Welcome to our atmosphere
An old pop song tells us that “love is like oxygen”. A politician concerned about the environment is accused of being “out in the ozone”. Computer programs that exist only in a promoters press releases are dubbed “vapourware”. Oxygen, ozone and water vapour are three of the most familiar ingredients in the life-sustaining soup we call the atmosphere. It's a complicated brew: some of its elements weren't even discovered until well into the twentieth century. You'll find only a tiny bit of molecular difference when you compare the air that hangs over an industrial city, simmers in a sauna or lies atop ice at the South Pole. However, just as a pinch of cayenne pepper can transform a recipe, several of the most scant ingredients of the atmosphere – which can show up at one part in a billion or even less – make a huge difference to how the atmosphere behaves and how it feels to us.
Fiery and foul
If there's more to the atmosphere than we once thought, there's also less. Aristotle believed in four elements – earth, air, fire and water – but he also thought the Sun's heat interacted with water and earth to produce “vapours” that caused rain and snow and “exhalations” that fuelled the wind. Over a thousand years later, Shakespeare's Hamlet bemoaned a “foul and pestilent congregation of vapours”, and his King John fretted about an “exhalation in the sky”!
With the arrival of the seventeenth century and the scientific renaissance, people acquired the tools to discover just what constituted air. One of the earliest ideas came from Leonardo da Vinci and several followers. They pushed for the notion of “fire air”, which they thought helped to fuel blazes and support life, and “foul air”, which they believed inhibited the same processes. In the 1770s, oxygen was discovered and identified as “fire air”. Before he literally lost his head in the French Revolution, pioneering chemist Antoine-Laurent Lavoisier demonstrated that living creatures took in oxygen and released carbon dioxide, just like a slow-burning fire. (Plants do the opposite, an important aspect of the climate-change debate.)
Although its probably the main component that springs to mind when we think of air, oxygen constitutes only about 21 percent of the atmosphere. A far greater portion, around 78 percent, is made up of nitrogen, identified (also in the 1770s) as “foul air” Nitrogen is actually a rather benign element of our atmospheric blend; it cant support life, however. Nitrogen is such a stable molecule that only a few natural processes can remove it from the air, including the intense heat of a lightning bolt. Once atmospheric nitrogen returns to the soil, it helps nourish plant life. At the same time, bacteria in the soil and ocean can both remove nitrogen from the air and help put it back. This is just one of many important cycles that link the atmosphere to what scientists call the biosphere – the world of plants, animals and other organisms.
Oxygen and nitrogen are the biggies of the atmosphere. Together, they constitute about 99 percent of it. Water vapour accounts for much less than a percent overall, although in humid regions it can occupy as much as 3 percent of the ground-level air. The rest of the atmosphere is divided into dozens of players. Some of these are amazingly stable. The so-called noble gases – argon, neon, helium, krypton and xenon – can stay in the air for many millions of years.
The air also includes solid particles, or aerosols, ranging from the microscopic to the visible. Salt escapes from the ocean in sea spray, dust is scoured from the ground by wind, and a variety of particles are ejected from volcanoes. Most of these take only brief trips of a few hours to a few days through the atmosphere before they settle to earth or sea, although volcanic debris can stay airborne for several years if its shot high enough. Also, meteorites deposit a few million tonnes of dust each year as they whisk into our atmosphere and disintegrate.
There's also a group of sparse, but highly reactive, molecules that, like rabble-rousers at a gathering, tend to influence others. For instance, the hydroxyl radical (OH) makes up less than a trillionth of the atmosphere, yet OH is critical to clearing the atmosphere of hydrocarbons and other pollutants. Molecules like these have far more of an influence on the air than their numbers might suggest, and it's their enhancement – or destruction – by human activity that adds a question mark to our atmosphere's future.
How the atmosphere evolved
In the billion or so years after the cataclysmic Big Bang, as Earth coalesced out of cosmic debris, the lighter elements found in the Sun (hydrogen and helium) gradually escaped. Volcanoes spewed out much of the stuff now found in our atmosphere, including methane, nitrogen, the noble gases and carbon dioxide, as well as the water vapour that condensed to form the world's oceans. In fact, carbon dioxide probably dominated our atmosphere for billions of years, much as it does on Mars and Venus, where it makes up more than 90 percent of the air.
Oxygen wasn't a major player until life came along in the form of primitive bacteria. Like the plants that would follow them eons later, these bacteria carried out photosynthesis: using sunlight, they took in carbon dioxide and emitted oxygen. Once some of the oxygen was converted to ozone, it formed a shield for ultraviolet light and allowed more forms of life to develop. A host of chemical readjustments followed. Gradually, the atmosphere took on its present shape – or, at least, the shape it had for roughly a billion years before industrial society arrived. Oxygen and nitrogen still take up the same 99 percent of dry air they did before industry arrived, but the rest of the picture has changed notably.
Humans are a large part of this changing picture. Of course, we've burned wood, coal and other substances for millennia, but from the late-eighteenth century onward, industrial culture spread widely enough to affect Earth's air quality on a regional and global scale. Before the most obvious pollutants from coal were addressed, they triggered horrific smog episodes in Europe and North America that killed thousands in the first half of the twentieth century. The term “smog” was coined in Britain to describe a combination of smoke, mainly from coal burning, and fog.
Another source of burning, the internal combustion engine, has made possible a different kind of nasty brew. Compounds emitted by cars interact with sunlight to form ozone and other troublesome chemicals. Although ozone in the stratosphere helps protect us from ultraviolet radiation, it causes health problems when it's at ground level. This type of pollution has become endemic to sunny, lower- and mid-latitude cities from Los Angeles and Mexico City to Rome and Bombay.
Many kinds of pollution, including the soot produced from coal and heavy industry, wash out of the air in only a few days (although acidic rainfall has produced its own set of ill effects on forests and lakes). Many cities and regions have been able to make substantial improvements in their air quality simply by finding cleaner ways to burn. However, the key byproduct of internal combustion remains carbon dioxide, and it's quite tough to eliminate. A very stable molecule, carbon dioxide can stay in the atmosphere for over a hundred years. About 25 percent of the carbon dioxide produced by humans is taken up by the oceans. Plants remove some of the rest, but they aren't doing so quickly enough to keep up with the expansion of human industry. Together with other greenhouse gases, increasing levels of carbon dioxide have taken the concept of pollution to a new level by raising the spectre of human-triggered global warming.