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Chapter 11 Atmospheric science and air pollution
Natural atmospheric conditions and human-made pollutants contribute to the destructive actions of air pollution. Some properties influencing our dynamic atmosphere are pres- sure, density, relative humidity, and temperature. The four layers of the atmosphere have boundaries invisible to the human eye, but they can be delineated by changes in temperature, density, and composition. The oceans and their interaction with the atmosphere also affect weather and climate. Because human pollution of the atmosphere affects ecological systems and health, the Environmental Protection Agency (EPA) pays special attention to six "criteria pollutants" that pose the greatest threats to health and welfare. Most outdoor air pollution impacts are due to chronic low-level emissions that result from recurrent conditions such as urban smog. The depletion of stratospheric ozone threatens to be a great global problem. Recently, scientists found that the effects of acid precipitation are worse than first predicted, and the mandates of the 1990 Clean Air Act do not adequately address this problem. The health effects from indoor air pollution are worse than outdoor air pollution, and cause, 14 times as many deaths. The highest health risks in the developing world are particulate matter and chemicals from wood and charcoal smoke, while the top risks in developed nations are cigarette smoke and radon. Outdoor air pollution has been reduced in many countries because of limits on toxic emissions through legislation. Indoor air pollution is minimized by reducing toxic materials, increasing ventilation, and using efficient stoves.
The 1952 "Killer Smog" of London .
On December 5, 1952, people in London stoked their coal stoves to keep warm during that very cold day, which caused smog, a mixture of smoke and fog, to settle over the city, causing the city's air quality to be ten times worse than usual.
The visibility was so poor that pedestrians could not see across the street, roads became clogged with abandoned cars, schools closed, flowers wilted, and cattle died.
Modem researchers estimate that the actual death toll of the four-day smog event, including delayed cases that appeared over the next two months, reached 12,000.
Similar, but less severe, smog had occurred in London since the early 1800s, and also in the United States, Mexico, and Malaysia.
Before the 1950s, smog was considered a necessary burden, but today, we view air pollution as an environmental challenge.
Limits on toxic emissions through legislation have caused declines in air pollution.
The success of environmental advocacy and policy in reducing air pollution has made the air in many American cities and London cleaner than in the 1950s.
However, people and cities around the world still suffer from air pollution, especially in developing nations. For example, in 1995, airborne pollution in Delhi, India, was measured at 1.3 times the level of London's average for the year 1952.
Atmospheric Science
The atmosphere, the thin layer of gases that surrounds Earth, supports life by providing needed chemicals (oxygen and carbon dioxide), absorbing dangerous radiation, burning up incoming meteors, transporting and re- cycling water and other chemicals, and moderating climate.
Today, about 78% of the atmosphere consists of nitrogen gas and 21 % oxygen gas.
Human activities have added small but significant quantities of artificial gases, as well as changing the quantities of some atmospheric gases, particularly carbon dioxide (CO2), methane (CH4), and ozone (03),
Important atmospheric properties include temperature, pressure, and humidity Movement of air within the lower atmosphere results from differences in the physical properties of different air masses, which include pressure, density, relative humidity, and temperature.
The air's density is greater near Earth's surface, and decreases with altitude because gravity pulls gas molecules toward the surface.
Atmospheric pressure, which measures the weight per unit area produced by a column of air, also decreases with altitude.
At sea level, atmospheric pressure is equal to 14.7 Ib/in2 or 1,013 millibar (mb).
At the top of Mount Everest (29,035 ft), the "thin air" is just over 300 mb, and a mountain climber is standing higher than two-thirds of the atmosphere's air molecules.
Air also has a relative humidity, the ratio of water vapor a given volume of air contains to the maximum amount it could hold, so a relative humidity of 50% means the air contains half the water vapor it possibly could hold.
When relative humidity is high, sweat evaporates slowly and the body cannot cool itself efficiently, making it feel hotter than it really is.
Temperature differences affect air circulation; variation over Earth's surface is due to the sun's rays striking some areas more directly, while altitude through the layers of the atmosphere also causes temperature differences.
The atmosphere consists of several layers
The atmosphere, about 1/100 of Earth's diameter, consists of four layers whose boundaries are not visible to the human eye, but that can be delineated by changes in temperature, density, and composition.
The troposphere, nearest Earth's surface, most directly affects living things because it provides gases for breathing, and cycles nutrients from terrestrial and aquatic ecosystems.
Movement of air within the troposphere is also responsible for the weather.
Although only 7 miles high, the tropospheric air temperature declines with increasing altitude, until it suddenly stops declining at the tropopause, the top of the troposphere.
The tropopause prevents mixing between the troposphere and the next layer, the stratosphere.
From 7 to 31 miles above sea level, the stratosphere's temperature rises with altitude.
The stratosphere is drier, less dense, and calmer than the troposphere, so there is little vertical mixing, and substances entering it stagnate and re- main for a long time.
Most of the minute amount of ozone is concentrated in the bottom stratosphere, in a layer called Earth's "ozone layer."
Ozone absorbs or scatters the sun's damaging ultraviolet (UV) radiation, so this layer is vital to the maintenance of life on Earth.
In the mesosphere (31-53 miles above sea level), air pressure is extremely low and temperatures fall with altitude.
The thermosphere extends to an altitude of 300 miles, where solar rays produce extremely high temperatures and ions from the sun react with atmospheric molecules to produce the beautiful, haunting displays known as the aurora.
In the thermosphere, the molecules are so few and far between that they rarely collide, so that heavier molecules (nitrogen and oxygen) sink and light ones (hydrogen and helium) end up near the top.
Solar energy heats the atmosphere, helps create seasons, and causes air to circulate
Radiation from the sun plays a major role in our atmosphere by driving most of its air movement, creating seasons, and driving both weather and climate.
About 70% of the sun's energy is absorbed by the atmosphere and planetary surface, while the rest is reflected back into space.
Given Earth's curvature, solar radiation intensity is highest near the equator and weakest near the poles.
Because Earth tilts on its axis, the northern and southern hemispheres each face the sun for half the year and create seasons, which are especially pronounced near the 'poles.
Land and surface water absorb solar energy, reradiating some heat and causing some water to evaporate, making the air near Earth's surface warmer and wetter than at higher altitudes.
Warm air rises into regions of lower atmospheric pressure, expands, and releases heat, which cools the air, which then descends, becoming denser and replacing the warm air.
This type of circular, or convection, current, with warm air rising to be replaced by colder air descending, plays a key role in guiding both weather and climate.
The atmosphere drives weather and climate
Weather consists of the local physical properties of the troposphere such as temperature, pressure, humidity, cloudiness, and wind over relatively short time periods and in a relatively small geographic area.
Climate describes the pattern of atmospheric conditions found across a relatively large geographic region over a long period of time, typically seasons, years, or millennia.
Weather is produced by interacting air masses
Weather changes occur when air masses with different physical properties meet.
A front is the boundary between two air masses that differ in temperature and density.
A warm front is the boundary where warm air displaces colder air, and
the warm front rises over the cold air mass, cools, and condenses to form clouds that produce rain.
A cold front is the boundary along which denser cold air wedges beneath warm air, pushing the warm air upward, where the warm air cools and expands to form clouds and potentially produce thunderstorms.
After a cold front passes through, the sky clears and the temperature and humidity drop.
A high-pressure system, which is an air mass with high atmospheric pres- sure, contains air that descends and typically brings fair weather.
In a low-pressure system, air moves toward the low atmospheric pressure at the center of the system and spirals upward, causing the air to expand and cool, followed by clouds and precipitation.
Usually, the air in the troposphere gets cooler as altitude increases, and warm air rises, causing vertical mixing.
Sometimes, however, colder air is trapped near the ground, with warmer air above it, which is called a temperature inversion or thermal inversion. Thermal inversions trap pollutants near the ground, which can cause episodes of smog buildup in urban areas that can kill thousands of people.
Global climate patterns result from the differential heating of Earth's surface
Air movements on a larger geographic scale can create climatic patterns that are maintained over long periods of time.
Near the equator, Hadley cells are planet-wide patterns of convection cur- rents caused by sunlight.
The intense sunlight warms surface air, causing it to rise in two columns toward the poles.
As the air is warmed, it rises and expands, which causes moisture to be released, resulting in tropical rainforests near the equator.
As the columns of air cool, they descend at about 30° latitude and absorb moisture from the land, resulting in desert areas.
Two other convection currents, Ferrel cells and polar cells, force air upward around 60° latitude, resulting in precipitation in these areas.
Global wind patterns are influenced by Earth's rotation
Horizontal air currents, called wind, are caused by cooling air replacing rising warm air in a convection current.
On a global scale, the Hadley, Ferrel, and polar cells produce wind patterns that extend across the planet, but do not cause north-south surface winds, because of the Coriolis effect
The Coriolis effect is caused because regions near the equator spin more quickly than polar regions, so the north-south winds are deflected from a straight path, and seem to travel partly in east-west directions.
The interaction of the convection currents with Earth's rotation produces global circulation patterns that sailors used for centuries to cross oceans. A region with few latitudinal winds near the equator is known as the doldrums.
Between the equator and 30° latitude lie the west-blowing trade winds, and from 30° to 60° latitude are the westerlies (which blow toward the east).
Outdoor Air Pollution
Throughout human history, humans have generated significant quantities of air pollution that can affect climate and/or harm organisms.
However, in recent years a number of air pollution problems have been greatly diminished as a result of government regulation and improved technologies.
The majority of outdoor air pollution comes from natural sources
Outdoor air pollution consists of volatile chemicals or particulate matter mixing in the troposphere.
The majority of outdoor air pollution is caused by natural sources such as the metabolism of plants, the decay of dead plants, salt from sea spray, dust storms, volcanic eruptions, and forest fires.
Although dust storms are a natural occurrence, poor farming and grazing practices allow wind erosion that contributes to the hundreds of millions of tons of dust blown across the oceans each year, carrying fungal and bacterial spores that have been linked to die-offs in Caribbean coral reef systems.
Volcanic eruptions release large quantities of particulate matter and sulfur dioxide into the troposphere and stratosphere, where it can remain for months or years.
The 1980 eruption of Mount St. Helens in Washington produced dust that circled Earth for 15 days, while the dust from the massive 1883 eruption on the Indonesian island of Krakatau produced gorgeous sunsets and caused global temperature to drop.
The burning of vegetation pollutes the atmosphere with smoke and soot from both natural and human-caused fires.
In 1997, a severe drought caused fires in Indonesia, Mexico, Central America, and Africa, releasing more carbon monoxide than the worldwide burning of fossil fuels.
Human activities create various types of outdoor air pollution
Human activity can increase the severity of natural air pollution, as well as introduce new sources of air pollution. .
Point source pollution describes a specific spot-such as a factory's or smokestacks, where pollution is discharged.
Non-point sources are more diffuse and consist of many small sources, such as fireplaces.
Primary pollutants such as soot and carbon monoxide are emitted into the troposphere in a form that is directly harmful.
Secondary pollutants are hazardous substances produced when chemicals added to the atmosphere react with chemicals normally found in the atmosphere.
Six pollutants are closely tracked by the U.S. EPA
The U.S. EPA gives special attention to six criteria pollutants judged to
pose especially great threats to human health and welfare: carbon monoxide (CO), lead (Pb), nitrogen dioxide (N02), ozone (03), sulfur dioxide (SOJ, and particulate matter.
The EPA has established national air quality standards and monitors levels of these pollutants throughout the country, particularly in urban areas. According to the EPA, in 2001 almost half of the population lived in counties where at least one of these six pollutants reached unhealthy levels, although the percentage of days citizens were exposed to unhealthy air dropped.
Carbon monoxide: This colorless, odorless gas, produced primarily by the incomplete combustion of fuels, is the most abundant air pollutant in terms of emissions.