easily exceed [50%, the data can be very sensitive to this correction.
In aquatic systems, concentrations can also be expressed as mass per unit mass and in the oceans some trace constituents are present at concentrations of µg kg-1 or µg kg-1. More often, however, sample sizes are measured by volume and concentrations expressed as µg 1-1 or µg 1-1. In the case of freshwaters, especially, concentrations expressed as mass per litre will be almost identical to those expressed as mass per kilogram. As a kind of shorthand, however, water chemists sometimes refer to concentrations as if they were ratios by weight, thus, mg 1-1 are expressed as parts per million (ppm), µg 1-1 as parts per billion (ppb) and µg 1-1 as parts per trillion (ppt). This is unfortunate as it leads to confusion with the same units used in atmospheric chemistry with a quite different meaning.
Concentrations of trace gases and particles in the atmosphere can be expressed also as mass per unit volume, typically µg m-3. The difficulty with this unit is that it is not independent of temperature and pressure. Thus, as an airmass becomes warmer or colder or changes in pressure so its volume will change, but the mass of the trace gas will not. Therefore, air containing 1 µg m-3 of sulfur dioxide in air at 0 ° will contain less than 1 µ g m-3 of sulfur dioxide in air if heated to 25 °C. For gases (but not particles) this difficulty is overcome by expressing the concentration of a trace gas as a volume mixing ratio. Thus, 1 cm3 of pure sulfur dioxide dispersed in 1 m3 of polluted air would be described as a concentration of 1 part per million (ppm). Reference to the gas laws tells us that not only is this one part per 106 by volume, it is also one molecule in 106 molecules and one mole in 106 moles, as well as a partial pressure of -6 atmospheres. Additionally, if the temperature and pressure of the airmass change, this affects the trace gas in the same way as the air in which it is contained and the volume mixing ratio does not change. Thus, ozone in the stratosphere is present in the air at considerably higher mixing ratios than in the lower atmosphere (troposphere), but if the concentrations are expressed in µg m-3 they are little different because of the much lower density of air at stratospheric attitudes. Chemical kineticists often express atmospheric concentrations in molecules per cubic centimetre (molec cm-3), which has the same problem as the mass per unit volume units.
Worked Example
The concentration of nitrogen dioxide in polluted air is 85 ppb. Express this concentration in units of µg m-3 and molec cm-3 if the air temperature is 20°C and the pressure 1005 mb (1.005 x 105 Pa). Relative molecular mass of NO2 is 46; Avogadro number is 6.022 x 1023.
The concentration of NO2 is 85 µl m-3. At 20 °C and 1005 mb,
85 µl NO2 weight 46 x 85 x 10-6/22.41 x 273/293 x 1005/1013 = 161 x 10-6 g
NO2 concentration = 161 µ g m-3
This is equivalent to 161 pg cm-3, and
161 pg NO2 contain 6.022 x 1023 x 161 x 10-12/46 = 2.1 x 1012 molecules
and NO2 concentration = 2.1 x 1012 molec cm-3.
4 THE ENVIRONMENT AS A WHOLE
A facet of the chemically centred study of the environment is a greater integration of the treatment of environmental media. Traditional boundaries between atmosphere and waters, for example, are not a deterrent to the transfer of chemicals (in either direction), and indeed many important and interesting processes occur at these phase boundaries.
In this book, the treatment first follows traditional compartments (Chapters 2, 3, 4, and 5) although some exchanges with other compartments are considered. Fundamental aspects of the science of the atmosphere, waters, and soils are described, together with current environmental questions, exemplified by case studies. Subsequently, quantitative aspects of transfer across phase boundaries are described and examples given of biogeochemical cycles (Chapter 6). Monitoring considerations are covered in Chapter 7, with the effects of chemical pollution in Chapter 8, and finally the regulatory aspects in Chapter 9.
CHAPTER 2
The Atmosphere
A. G. CLARKE AND A. S. TOMLIN
1 THE GLOBAL ATMOSPHERE
1.1 The Structure of the Atmosphere
1.1.1 Troposphere and Stratosphere. The vertical structure of the atmosphere, showing the features that are most relevant to the problems covered in this chapter, is illustrated in Figure 1. The figure shows the stratosphere, troposphere and boundary layer (that closest to the earth's surface). The difference between the layers is characterized by changes in temperature with height, and with changes in structure of the layers such as cloud cover and turbulence. The depth of the troposphere is 8-15 km, the lowest values occurring at the poles and the highest at the equator with some seasonal variations. Within this layer occurs most of the variability of conditions which leads to 'the weather' as the layman experiences it. The stratosphere is relatively cloud-free and considerably less turbulent — hence long distance passenger jets fly at stratospheric altitudes. Within the troposphere temperature decreases with height owing to the decreasing influence of radiation from the earth's surface, but as we enter the stratosphere the temperature starts to increase again. The turning point is called the tropopause. This situation of a layer of warmer, less dense air over a layer of cooler, denser air is quite stable. Consequently air is mixed across the tropopause very slowly unless special events such as tropospheric folding occur.
We normally think of 'air pollution' in terms of the troposphere, within which most pollutants have a fairly limited lifetime before they are washed out by rain, removed by reaction, or deposited to the ground. However, if pollutants are injected directly into the stratosphere they can remain there for long periods because of slow downward mixing, resulting in noticeable effects over the whole globe. Thus major volcanic eruptions injecting fine dust into the stratosphere can lead to a reduction in the amount of solar energy reaching the ground for more than a year after the event. Other global problems relating to events in the stratosphere such as the possibility of damage to the ozone layer are discussed later.
1.1.2 Atmospheric Circulation. To understand both global and local environmental problems we must first understand how pollutants circulate throughout the atmosphere. The main driving forces for the circulation of the atmosphere are the incident solar radiation and the earth's rotation. Because of the sun's angle, the amount of solar energy falling on a given area varies with latitude so that the poles are cold and the equatorial regions warm. Warm air rises at the equator and cold air flows inwards from both North and South. A similar situation occurs at the poles where warm air flows towards them and falls in the cold regions there. The rotation of the earth affects the circulation patterns in a fundamental way due to an effect called the Coriolis force. For example, air moving south towards the equator gives the impression of being influenced by a force in the westerly direction. The net result is the tendency for air to circulate in large-scale eddies around the 'low' and 'high' pressure regions on synoptic weather charts.
The proportions of incident energy reflected back to space, absorbed by the land or sea, and re-radiated at a longer wavelength all vary from place to place and affect the temperature distribution and circulation patterns. This energy balance is crucial to the determination of the global climate and is considered in more detail in Section 1.2. The processes of evaporation of water, cloud formation, and precipitation also affect the energy balance and circulation patterns. The presence of the ground has only a small effect on the overall pattern of atmospheric circulation and at most altitudes air movements approximate to those of a non-viscous fluid. The theoretical wind speed can be calculated from the pressure gradient and the rotational velocity of the earth — the so-called geostrophic wind speed. The pressure gradient is reflected on a weather chart by the closeness of the isobars, lines of constant pressure. If the isobars are close together the wind speed will be high.
1.1.3 The Boundary Layer. Near to the ground the situation is more complicated due to the effects of frictional and buoyancy forces. Turbulence is generated by mechanical forces as air flows over uneven ground features such as hills, buildings, or trees. The ground may also warm or cool the air next to it resulting in upcurrents and downcurrents. In the language of fluid mechanics we have turbulent transport of momentum and energy with corresponding velocity and temperature gradients in the vertical direction. Consider the variation of wind speed with height over the lowest few hundred metres of the atmosphere. This variation is greatest over rough surfaces (e.g. a city) where the effect could be a reduction of 40% of the wind speed aloft, that is, the geostrophic wind. Over smooth surfaces (e.g. sea, ice sheets) the effect is less and the reduction may be only 20%. The changing effect of friction with height also causes a variation of wind direction as we move away from the earth's surface, i.e. 'wind shear'. A plume from a tall chimney may therefore appear to be travelling at an angle to the ground level wind.
