(Photo by Carlye Calvin.)

Something in the Air

When it comes to air quality, we all live downstream

People have dealt with air quality issues since the first fire was lit within the close confines of a cave. In the modern world, the ways we make products, grow food, travel, and heat our homes all add pollution to the air. Vegetation, volcanoes, and other natural sources also play a role in air quality, especially when their emissions are mixed with what people produce.

Many air quality problems, such as those brought on by temperature inversions, are local. But clues to air pollution's regional and global reach have been piling up since our view of the atmosphere expanded with the explosion in air and space flight in the mid-20th century.

Researchers at NCAR are collaborating with colleagues around the world to better grasp what's in our air, where those ingredients came from, and where they are heading next. Here's a look at a sample of local, regional, and global investigations.

Local pollution lockdown: the temperature inversion

Urban smog can be trapped and held near the ground during summer or winter by a temperature inversion. When dust particles are mixed in, the layer of pollutants becomes visible, earning the name "brown cloud" in Denver and other locales.

As altitude increases and the air gets thinner, air temperature usually decreases—which is why Hawaii's mountaintops can get snow each winter. Sunlight hitting the ground helps keep warm surface air bubbling upward. The air expands and cools as it rises and its molecular motion slows down.

When a layer of cold air hugs the ground, flipping the normal pattern upside down, it's called an inversion. Once in place, an inversion can be quite stable, as its cold-below-warm structure inhibits the usual vertical mixing of air. The longer a temperature inversion lasts—sometimes for days or even weeks—the more pollutants accumulate in the stagnant air. Relief comes when a new weather system arrives with horizontal winds strong enough to blow away the smog. Inversions happen year round. In winter they often set up when cold, calm air masses shed heat easily into cloudless night skies.

Inversions pose particular air quality problems for cities, especially those where the horizontal movement of air is restricted by mountains, such as Los Angeles, Mexico City, Salt Lake City, or Denver. If your morning weather report includes an air quality alert along with a request to limit driving or wood burning, the odds are it's because your area is experiencing a temperature inversion.

During October 2002, NCAR scientists launched weather balloons and pointed a slew of sensing devices into the skies of the Salt Lake Valley during the Vertical Transport and Mixing Experiment. The project brought together nine experiments coordinated by 14 institutions for an intensive study of how air moves in the valley, especially overnight during colder months. The effort is still yielding results that will be applied to improving the computer models employed in forecasting weather and tracking air quality.

A regional recipe for smog: add vegetation, hydrocarbons, and stir

It may come as a surprise that some types of vegetation can make pollution worse. Ground-level ozone, a major smog component, forms when nitrogen oxides and volatile organic compounds (VOCs) react in the presence of sunlight. Chemical solvents and the hydrocarbons in motor vehicle exhaust contribute to ozone formation, but when VOCs released by pine, poplar, or other trees are added in, ozone production goes up. This makes understanding the interactions between human-produced chemicals and those produced by nature an important key to understanding regional air quality.

More than a dozen researchers from NCAR were joined by university and government colleagues this past July for CELTIC , an intensive study at an experimental site in the Duke Forest, a natural woodland maintained for recreation and research by Duke University in North Carolina. The scientists are analyzing their measurements of the exchange of gases and particles between the forest canopy and the atmosphere to understand how pollutants are affected by the interactions. They hope their work will lead to better air quality prediction across urban and forested areas.

What happens to regional air quality if the global climate continues to warm? Research has shown that VOC emissions from vegetation increase by 15% to 25% with every 1 degree Celsius (1.8 degree Fahrenheit) temperature increase. NCAR is involved in a multiagency project to help the government keep polluted areas in compliance with Clean Air Act standards in the event of rising global temperatures. The result will be a greater understanding of how future climate may affect urban and regional air quality.

Megaproblems in megacities

A 2001 United Nations report predicted that by 2015 at least 27 cities will be home to more than 10 million inhabitants each, and some of those will see unprecedented population levels as high as 20 or 30 million. In developing countries, where the vast majority of these megacities are growing, soot and smoke are currently the dominant forms of air pollution. As cities become more prosperous, motor vehicles become a more significant source of emissions. In either case, the pollutants don't stay within city limits. As they travel downstream, they change and are changed by the natural and human-altered environments they encounter.

NCAR is leading a major project to study what happens to big-city chemical soup as it moves downstream. The Megacity Impacts on Regional and Global Environments initiative will shift into field campaign mode in early 2006, when an international team will focus on the air above and around Mexico City. MIRAGE is using computer models, laboratory studies, and field projects to learn what physical mechanisms produce and control urban pollutants, how the various pollutants interact, and especially how these chemicals are transported outside urban areas and affect the larger environment.

What goes around, comes around

The summer of 2004 saw a high level of cooperation among international teams of researchers studying the flow of polluting gases and particles across continents and oceans. One major component focused on the transport of pollution across the North Atlantic Ocean. As part of that effort, NCAR researchers brought advanced instruments to the New England Air Quality Study to hunt for formaldehyde, peroxyacetyl nitrate (PAN), and other key chemicals involved in the lifespan of ground-level ozone.

NCAR scientists were also part of a multi-institutional team that flew aboard a NASA research aircraft as it made its way across the United States for the North American Intercontinental Chemical Transport Experiment. They began by measuring the arrival in the West of pollution from Asia and then followed the flow to the Midwest and South, measuring the emissions from cities, industrial sites, and natural vegetation along the way. When they arrived in New England several weeks later, they joined forces with the other groups gathered there to observe the pollutants exiting North America and heading toward Europe.

Creating computer models of this eastward flow will be one of the next steps in this collaboration between North American and European researchers. In 2006 a second field campaign will focus on the quality of air flowing over the North Pacific Ocean from Asia to North America.

Tracking pollution from space and on the ground

A space-based instrument called Measurements of Pollution in the Troposphere has been orbiting Earth since late 1999. Developed at NCAR and the University of Toronto, MOPITT makes global observations of carbon monoxide, a pollutant in its own right and a tracer for tracking other harmful ingredients in the lower atmosphere. 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.

NCAR researchers have created seasonal global records of amounts and source locations of carbon monoxide using MOPITT data. This map shows January, February, and March for the years 2000 through 2004. Blue areas indicate no or very little atmospheric carbon monoxide; the levels increase as the colors shift from green, to yellow, to orange, to red. (Image courtesy Cathy Clerbaux, NCAR.)

The instrument, which flies aboard NASA's Terra satellite, has also been used to trace long pollution plumes originating in major fires on several continents. The plumes, which stretch for thousands of miles beyond their source, have been traced back to fires in China, Australia and Russia, to name a few.

On the ground, NCAR's Raman-shifted Eye-safe Aerosol Lidar (REAL) is one of the few lidars that can be used in highly populated areas. The eye-safe and scanning capability expands this laser-based radar's uses to include mapping urban smog ingredients and studying chemical dispersion very near Earth's surface.

REAL made its debut in the field this spring in a package of high-tech instruments and weather forecasting models that was put through its paces at the Pentagon. Coordinated by NCAR scientists, the tests scanned for potential airborne toxins near the Pentagon and predicted the motion and impact on the building such contaminants might have during a hypothetical accident or attack. The knowledge gained from the tests will allow the development of improved systems for protecting facilities from chemical or biological toxins.