Something in the air – Natural and Industrial Air Pollution
The Earths Atmosphere
Air is a reservoir of breathable gases. Air is the window that shields life from deadly radiation. Air is the earth’s armor that protects against meteorite bombardments. Air is a blanket that keeps the earth warm. Air is the carrier of energy and water. Air is a garbage disposal system.
The six sentences above briefly describe the main functions of air rather than define what it is. Without air, the earth would be a lifeless, rock-strewn planet. So, what is this air you breathe?
Clean and dry air, that is, air without water vapour and airborne substances, is a mixture of several gases. Nitrogen (78%), oxygen (21%) and argon (0.9%) are the main constituents. The leftover space (0.1%) is reserved for carbon dioxide, methane, neon, helium, krypton, hydrogen, xenon, and ozone. All these gases generally mingle well amongst each other. The exception is ozone. It concentrates in a layer at
an altitude of between 10 – 50 km in the atmosphere, the ozone layer, and it may be present near the surface.
The air is hardly ever dry, though, as almost three quarters of the earth’s surface beneath it is water. Some of this water evaporates and forms an invisible, odorless gas – water vapor. The earth’s solid crust, its plants and animals and sweaty humans, also pass some moisture to the atmosphere. The amount of water vapor in the atmosphere determines humidity.
Water vapor doesn’t replace the other gases of the air completely. The maximum volume that is available is 4%. Once this amount is reached, the air is saturated and the water vapor has filled all available space (100% relative humidity).
Excess water vapor condenses and forms tiny water droplets in the air. At sufficiently low temperatures, the droplets freeze into small ice crystals. The suspended droplets and ice crystals appear as clouds, fog or mist.
Like the skin of an apple that tightly surrounds the fleshy interior, the air also hugs close to the surface of the earth. Almost all air resides within 80km of height. Compare this with the earth’s diameter of 12,756km and the comparison to an apple’s thin skin is well justified. The ancient Greeks called this skin atmos sphaira (vapour ball); today we call it atmosphere. Because air is highly compressible, its own weight squeezes about 50% of its content to within the first 5km of height. Mountain climbers who venture above this height have to carry compressed air. Aircraft compensate for the lack of air by pressurizing cabins.
Weather and Pollution
The weather can be held accountable for the fact that pollution has become an international problem. Wind carries the waste of an industrial region to otherwise pristine regions. Fish die in clean Swedish lakes as a result of pollutants from Central Europe. Similarly, remote Canadian regions suffer from polluted air created in US industries hundreds of kilometers away. One estimate claims that 50% of sulphate deposits in Canada come from its southern neighbor. Ironically, the complaints of residents near industrial sites led to taller smokestacks that distribute the pollutants over a much larger area, letting neighboring provinces suffer for a while.
For years, scientists believed that pollution would dilute into the atmosphere to negligible levels. Recent studies, however, showed otherwise. Certain wind patterns can carry the pollutants as a ‘parcel’ and deliver it many hundreds of kilometers away. At its destination, it may arrive as acid rain or snow over a limited area. Even the Arctic isn’t spared. Mercury concentration in Arctic spring rain or snow has more than tripled over the last two centuries.
Pollutants undergo some chemical changes while in the atmosphere and can reside there for a long time. Vertical mixing, i.e. updraft and downdrafts, distribute the pollutants within the whole troposphere. Therefore, a European or Australian chemical can catch a ride in the global wind pattern and contribute to international pollution problems.
Strong winds and vertical mixing are also to blame for natural pollution – dust and sand. For citizens at the edge of deserts, dust storms and sandstorms are a regular occurrence. Sahara dust and sand sometimes rise up to 5km into the atmosphere and drift across the Atlantic. Several hundred million tons of dust leave the West African coast annually. The heavy particles settle in the Atlantic but lighter dust creates haze in the Caribbean, and meteorological authorities need to declare pollution alerts.
What does nice weather mean to you: light wind, no rain and glorious sunshine? Millions, however, relate nice weather with smog alerts and high pollution levels. In light or no wind conditions, the pollutants remain at home. Without vertical mixing, they accumulate and can exceed acceptable levels. If this is not enough, sunshine converts chemicals such as nitrogen dioxide into toxic ozone. So much for nice weather.
A stable atmosphere – one without updraft – occurs frequently in winter during high-pressure situations. Instead of rising, the air actually descends and traps pollutants. A high-pressure system generally covers a large area and often remains active for many days, sometimes weeks. Thus, pollution gradually spreads over hundreds of kilometers.