7
AIR POLLUTION AND ACID RAIN
We move now into areas of more direct local effect—air pollution and acid rain. One difference between these topics and the global effects of ozone depletion and greenhouse warming is that ordinary meteorology is crucial to many aspects of local pollution. Until now, we've managed to discuss global issues without getting into the basics of weather, without talking about fronts and storms and wind and rain. To understand air pollution, we must become a little more informed about what makes daily weather.
In simplest terms, if you analyze a pollution problem like Los Angeles smog, which is a classic urban air-pollution problem, you can think of it as a process that begins with sources of emissions. The most important source of Los Angeles smog is automobile exhaust, but of course there are other important sources, too. Each source emits its own mix of pollutants into the atmosphere, and the atmosphere is like a great big pot that contains this evil stew. It allows the pollutants to mix together and interact with one another. It exposes them to sunlight and transports them from place to place. And along the way it allows various changes to occur. The result is that pollution, that atmospheric stew, influences "receptors" like your lungs, the paint on your car, and crops in the field.
What goes on in the atmosphere is absolutely crucial to the situation. But so is location. If Los Angeles weren't where it is and if it didn't have the kind of weather regimes it has, it wouldn't have the air pollution it has, even though the main cause is people driving cars.
Health effects, an aspect of air pollution I'm not going to say very much about, is in some ways the most crucial. Although a great deal is known about the effects of air pollution on human health, like much medical knowledge our understanding is still incomplete in biophysical terms. We don't know with any certainty what happens to someone's lungs when sulfate particles invade them. It's true that air pollution can increase the chance of bronchitis and emphysema and so on, and there is a great deal of statistical evidence to support such claims. Smog alerts, in short, are a good idea. But the deeper you delve into the subject, the more you realize how incompletely understood it is at a fundamental level.
Here we'll concentrate on the meteorology and chemistry, of air pollution. We begin with some aspects of meteorology. Let's consider circulation in the atmosphere, first in global terms, because it's relevant to what happens in Los Angeles.
What drives the large-scale circulation of the atmosphere is heat from the Sun. In the tropics, near the Equator, a given area of the surface of the Earth receives much more solar radiation on average than does an equivalent area at high latitudes, near the poles. Therefore, the warmest places on Earth arc in the tropics. Especially over the tropical oceans, this warmth leads to convection in the atmosphere: the air closest to the surface absorbs heat from the surface, which is warmed by sunlight, and as that air is warmed, it becomes buoyant, and rises. A visible manifestation of that process, noticeable if you are in the tropics lying on the beach or flying over the area in a satellite, is impressive towers of convective clouds, which are typical, for example, of thunderstorms: tall, puffy white clouds, cumulus and cumulonimbus clouds. The presence of convective clouds is a sign of rising air. The rising air in this convection transports moisture and heat to higher altitudes.
The air that rises in the tropics has to come down somewhere. Thus, a circulation is established: the air rises near the Equator, flows poleward (northward in the Northern Hemisphere, southward in the Southern Hemisphere), descends in the subtropics (latitudes between about 20° and 30°), and returns to the Equator near the surface. This circulation, which resembles a gigantic sea breeze, is called the Hadley cell, after a scientist who studied it long ago.
This circulation is a statistical construct. What do we mean by that? If you go out one fine day to some point in the Northern Hemisphere tropics, say Martinique, and take measurements, you will not necessarily
find that the wind that day blows from north to south (toward the Equator) near the surface and from south to north (toward the North Pole) at higher altitudes. But if you take many measurements, not just in Martinique but at many tropical locations, and you average them over days and weeks and months, you'll find that on average the circulation behaves in the sense implied by the Hadley cell, the motions being upward at the Equator, poleward aloft, downward in the subtropics, and toward the Equator near the surface. The statistical, long-term, average property of the atmosphere is such that in the tropics the large-scale north-south circulation is a Hadley cell, circling the globe.
A typical latitude of this descending motion in the subtropics is in the range from 20° to 30°. The reason why it is often sunny in these latitudes is that the descending motion of the Hadley cell suppresses atmospheric convection. Clouds generally require upward motion for their formation, and it is more difficult for a cloud to form if the large-scale vertical motion of the atmosphere is downward. Thus, the absence of convective clouds is a typical sign of descending air motions. All the great deserts of the world are in the subtropical latitudes. It's not an accident that the desert Southwest in the United States is about at this latitude rather than at, say, 45° north. It's no accident that the Sahara Desert is mainly in the 20°-30° north-latitude belt. The great Australian desert and Chile's bone-dry Atacama Desert are in the corresponding latitude belt in the Southern Hemisphere.
Though there are many variations from one longitude to another— not every place in the 20° to 30° latitude belt has a sunny, dry climate—on average, over the whole Earth, over many years, the basic feature of the large-scale, global-average, north-south circulation of the tropical atmosphere is the Hadley cell.
Why doesn't the Hadley cell extend all the way to the poles? Why don't we have a simple picture of rising motion in the tropics, with air flowing toward the poles at some altitude, descending near the poles, and returning toward the Equator close to the surface? Why does the Hadley cell extend only to about 30°? That's a profound question, the answer to which has been debated for centuries. Many wrong answers have been given by many thoughtful people. We know now that the answer is not simple. There is no relatively short statement you can make that shows exactly why the Earth causes this circulation to be what we observe it to be, rather than what we might think it ought to be.
We do know that the answer has to do with the fact that the Earth rotates: if the Earth didn't rotate, or if it
rotated much more slowly, the
circulation might well descend near the poles, instead of near 30°. The answer also has to do with other quantitative aspects: how far the Earth is from the Sun, and therefore how much energy it receives; how much moisture we have in the atmosphere; and how likely it is that water will change from one phase into another (solid, liquid, and gaseous water all occur in the atmosphere). The main factors causing the circulation of the Earth to be what it is are the temperature contrast between the poles and the Equator, and the rotation of the Earth. If the Earth rotated at a very different rate, there could be a very, different circulation.
We do know that the circulations on other planets are markedly different from the Earth's circulation. This is a fascinating topic, one that really has come into its own only since the advent of space exploration. Analyzing the atmospheres of other planets makes us realize how different the circulations can be. In fact, no other planet in the solar system has anything like the same meteorology, as the Earth. Why these things are as they are is another of those questions to which there is no simple answer. Rotation and heating are the critical elements. In these two respects, the planets all differ greatly from one another.
What we observe, first of all, is that the air that flows back near the surface of the Earth from the subtropics toward the Equator doesn't flow directly from north to south. Instead, it gets turned to the right (in the Northern Hemisphere) by a deflecting effect due to the Earth's rotation. This effect, called the Coriolis effect, arises because the rotation of the Earth produces an apparent acceleration that doesn't change the speed of the wind, but does affect its direction. The Coriolis effect needs to be taken into account in understanding many large-scale motions on the Earth, not just the meteorological patterns. For example, to calculate accurately how to launch a projectile, such as an artillery shell, from one location to another one far away, it's necessary to take into account the fact that, while the projectile is in flight, the target will appear to have moved because of the rotation of the Earth. Allowing for the Coriolis effect is unnecessary for rapid, short motions, but it's essential to slow, long ones, such as the large-scale winds.
North of about latitude 30° in the Northern Hemisphere, and south of about latitude 30° in the Southern Hemisphere, we have the belt that includes most of the United States, sometimes miscalled the temperate latitudes (it's often not very temperate there). These are the latitudes where we find the great migrating storms that characterize the weather of many parts of the United States. It's an area of complex atmospheric
flow where the big highs and lows on the weather map migrate slowly from west to east.
The tracks of these storms often tend to follow the jet stream, an intense west-to-east band of rapidly moving air. The jet stream is found not near the surface, but at the top of the troposphere. It's the reason why it routinely takes less time to fly from the West Coast to the East Coast than it takes to fly back to the West Coast.
The jet stream can be thought of as resembling a river. In the river are eddies, and those eddies are associated with the storms, the highs and lows moving across the weather map. The storm systems tend to follow the jet stream. For a long time meteorologists called it the steering current, because it allows you to predict where today's cyclone will be tomorrow. On average, the jet stream appears as a narrow ribbon of fast-moving air, but in fact it meanders; its position changes a lot from day to day.
The weather in some areas is especially susceptible to variations in the jet stream. California is one. Often, whether a particular winter in California is a wet season or a dry season depends on the position of the jet stream. Southern California usually doesn't get very much rain. Most of the rain that does fall there comes from a few intense winter storms. There are winters when all the storms seem to go north, leaving Southern California dry. Sometimes you'll hear that called a blocking pattern. That means that a region of high pressure diverts the jet stream to the north. Why this happens is complicated, but that's a typical circulation during dry, California winters.
The concept of fronts is more straightforward. Imagine that cold air is coming down from the north. As the cold air mass advances, it displaces warmer air. A front is just a boundary between air at one temperature and air at another temperature. So if this cold front is moving southward and you're in its path, then as time goes on the front will cross over you, and instead of being in the warm air, you'll be left in the cold. That's why it's called a cold front. Similarly, if the movement of the air mass is in the opposite direction, so that warm air is advancing, that's a warm front. If you're standing still and are initially in cold air, then after a warm front passes you, you'll find yourself in warm air.
In winter the temperature contrast between the Equator and the high latitudes, between the tropics and the polar regions, is strongest. For although the temperature in the tropics doesn't change very much
all year, it's of course coldest in high latitudes in winter. It is that temperature difference between loss, latitudes and high latitudes that drives the entire atmospheric circulation. The Hadley cell, for example, is much more intense in winter than it is in summer. Storms in middle latitudes are, in general, also more intense in winter than in summer. These storms derive their energy from the temperature difference between high latitudes and low, latitudes.
Land masses and mountain ranges have a powerful effect on all these weather patterns. We've been talking about the average conditions over the whole Earth, but it's clear that where you're located—whether, for example, you're on the west side or the east side of a continent—greatly affects your local weather. Storms generated in regions of great temperature contrast tend to form off the east coast of the continents. Air in middle latitudes usually moves from west to cast, and when an air mass remains over a continent for a long time—in winter, when the continents are colder than the oceans—the air mass gets cold. Then, when the air moves eastward and hits the relatively warm ocean, it presents a strong temperature contrast. That change in temperature can produce an intense storm. That's why such storms form off the cast coast of the United States in the region of the Gulf Stream and off the cast coast of Japan and Asia in the region of the Kuroshio current. These are the regions where the greatest heat transfer from the ocean to the atmosphere occurs, and the regions where many of the storms in the middle latitudes tend to originate. The effects of mountains, like the effects of continents and oceans, are important as well.
The part of the Hadley cell that flows toward the Equator in the tropics at low altitudes is called the trade winds. In general, trade winds blow from cast to west. By convention, we call a wind blowing from cast to west an easterly wind. An easterly comes from the east; it doesn't go toward the east. So trade winds are easterlies. Because of the Coriolis effect, trade winds blow generally from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere, converging near the Equator.
The modern name for this region near the Equator where the trade winds from both hemispheres converge is the Intertropical Convergence Zone (ITCZ). You can see it very clearly in satellite photographs. Air arrives from the northeast in the Northern Hemisphere, from the southeast in the Southern Hemisphere, and where it meets it has nowhere to go but up. The result is intense rising motion. On a satellite photo, it is marked by cumulus convection.
The average position of the ITCZ is very near the Equator, but the ITCZ is almost never found at its average position! In the Northern Hemisphere summer, when the Sun is, on average, north of the Equator, this convergence zone shifts to the north. The Northern Hemisphere summer might have an ITCZ at, say, 5° or 10° north latitude. In the Southern Hemisphere summer (Northern Hemisphere winter), the ITCZ might be located at around 5° or 10° south latitude. So this pattern of deep cumulus convection, representing the upward-moving branch of the Hadley cell, migrates north or south of the Equator with the seasons. The annual average position is near the Equator.
At this point I need to remind you of something that's crucial when we come to talk about Los Angeles smog. In the troposphere, approximately the lowest 10 to 15 kilometers of the atmosphere, temperature falls off with height, usually. It's warmest at the surface, where the atmosphere is heated from below. Temperature decreases with height until at some altitude, typically around 10 to 15 kilometers, it reaches a minimum and begins to increase with height. The altitude where this temperature minimum occurs is called the tropopause. It's the boundary, between the troposphere and the stratosphere. The lower region, the troposphere, is where all the weather is, where about 80% of the mass of the atmosphere is, and where all the people are.
The stratosphere, the layer just above the troposphere, contains nearly all the rest of the mass of the atmosphere. Temperature rises with height in the stratosphere, largely because sunlight is being absorbed by ozone. Up at 50 kilometers, 99.9% of the mass of the atmosphere is below you. Pressure at the surface is near 1,000 millibars; pressure at 50 kilometers is about I millibar. In this context, pressure is simply one way to measure the mass of air above a given altitude.
There are other regions above the stratosphere. The concept of temperature itself becomes strained at sufficiently high altitudes, because there are so few molecules of anything at those levels. The ionosphere, for example, is very important for radio communication, but there are very few molecules there, compared to the lower atmosphere.
In the lower atmosphere, in general, temperature decreases with height, typically at a rate of about 6.5°C per kilometer of altitude (or about 3.6°F for every 1,000 feet in elevation). But just as the circulation we looked at in discussing the Hadley cell was statistically an average circulation—the average of many observations over the whole atmosphere over many days, weeks, and months—so this picture of temper-
ature is an average as well. That is, temperature doesn't always decrease with height at the same rate. Though the global long-term average decrease is about 6.5°C per kilometer, there are times and places where temperature decreases more rapidly or less rapidly with height, and there are times and places where it actually increases with height. And one of those is just above Los Angeles on a typical summer afternoon.
This situation has a name: inversion. The name comes from the fact that such a configuration, with cooler temperatures at the surface and warmer temperatures aloft, is the inverse of the normal configuration.
An inversion is a very stable configuration. It's very difficult for upward vertical motions to occur in an inversion. Convection frequently occurs in the tropics, because the air is hottest near the ground and is therefore buoyant. If air is coldest near the ground, with hotter air above, then the cold, heavy, dense air is already near the surface, and it will tend not to rise. An inversion thus effectively forms a lid that traps pollutants in the lowest level of the atmosphere and prevents them from dispersing. Keep in mind the difference between the long-term global-average picture and short-term local variations like this one.
Where you define the top of the atmosphere, by the way, depends on what your purposes are. By the time you reach an altitude of about 30 miles, or 50 kilometers, as we've seen, about 99.9% of the atmosphere is below you. From the point of view of someone who wants to breathe, if you're above 50 kilometers, you're in outer space. You need to carry an oxygen supply in order to breathe. In fact, you need pressurized air in commercial aircraft, which typically fly at a height of around 6 miles, which is only about 10 kilometers. At that point, if the pressure in the cabin fails, you need an oxygen mask to breathe. From that point of view, as soon as you're much above that height, you're outside the atmosphere.
But perhaps your point of view is that of a satellite, and you're interested in having a long lifetime. What brings satellites down is friction in the atmosphere. The higher the satellite, the less the friction. It's the occasional collisions of the satellite with the relatively few molecules that are up there that eventually slow it down. There's not much air there, but it's enough. In fact, the height of the atmosphere, or the height at which a given pressure level occurs, depends on the solar cycle: how much solar wind is coming from the Sun. Satellite-orbit designers actually have to take into account the 11-year sunspot cycle, because it affects the density of the upper atmosphere and hence the friction
experienced by the satellite. Once a satellite begins to slow down because of friction, it will spiral downward into the Earth's atmosphere and eventually burn up.
So where you define the outside of the atmosphere is arbitrary. By the time you're at satellite altitudes, typically a few hundred kilometers, there's virtually no trace of atmosphere. There's gradual blending between an extremely thin atmosphere and the virtual emptiness of outer space. Yet that trace atmosphere can eventually destroy the satellite.
Remember how shallow this atmosphere is. The Earth's circumference is about 40,000 kilometers, around 25,000 miles. The thickness of the atmosphere, if we take it to be the altitude that includes 99.9% of the mass of the atmosphere, is only about 50 kilometers. We're clearly talking about a very thin skin compared to the size of the Earth itself.
As for the weather, almost everything that's interesting occurs in approximately the lowest 10 miles, or 16 kilometers. All the clouds we've been talking about occur in the troposphere. The troposphere is characterized by convection, or overturning. Tropos is Greek for "turning."
Occasionally, a powerful thunderstorm with intense upward air motion can reach the stratosphere, actually penetrating the tropopause, the boundary between the troposphere and the stratosphere. Because temperature increases with height above the tropopause, you might think of the stratosphere as one gigantic inversion. It's a stable region, with generally weak vertical motions, because temperature increasing with height means light air is found above heavier air.
We need now to learn more about the physics of radiation. There are many different kinds of electromagnetic radiation, which we distinguish according to their wavelengths. The kind you can see, visible light, occupies a small region of the electromagnetic spectrum that includes wavelengths between about 0.4 and 0.7 micrometers. A micrometer (formerly called a micron) is a millionth of a meter. Light can be thought of as vibration, a rapidly fluctuating electromagnetic wave. Similarly, very long wavelengths we call radio waves, and very short wavelengths we call X-rays and gamma rays.
We know that each of these bands of wavelengths has its own properties. X-rays, for example, unlike visible light, can penetrate people. But it's a very small portion of the spectrum that we're concerned with when we talk about climate.
The distribution of the radiation that the Sun gives off has a maximum at around 0.5 micrometers in the visible. It extends down to small
fractions of a micrometer, and it extends to longer wavelengths too, but it drops off to very little amplitude when the wavelength is longer than about 2 micrometers. There's a lot of energy in the visible part of the spectrum. That's why the sun looks so bright!
The Earth, on the other hand, gives off radiant heat, or infrared (IR) radiation. It peaks at around 12 micrometers, in the infrared. Your eyes are not sensitive to IR, so you can't see the Earth at night. Of course, you could see the Earth from space in the daytime, if you were fortunate enough to be an astronaut, but what yon would be seeing is not the radiant heat the Earth gives off, but rather reflected sunlight. At night, with the Earth between you and the Sun, you could not see the Earth.
When you look up at the Moon, you're not seeing radiation that the Moon itself gives off. You're seeing sunlight reflected from the surface of the Moon.
The science of light, incidentally, began in antiquity. Infrared radiation, however, was discovered only in 1800 by Sir William Herschel, an English astronomer. A good deal later, a remarkable pair of physical laws came along to explain why the Earth radiates mainly in the infrared and the Sun radiates mainly in the visible. The difference has to do with temperature. One law says that colder bodies emit radiation with longer wavelengths than do hotter bodies. The quantitative statement of that law is that the maximum wavelength is inversely proportional to the temperature. You can find out what that wavelength is by dividing the number 3,000 by the temperature in degrees Kelvin. That gives you the wavelength in micrometers. The law is called Wien's displacement law. With, a German, formulated it in 1893.
The temperature of the outside of the Sun is about 6,000° Kelvin. So if you divide 3,000 by 6,000, you get about 0.5 micrometers, which is the wavelength at which the Sun's radiation is maximum. The temperature of the surface of the Earth is about 288°K, or 15°C, roughly 60°F. The Earth, however, emits not only from the surface but also from the atmosphere, especially the troposphere, which is colder than the surface. The Earth's average effective radiating temperature thus turns out to be a little more than 250°K. Divide 3,000 by about 250 and you get a number around 12. That's why the maximum radiation of the Earth occurs at a wavelength of around 12 micrometers.
Wien's law is a nifty formula. It works backwards, too. You can take any body in the universe, and if you know the wavelength at which its radiation is at a maximum, you can figure out its temperature. You can discover that some stars are hotter than the Sun, because they radiate at
a still shorter wavelength. You and I are radiating, incidentally. People give off about the same amount of energy as a 60-watt incandescent light bulb.
The other law is one we came across earlier. It tells you how much energy a body—actually a "black body," a perfect radiator—gives off in terms of its temperature. This equation says that the flux of energy is some constant times the fourth power of temperature. It means, for example, that if you double the temperature of a body, you don't just double the amount of radiation it gives off, you multiply it by 24 , or 16. This law, by the way, is called the Stefan-Boltzmann law. Stefan formulated it first, in 1879, from experiments; Boltzmann deduced it theoretically five years later.
Because 6,000°K is much more than 250°K, 6,000 to the fourth power is a great deal more than 250 to the fourth power—more than 300,000 times as much, if you do the arithmetic. So each square meter of the Sun gives off more than 300,000 times as much energy as each square meter of the Earth. Hotter bodies give off more energy, than colder bodies, and they give it off in shorter wavelengths.
Sunlight is intercepted by the Earth. Remember, about 30% of the sunlight that hits the Earth is reflected away—that's the albedo—mainly by clouds, which account for about 20%. Another few percent are scattered away by particles in the atmosphere. A tiny bit of what gets all the way down to the surface isn't absorbed but is reflected. Most of what gets down to the surface is absorbed. In round numbers, 30% is reflected away, 50% is absorbed at the surface, and the other 20% is absorbed in the atmosphere.
When we say that the greenhouse effect operates because the atmosphere is transparent to incoming sunlight, that's a rough approximation. Twenty percent of the incoming sunlight does not pass through the atmosphere, but instead is absorbed in the atmosphere. Some of the incoming sunlight powers chemical reactions: some of it photodissociates oxygen, some photodissociates ozone, and, in Los Angeles, some makes smog. Sunlight is a key ingredient in making smog.
The structure of many of the molecules in the Earth's atmosphere is such that they can absorb light of particular wavelengths. For example, when oxygen absorbs sunlight, a molecule of oxygen absorbs some of the energy from the sunlight, and that energy can sometimes serve to break the bond that holds one oxygen atom to the other, resulting in two free atoms of oxygen. Not just any old sunlight will do that, but
only sunlight in particular wavelengths, determined by the structure of the oxygen molecule.
What about the other 78% or so of the atmosphere that consists of nitrogen? Is there a reaction that breaks ordinary nitrogen into N + N? No, there isn't. The chemical bond between the two atoms of nitrogen in the N2 molecule is so strong that no sunlight, not at any wavelength that the Sun emits, is sufficiently strong to break up that bond. Another way to describe this property of nitrogen in the atmosphere is to say that it's photochemically inert.
Some of the infrared radiation emitted by the Earth has a good chance of being absorbed by gases on the way out. The Earth emits energy chiefly in the wavelength range from around 6 micrometers up to 14 micrometers. Between those two wavelengths, there is a "window" in the spectrum, so called because the gases in the atmosphere don't absorb much energy from the radiation in that part of the spectrum. The atmosphere can thus be said to be transparent to IR radiation in that band of wavelengths, and it is in that window that the Earth does most of its emitting to space.
There is another window in the visible part of the spectrum, which is why sunlight reaches Earth's surface. That's why we see the Sun. In a part of the electromagnetic spectrum where the Sun emits strongly— from 0.5 or 1 up to about 2 micrometers—the atmosphere is partially transparent. There are regions of spectral absorption by water vapor, and where a little bit is absorbed by oxygen and ozone, but most of this "visible window" is clear for the penetration of sunlight.
It's because the Earth is emitting in the infrared part of the spectrum, and because water vapor and carbon dioxide and other gases are absorbing in the infrared, that we have a greenhouse effect. The more of these gases there are in the atmosphere, the stronger the effect. That's why these gases—methane, nitrous oxide, ozone, carbon dioxide, water vapor—are the gases we talk about in reference to the greenhouse effect. They just happen to have a molecular structure that absorbs in the part of the spectrum where the Earth emits. Perhaps it's a coincidence. If we had different gases so that this was a clear window, or if we had an Earth at a different temperature so that it radiated at considerably different wavelengths, we wouldn't have the same greenhouse effect.
Consider the reaction by which O2 + sunlight results in O + O by means of photodissociation, the breaking apart of the oxygen molecule by light. Photodissociation of oxygen tends to occur in the high atmosphere, in
the stratosphere and in the region above it called the mesosphere. Why? Because that's where the sunlight first contacts the atmosphere, and because the reaction absorbs sunlight at just the wavelengths needed to break up the oxygen molecules, so that down in the lower atmosphere not much sunlight in those particular wavelengths remains.
Photodissociation of the O3 ozone molecule occurs in the same way, but requires a different band of wavelengths of sunlight. Ozone can photodissociate in the presence of sunlight into ordinary oxygen (O2 ) and a free oxygen atom (O). The free oxygen atom gains some energy from sunlight in the process, and is therefore at a higher energy level than before. For it to recombine with O2 oxygen to form ozone requires the presence of a third molecule, a mediator molecule, which has a purely mechanical role in this process. The mediator, which might be oxygen or nitrogen, is present when the free oxygen atom and the oxygen molecule collide, essentially in a three-body collision. The mediator molecule's function is to carry away the extra energy. If the free oxygen atom and the ordinary oxygen molecule just slammed together, they would immediately fly apart again because of the extra energy carried by the oxygen atom, unless a third body were present to carry away the energy.
Ozone photodissociation also occurs mainly in the stratosphere, where most of the ozone is. Many of the free oxygen atoms resulting from the photodissociation reaction combine quickly with ordinary oxygen to form new ozone. In other words, this reaction is reversible. The rates at which these reactions occur depend on the availability of the molecules, the availability of sunlight, and the availability, in this case, of the mediator molecule, whose only function is to carry away energy.
Because ozone is very rare, while oxygen and nitrogen are abundant, the mediator molecule is often oxygen or nitrogen. Since there are many, many more of those molecules than there are ozone or odd oxygen molecules, the chances are good that an oxygen or nitrogen molecule is nearby when this reaction occurs.
This same energized or excited oxygen molecule can also interact with water vapor. Water is a stable molecule—it doesn't fly apart of its own volition—but in the presence of an excited oxygen atom, water can break up into two OH molecules. These are called hydroxyl molecules. Hydroxyl, a very reactive molecule, is the garbage collector of the atmosphere. It has the function of scavenging many molecules. It can react with all sorts of hydrocarbons, for example, such as methane. It can also react with NO2 to form nitric acid (HNO3 ).
Another thing that the energized odd oxygen molecule can do is attack nitrous oxide (N2 O) to form two nitric oxide (NO) molecules. And that's an important reaction too, as we shall see.
Remember that the atmosphere is made up of roughly 78% nitrogen (N2 ) and 21% oxygen (O2 ). Nitrogen cannot be photodissociated by sunlight—there is no sunlight energetic enough to break apart nitrogen molecules. But an ordinary molecule of nitrogen can, in the presence of heat, break apart and recombine to form two nitric-oxide molecules. So we have here another way of forming nitric oxide.
Where does this commonly occur? It happens all the time in your car's engine. There's plenty of this stuff around, because your engine takes in a lot of air. Gasoline is mixed at roughly the ratio of one part gasoline to 15 parts air in your carburetor or fuel-injection system before it's ignited. So your car processes an enormous amount of air. One of the things that happens in that engine is that combustion breaks apart the molecules and recombines them in this way. The temperature inside the combustion chamber of your car is about 2,000°K, hot enough fur this reaction to occur. In fact, this is a crucial reaction in smog.
What you need to make smog is lots of tailpipes and other sources putting out products like nitric oxide and carbon monoxide; you need sunlight providing the energy for reactions; and you need a vessel, a cooking pot, to keep these pollutants from escaping.
The sad fate of Los Angeles is that it offers all three of these things. The vessel that it lies in is part meteorology and part topography. Above Los Angeles you often find temperature inversions, in part because the city has cold air blowing inland from the ocean on sea breezes. So cold air often underlies warm air there, with temperature increasing with height.
The sea breeze brings winds blowing toward the land at low levels— where the people are—during the daytime, when tailpipes are putting out exhaust, and pushes everything from the ocean onto the land. There is also a wall in the form of the mountains on the cast side of Los Angeles, which keeps things from escaping. You thus have a situation in which the pollutants can't escape. If you took the inversion away, they could escape by convection. If you took the mountains away, the winds could disperse the pollutants. (This actually happens, to some extent: there is Los Angeles smog in the Grand Canyon, because there are gaps in the mountains—Los Angeles basin is not a perfect containing vessel.)
The situation is different in different places at different times of the day. Los Angeles has not only pretty bad smog but pretty variable smog. We know a lot about how much it varies from hour to hour and from place to place. The thing to keep in mind is that because the pollutants are produced during different parts of the daily cycle, and because it takes time for all these things to occur, different effects happen at different times of the day.
Carbon monoxide (CO) is a direct pollutant—it comes straight from a source, like your car. It's poisonous, and it affects people. The worst place in Los Angeles for carbon monoxide is where the biggest concentration of vehicles is: on the west side, for example, where several freeways meet. The worst time of day for CO is early in the morning, during rush hour.
The other pollutants occur later in the day and in different regions. For example, the worst place for ozone in Los Angeles is on the east side of the basin. So if you think you are sensitive to a given pollutant, you might try to locate yourself so as to maximize your own well-being. That's a macabre calculation, but there are plenty of data to rely on.
The inversion acts as a lid all the time, but as people go to work in the morning, there's a maximum in emissions. The sea breeze generally packs the pollution up against the mountains, but in the morning the sea breeze hasn't fully developed yet, because it depends on the temperature contrast between the land and the ocean. Hence, early in the morning, there's no big transport of pollutants out from the most densely populated areas, such as along the coast or downtown Los Angeles. In the early morning, then, the worst area to be in is where the traffic concentration is, and the pollutant to worry about the most is carbon monoxide.
By midmorning, however, the sea breeze has picked up, because the Sun has warmed the land a little, and the pollutants start to move inland, from west to east. At that point, the secondary pollutants, known usually as NOx (nitrogen combined with an odd number of oxygen atoms), begin to pick up, because it takes time for them to be produced. In general, they aren't the primary pollutants that come out of tailpipes, because to form, they require reactions.
The region of maximum pollutant concentration moves inland as the day progresses and as the sea breeze pushes the whole ugly stew from west to east. By midafternoon, there's typically a very strong sea breeze, and the most heavily polluted air gets pushed fight up against the mountains. With the Sun shining on a typical warm, smoggy August
day, ozone, which depends on sunlight to form, is at a maximum. That explains why cities on the east side of the Los Angeles basin have reputations for very bad air quality.
Such is the typical pattern of different pollutants occurring at different times of the day. Nighttime, incidentally, is the best time. Because traffic declines then, there's less source. Sunlight departs, thereby eliminating the reaction that forms ozone. And what was a sea breeze reverses and blows out to sea. But in the morning, the cycle repeats.
Turning now to another form of airborne pollution, acid rain, let's begin by defining pH, which is a measure of the acidity of a substance. The lower the pH number, the more acidic the substance is. The pH level is measured on a logarithmic scale, which means that every step on the scale represents a factor of 10. Thus, something with a pH of 8 is 10 times more acid than something with a pH of 9, which in turn is 10 times more acid than something with a pH of 10.
The difference between natural rain and acid rain is a factor of between 10 and 100 in acidity. Natural rain has a pH of between 5 and 6 and the most acidic acid rain has a pH of around 4. Even natural rain is slightly acid; the basis of its acidity is partly carbonic acid, H2 CO3 , which is formed from the same CO2 that's in the atmosphere. The CO2 gets washed out of the atmosphere and combines with water to form the carbonic acid. The neutral number on the pH scale is 7. That's the number that separates acids (lower numbers) from alkalines (higher numbers), sometimes called bases. Distilled water, actually pure water, H2 O—nothing else but the water you would find in a laboratory, which is close to the water you buy in the store—is neutral, with a pH of 7. Keep in mind that naturally occurring acids (sulfuric, nitric and others) make natural rain somewhat acidic.
The state of the science of acid rain is better than that of the ozone hole or the greenhouse effect. Our knowledge of acid rain is incomplete but still less fragmentary than our understanding of ozone loss or climate change. Yet, we need to learn more about acid rain. Ironically, the pace of research in this area has slowed in recent years, in part because new laws have reduced the emissions of chemicals that cause acid rain. These laws make the subject seem less urgent.
There's obviously research to be done. But the question of "solving" the acid-rain problem is considered largely a problem of politics, economics, and human behavior. It's not a mystery, and in that sense it's unlike any of the other things we've addressed thus far. The ozone hole,
remember, was unsuspected for years. It was discovered 50 years after we put into the atmosphere the first chlorofluorocarbons, these chemicals which, when first invented, we thought were a miracle chemical, whose harmful side effects were not even suspected. What we know about ozone depletion we have learned only in recent years. And each year we're learning more about how it works. For the greenhouse effect, too, many of the answers are not yet at hand.
Acid rain is not in that category at all. We understand exactly how it's formed. It occurs because we've been interfering with the natural workings of nature. Consider the role of sulfur, for example. Sulfur gets into the atmosphere naturally, partly from volcanoes. When a volcano erupts, it brings up a great deal of sulfur from deep in the Earth. Sulfur also comes from the decay of organic matter, dead plants and animals and compost heaps. It comes out of the ocean too, partly as hydrogen sulfide, a chemical that has an intense smell of rotten eggs when present in high concentrations, and partly as dimethyl sulfide.
The sulfur that goes into the atmosphere eventually gets rained out. It combines with water in the atmosphere, forms sulfuric acid, rains out, returns to the ocean, and so completes its cycle.
The anthropogenic, or human-caused, sources of sulfur lie almost entirely in the burning of fossil fuels, and the greatest contributor is high-sulfur coal. Coal has a substantial sulfur content, ranging from about 0.5% in low-sulfur coal, which happens to be expensive, to as high as 7% in plentiful low-grade coal.
Sulfur dioxide, SO2 , is the major pollutant that leads to acid rain. In the global aggregate, we annually release about 100 million tons of sulfur dioxide and about half that much, about 50 million tons, of NOx , which also creates acid precipitation. The natural production of sulfur is smaller than the anthropogenic, or human-caused, source. There is a potentially important climate connection too. The sulfur emissions form particles (aerosols) that can reflect sunlight and alter clouds.
Furthermore, sulfur dioxide and NOx emissions are strongly localized. Unlike carbon dioxide, which gets mixed around the whole world very rapidly, acid rain can have a very localized effect. There are well-documented cases of strong ecological damage in the vicinity downwind of power plants.
The effects of acid rain are much more severe in some areas than in others. This has led to many of the political problems that accompany acid rain. The smokestacks from power plants in the Midwest, burning
sulfur rich coal, can produce acid rain in the lakes of Maine. A few trace studies have been done; people have tagged particles, followed them in the wind, and modeled them numerically. So we can know where the source of acid rain is, and that leads to heated disputes. One that has gone on for many years is between the United States and Canada. There are many such disputes in Europe. There is acid-rain damage to lakes and forests in Sweden that has sources in England. Acid rain brings heavy political costs, for the benefits of the power and the costs of the acidification often fall on very different people.
There's a long list of things that can be done about the acid-rain problem. The right mix depends on many factors, including local politics and global economics.
One solution is to use low-sulfur coal. But high-sulfur coal is far cheaper than low-sulfur coal, as it happens. That has to do with the economics of mining it and transporting it and so on. If you want low-sulfur coal, you must pay more.
Another possible solution is to treat high-sulfur coal chemically before you burn it. And an enormous technology has been developed in so-called scrubbers, which are mechanical and chemical devices for removing the sulfur, not from the coal before it's burned, but from the stuff that goes up the smokestacks after it's burned and before what remains is released to the atmosphere.
It's also possible to disperse the emissions. One way to do that is just to build taller smokestacks. Part of the acid-rain problem has been that in some regions where powerplants are located, there are consistent inversions. Recall that inversions are temperature structures in which light, warm air overlies cold, dense air. When that kind of meteorology settles on a region where a smokestack is situated below the level of the inversion, the emissions are trapped below the inversion. But if you build a taller smokestack, get its top above the inversion, you improve the ability of the stack to disperse the emissions. And in general, the higher you are, the stronger the wind, which helps disperse emissions as well. Of course, this may only relocate the problem.
Alternative energy sources are of course the long-term solutions. If you don't want acid rain, don't burn sulfur-rich fuels. In many cases, the economics of coal-fired powerplants are very attractive, except for the necessity of dealing with emissions. In many cases, legal requirements have been placed on plants to encourage them to move away from the use of sulfur-rich fuels.
Locating powerplants strategically is a viable strategy, too, because in many cases, where you put the powerplant determines where the stuff that emerges from the stack ends up. You can make wise or foolish choices regarding the location of each powerplant.
Another possibility is to buffer the lakes, treating the problem at the other end. That is, take a lake that has already had its acidity increased by acid precipitation—or acid deposition, technically, since the compounds can come down either in the form of rain or as solid particles— and add some substance such that when you mix the whole stew together, you get something with a more nearly neutral level of acidity.
Interestingly, and fortunately, when you buffer a lake, over time an amazing number of things often get restored. In many lakes that are otherwise healthy ecosystems, once you have done away with the cause of the problem, you can engineer the lake back to health. However, lakes are complex, and buffering is often not effective. There's an enormous range of experience with the problem, and success depends on a great many factors other than the acid.
It's much harder, by the way, to "buffer" a forest. Damage to plant life comes from precipitation that falls, but it also comes from ground water drawn up in the root system, which carries nutrients and minerals and the rest of the things that come in solution. But it's really hard to get in and change ground water or change precipitation. So fight now, these end-use mitigation strategies are applicable to lakes but not to forests. A great deal of research remains to be done on the means of dealing with acid rain.
Often the fights about acid rain are not so much scientific in nature as they are political and economic. The challenge is to find the right combination of balancing the interests of the generators and users of affordable and reliable electricity with the interests of the people who suffer the consequences of acid deposition. It becomes very straightforward: How much are you willing to pay?
It's like the old story in stock-car racing. A wealthy playboy decides he wants to take up stock-car racing, so he asks, "How do you get into this?" He is told, "Well, you go buy a car and find an engine builder and he builds you an engine, and you're in business." He then asks how much it will cost. The reply is, "Speed costs money; how fast do you want to go?"
That's the situation here. Neutral lakes cost money; how neutral do you want to go? The difficulty in the political arena is that in many cases there are interjurisdictional equity arguments. The laws that govern the
polluter are from a different entity or even a different country than the laws that govern the polluted.
One of the issues in solving the acid-rain problem is the argument about what an affected lake would have done by itself anyway. Fish in a lake die, and the fellow who is producing energy from high-sulfur coal in the Midwest says, "You can't pin that on me, because a lot of other things can happen in lakes too." A lake 10 miles away from a sick lake can be healthy, for instance. So you can argue about that.
You can't argue, in general, that acid rain has no effect. That is a flat-Earth kind of argument. Damage from acid rain is clear in many places. But it's also true, when you argue about damage suffered by one particular area, that although the atmospheric chemistry may not be very complicated, the biology, of the lake or the region or the forest is complicated. So when it comes to taking a sick tree and tracing back to which pest or which disease caused that sickness, and how the progress of that pest or disease was or wasn't influenced by all the chemical consequences of what kind of worm grew up in its roots or what kind of rain fell on its leaves, that can be tricky.
Thus, there are points over which there are legitimate controversies and unsettled issues. But in general, the argument that acid rain has no effect, that there's no large-scale damage anywhere on the planet from sulfur emissions or smokestacks, is untenable.
Acid rain often gets mentioned in the same breath as ozone depletion and the greenhouse effect. And in some sense it belongs there; after all, why is there acid rain? Because there are so many people using so much energy. But acid rain differs from the ozone and greenhouse problems in the sense that we do know what to do about it. We simply have to find the political will to do it. If you don't want to have acid rain, don't put sulfur dioxide and NOx into the atmosphere. That's really the best solution. In many cases, a great deal has been done about acid rain. There are examples of lakes that were once acidified and are now restored to a neutral state and good health. The problems that remain are problems of economics and politics.
The issues we've examined in this chapter may seem somewhat out of place in a discussion of global change. After all, loss of stratospheric ozone and the possibility of climate change due to an enhanced greenhouse effect are issues that are truly planetary in scope. But air pollution in Los Angeles and acid rain in Sweden or Canada are purely local problems, aren't they?
No, they aren't. Not at all. They are genuinely global problems, because there are so many cities with polluted air, and because there are so many sources of acid rain. Humankind is the only animal that fouls its own nest, and it's been exceptionally ingenious in devising ways to do so.
The sad fact is that Los Angeles is not an unusual example. Urban air pollution affects many cities. Traffic police in Tokyo have sometimes sought relief by wearing oxygen masks. Mexico City, with the biggest urban concentration of people on the planet, perhaps 20 million inhabitants, has some of the dirtiest air on Earth. It has the triple misfortune of high altitude, overpopulation, and few effective controls on emissions. The growing prosperity of many densely populated cities in the developing world is vividly apparent in their filthy air. The traveler whose plane descends toward Delhi or Calcutta or Bombay sees not the India of legend promised by the guidebooks, but a dark cloud of pollution.
Everything we know about current trends tells us that many of these aspects of global change will inevitably intensify in the future. Almost all of the population increase in the developing world, which means the great bulk of all of the population increase on the planet, will occur in cities. The cities of 2025 may have three times the population of those of 1990. Planetary population grows, but urban population grows at a faster rate.
Acid rain is a problem in the United States and Canada, but in Eastern Europe it's a disaster. The former Communist governments there ranked environmental protection low in priority. Acid rain has affected more than half a million acres in Poland, even more in Czechoslovakia.
Local problems, if sufficiently numerous and severe, are global problems too.