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Although they are invisible to the human eye, chemical compounds in the atmosphere influence life on Earth. Even slight variations in the concentrations of water vapor or carbon dioxide, methane, ozone, and other greenhouse gases—not to mention variations in the two main atmospheric components, oxygen and nitrogen—can have dramatic repercussions.

NCAR scientists in the Atmospheric Chemistry Division of the NCAR Earth System Laboratory are focusing on understanding how these compounds influence, among other things, past, present and future climate, and regional and global air quality.

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The Chemistry of our Atmosphere

Common environmental problems – urban air pollution, the ozone hole, greenhouse gases – are drawing greater interest in understanding what goes on in the atmosphere. Home to aerosols, a variety of gases, and water vapor, among other things, atmospheric chemists strive to understand how these atmospheric substances interact with each other, as well as how they impact the Earth's ecosystems. Not surprisingly, the effects of human activity on atmospheric makeup drive much of today’s research.

At NCAR, our Atmospheric Chemistry Division’s (ACD) has concentrated on two main research challenges. The first is understanding and quantifying the impact of urban emissions on air quality. The second is understanding interactions between the physical climate system, the chemical climate system, and the biosphere.

In terms of understanding global air quality, scientific priority goes toward studying large-scale impacts of intense emissions originating from megacities, and the multiphase (gas-aerosol-cloud) processes that transform pollutants in the atmosphere. When it comes to climate, focus has been in two areas – simulating the recent past and future chemical climate states based on current climate simulations, and studying the role of the upper troposphere/lower stratosphere (UTLS) in the physical and chemical climate system.

The Terrain-induced Rotor Experiment (T-REX) is the second phase of a coordinated effort to explore the structure and evolution of atmospheric rotors (intense low-level horizontal vorticies that form along a mountain ridge crest) and associated phenomena in complex terrain. Researchers can view results on the T-Rex Mapserver.

What is Air Pollution?

The air we breathe contains many impurities, ranging from smoke that comes from open fires to invisible gases emitted by the tailpipes of cars. These and other pollutants affect human health, damage fragile ecosystems, reduce visibility, and even damage property. They also have profound impacts on climate.

Our atmosphere, which consists mostly of nitrogen (78%) and oxygen (21%), has always contained many trace chemicals and particles. Even before humans began to have an impact, volcanoes belched enormous amounts of ash and sulfur dioxide into the atmosphere. Winds whip up dust from deserts and smoke from forest fires. Even plants and animals emit subtle amounts of chemicals into the atmosphere—a process that scientists are just beginning to unravel.

But when people use the word “pollution,” they generally mean human-related emissions that degrade the atmosphere. We’re all familiar with the haze or brown cloud of smog that hangs over many of the world’s largest cities, obscuring the Sun and sometimes irritating our lungs and eyes. Common types of air pollution include:

  • carbon monoxide, an odorless gas that forms when carbon in fuels is not burned completely
  • ground-level ozone, the result of a chemical reaction when sunlight acts on nitrogen oxides and volatile organic compounds (more on ozone)
  • nitrogen dioxide, a brownish gas that comes primarily from vehicle emissions, electric utilities, and burning wood
  • particulate matter, which may contain a mix of suspended liquid or solid particles, including dust or soot or the sulfate form of sulfur dioxide. Tiny airborne particulate is also known as aerosol.
  • sulfur dioxide, a gas from burning fuels that contain sulfur (such as coal or oil).

Some pollution is the result of human-generated emissions interacting with naturally forming chemicals. At NCAR, for example, researchers study the ways that trees and other plants emit certain chemicals, known as volatile organic compounds. These chemicals can react with human-generated industrial emissions of nitrogen oxides to form ground-level ozone, a major component of smog.

Pollution's many negative impacts on human health and the environment have been well documented. Nitrogen dioxide, for example, can cause inflammation of the lungs and other respiratory ailments. A related pollutant, nitric oxide, produces haze, damages plants, degrades fabrics, and forms nitrate salts that can corrode metals. Acid rain, formed from emissions of sulfur dioxide and nitrogen oxide, kills life in lakes and streams and damages buildings and monuments.

Small airborne particles are especially dangerous because people can breathe them deep into their lungs, triggering inflammatory reactions. Some of the most innovative research into these ultrafine aerosols is being conducted at NCAR, where scientists have developed a tool to measure the chemical composition of particles as small as four nanometers, or one hundred-millionth of an inch. (This is so small that it’s equivalent to splitting the thickness of a human hair about 50,000 times.) The effort will help determine how such particles are formed.

Pollution Around the World

If pollution stayed in one place, scientists could easily determine where it formed. But, like so much of the atmosphere, pollution is in regular motion. Winds carry particles and gases aloft, blowing them around the globe until they disintegrate or fall back to Earth. This creates a challenge for public policy leaders trying to control the sources of pollution.

A major focus of NCAR research is tracking the paths of pollutants. For example, scientists using a computer model in 1998 found that 50 to 60% of sulfate aerosol in the Pacific Northwest appears to come from industrialized Asia—which may complicate efforts to keep the Northwest’s air clean. A large project in the summer of 2004 tracked pollution plumes as they leave the U.S. Northeast and head across the Atlantic.

In 1999, NCAR launched an Earth-orbiting monitor known as Measurements of Pollution in the Troposphere. MOPITT measures carbon monoxide from aboard NASA's Terra spacecraft as it circles Earth from pole to pole 16 times daily. Scientists at NCAR are blending the new data with output from a computer model of Earth's atmosphere to develop the world's first global long-term maps of pollution in the lower atmosphere.

To learn more about the impacts of pollution, scientists compare the atmosphere in relatively unpolluted places, like remote islands in the Pacific Ocean or Antarctica, with the air over industrialized regions. This way, scientists hope to understand the differences between “clean” and “dirty” air. But pollution is so far flung it even reaches areas once thought pristine. For example, NCAR scientists and their colleagues have sampled aerosols in the air over the southern Indian Ocean, far downwind of India.

This illustration shows a hypothetical plume and possible series of flight patterns during the PACDEX field project. When a major plume of dust and pollutants begins blowing off Asia, the G-V would fly from Boulder to Anchorage, where it would refuel, and then fly on to Yokota Air Base, Japan. It would then conduct a series of flights for about a week in and around the plume as the plume moves over the ocean to North America.

How does Pollution get out of the Atmosphere?

How does pollution get cycled out of the atmosphere? Some types of emissions, such as methane, remain in the atmosphere for years; others, such as carbon dioxide, stay for centuries. In contrast, sulfates can be thought of as sprinters: they stay in the atmosphere only a few days to weeks before falling or raining out. Typically, long-lived pollutants have global impacts, whereas shorter-lived pollutants have time only to cause regional impacts before cycling out of the atmosphere. Different pollutants have different lifespans based on the chemical reactivity of their molecules.

The atmosphere also contains cleansing agents that break up pollution molecules. Among the most important of these agents is the hydroxyl radical (often abbreviated OH). It prevents toxic buildups by oxidizing (essentially erasing) pollution and many naturally occurring chemicals in the air. Scientists at NCAR and the Georgia Institute of Technology have found exceptionally high concentrations of hydroxyl radicals over Antarctica, but it is not clear what the source of these radicals is or what the atmospheric impact might be.

Pollutants sometimes return to the atmosphere after being deposited on Earth, which complicates antipollution efforts. An example of this is mercury, a long-lived pollutant that is highly toxic. Gaseous elemental mercury, which comes from both natural and human sources, travels the globe for about a year before being deposited on land or water. It is then re-emitted through such events as wildfires, which release mercury stored in foliage and ground litter. NCAR researchers have measured the amount of mercury released by wildfires as part of an effort by scientists to understand the global sources of this toxin, as well as how much ends up in the food chain after deposition on land and water.

Air Chemistry and Climate

Scientists know that pollutants have profound impacts on climate. But untangling the exact picture is difficult because different pollutants sometimes have conflicting impacts.

When people talk about the climatic effects of pollution, the discussion inevitably turns to greenhouse gases, such as carbon dioxide (much of which is emitted by cars and trucks). Such gases trap solar radiation in the atmosphere and lead to an increase in temperatures. Without the greenhouse effect our planet would be covered in ice, and carbon dioxide occurs naturally—you're exhaling it in your breath right now. But atmospheric levels of the gas have risen by 31 percent since preindustrial times, from 280 parts per million by volume to over 370 ppmv today because of human activities.

Rising atmospheric levels of carbon dioxide and other greenhouse gases, such as methane and nitrous oxide, are believed to be largely responsible for the increase in global temperatures in recent decades. In the 20th century, the world’s surface air temperature increased by about 0.6º Celsius (1º Fahrenheit). Research at NCAR and other institutions has indicated there is a 90% probability that, between 1990 and 2100, global temperatures will rise by 1.7 to 4.9ºC (3.1–8.9ºF), because of human influences on climate. The warming is likely to occur unevenly, with polar regions far more affected than the tropics (see sidebar).

Other pollutants, however, have a cooling effect. Tiny particles of dust, soot, and other matter that are spewed by vehicles and factories shield parts of Earth from the Sun. Unlike greenhouse gases, they reflect more energy than they absorb. Although these particles can mitigate the impact of greenhouse gases, they can’t offset it. Sulfates and other aerosols remain in the atmosphere for just a few days, and they affect only a limited area near the pollution source. Greenhouse gases, in contrast, remain aloft for years and are distributed around the globe. Computer models at NCAR and other research institutions indicate that, on a global average, sulfates and other particles cause about half as much cooling as greenhouse gases cause warming.

In addition to an increase in temperatures, we may be in for other climate changes. Research by climatologists at NCAR and elsewhere points to more severe droughts, interrupted by heavy storms and flooding. Such extreme events would force changes in farming methods and in the ways that cities handle storm runoff. Pollution also can suppress rainfall, and some research indicates it is reducing the monsoon-driven rainfall so vital for survival on the Indian subcontinent.

Trees play an important role in the climate system by storing carbon.

Ozone in the Atmosphere

Without ozone, life on Earth wouldn’t exist as we know it. A naturally occurring form of oxygen with three atoms instead of the two in atmospheric oxygen, ozone can be found in the lower stratosphere (about 15–25 miles, or 25–40 kilometers, above Earth). There it intercepts ultraviolet light from the Sun that could otherwise prove deadly to people, animals, and plants.

Unfortunately, pollution is creating two ozone-related problems:

  1. Certain pollutants deplete ozone in the stratosphere, exposing us to unhealthy levels of ultraviolet radiation.
  2. Human-related emissions are spurring the formation of harmful ground-level ozone, which is a major contributor to smog.

Concerns over the first of these problems crystallized in the mid-1980s when a team of British scientists found an “ozone hole” in the stratosphere that occurred over the Antarctic during the Southern Hemisphere spring. As much as 70% of the continent’s stratospheric ozone was depleted for weeks at a time. Subsequent research has shown significant loss of stratospheric ozone over the high latitudes in the Northern Hemisphere. While less regular and dramatic than the Antarctic depletion, the Arctic ozone loss is still a cause for concern.

The main culprit behind ozone depletion is the emission of chlorofluorocarbons, which are used for refrigeration and a variety of industrial applications. Beginning in 1987, the international community has taken steps to reduce and eventually end the use of CFCs, which should lead to a restoration of stratospheric ozone levels by 2100.

What about ground-level ozone? This pollutant is difficult to regulate, partly because it is not emitted directly into the air. Instead, it forms in the atmosphere when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Thousands of sources contribute to ground-level ozone, including motor vehicle exhaust and chemical solvents. Emissions from natural sources also play a role. NCAR scientists study how emissions from such natural sources as trees and lightning interact and contribute to ozone levels in the lower atmosphere.

Ground-level ozone is one of the most harmful forms of pollution, causing respiratory problems and damaging plants. An important research focus at NCAR is determining how atmospheric processes lead to the formation of ozone, even in relatively pristine regions. NCAR scientists investigating ground-level ozone fluctuations in the Arctic have determined that photochemical reactions—chemical changes in the presence of sunlight—are largely responsible for springtime peaks of the gas, although seasonal intrusions downward of ozone-rich air from the stratosphere also play a role. Such information will lead to better understanding of this important pollutant and potentially assist in efforts to reduce it.

This diagram shows the physical phenomena and observing systems present at various heights in the atmosphere. At left is the height axis (kilometers on the left, miles on the right). At right is the temperature at various heights (Celsius on the left, Fahrenheit on the right). The color of the vertical bar shows cooling as one ascends through the troposphere and warming in an ascent through the stratosphere. High-flying planes are found near the tropopause, the cold, dry boundary region between the troposphere and stratosphere. Ozone is most concentrated in the lower stratosphere (bottom left).