Urban Ecology

Follow the Flow

by Diane Boudreau

The problem with studying air pollution is that it just won’t stay put. It blows in the wind. It rises with the sun's warmth. It skirts between buildings and slides along mountain slopes in a dusty dance of particles and gas.

Pollution doesn’t respect neighborhood divisions, state lines, or even national borders. The people making the stuff never notice its bad effects. But their neighbors downwind could face some nasty results.

Air pollution happens in the Phoenix area. For example, ozone from Tempe freeways bothers people living to the far east in Fountain Hills. Particles from a power plant miles out of town slink south into Phoenix.

Scientists at Arizona State University are tracing pollution’s trail throughout the Phoenix metropolitan area. They want to learn how it flows in urban areas.

“This is one of the areas where we can really excel,” explains Joe Fernando, an ASU professor of engineering. “We live in complex terrain. Because of that, we have a pollution problem. Here at ASU, we have experts in geography, mathematics, chemistry and engineering who are able to study the problem.”

Fernando leads a group of scientists who study air flow in cities that have what is called “complex terrain.” Complex terrain includes mountains and other irregular features.

The ASU researchers measured particulate matter and ozone, the main pollutants affecting the Phoenix area. Because pollution patterns change with the seasons, they conducted experiments in both summer and winter.

Summertime is ozone season. Ozone is a gas that can irritate your lungs with every breath you take. Phoenix has all the right ingredients for making ozone. Lots of cars and year-round sunshine make Phoenix a perfect ozone factory.

Ozone forms when chemicals called volatile organic compounds combine with oxides of Nitrogen (NOx). Gasoline is a volatile organic compound. NOx is produced by combustion engines like the ones that power cars, trucks, and airplanes.

These chemicals mix in a chemical reaction that is triggered by sunlight. The final result is ozone.

The researchers found that most of the ozone in the Phoenix area comes from the place where several major freeways—Interstate 10, U. S. 60, and the Loop 101—come together.

“This is the area where you expect a lot of morning traffic and a lot of production of nitrogen dioxide. We think that’s the source,” says Andrew Ellis, an ASU climatologist.

Ozone doesn’t form right away, however. The chemical reaction can take up to four hours. Jim Anderson is an atmospheric chemist at ASU. He describes the pollution plume as “a big reaction chamber” that moves eastward as the day progresses.

“Ozone is forming all along. By the time it gets to East Mesa, the ozone concentration is actually higher. It also goes up to Fountain Hills. At 6 p.m. in Fountain Hills we find a higher concentration of ozone than is found anytime in downtown Phoenix,” he says.

Phoenix is not a windy city. It also is surrounded by mountains. These facts make it hard for the ozone to escape the area once is has formed.

Did you ever see a bug trying to climb out of a plastic cup? It crawls up a little way, then slips and falls back down into the bottom. Because Phoenix is in a valley, it resembles a cup. The ozone starts to climb up the sides of the cup - the mountains - but it slides back down again before it can escape.

During the day, rising temperatures heat the slopes of the Superstition Mountains east of Phoenix. The heat causes the air around the mountains to rise. As the air rises out of the valley, it pulls more air in from the west.

Before the ozone can slip over the mountains, however, the sun goes down. This causes the mountains to cool. The air slides back down into the valley.

“As the air slides up the Superstitions we can watch the ozone level peak in Mesa at 4 in the afternoon. We also see a peak on high ozone days at about 10:30 at night. That is when the air slides down the Superstitions and back into the valley,” says Ellis.

The other major pollutant in the Phoenix area is particulate matter. Particulate matter is defined as tiny airborne particles that can be dangerous when they are inhaled. The researchers measured these pollutants during their winter experiments.

The Environmental Protection Agency (EPA) records the air concentration of all particles less than 10 microns wide. They have also started tracking particles under 2.5 microns. Scientists believe that these smaller particles may be the most dangerous to human health.

The EPA only tracks the size of particles, not what they are made of. But certain kinds of particles are more dangerous than others.

The ASU researchers collected particles from the air and studied them using a scanning electron microscope. The microscope bombards the samples with high-energy electrons. This makes the samples emit an X-ray signal. Each different element gives off its own X-ray “signature.” A computer connected to the microscope figures out what the particle is made of by “reading” the X-ray signature. It also measures the size and shape of the particle.

This information helped the researchers find out what harmful particles are out there, and where they might come from.

Exactly what kinds of particles are floating around in Phoenix area air? The scientists found some surprises.

“We saw distinctive particles that come from the burning of coal. But there aren’t any coal-fired power plants in Phoenix,” says Anderson.

The most likely source turned out to be a coal-burning power plant near Joseph City, Ariz., located more than 100 miles northeast of Phoenix.

“We suspect that’s where they come from,” says Neil Berman, a professor of chemical engineering. “The model shows that it’s possible.”

The team found that air from higher mountains north of Phoenix flows down into the Phoenix area after sunset.

Of course, local sources create plenty of particulates of their own. For example, the researchers discovered spheres of toxic iron oxide in the air.

“Some company is putting out a tremendous amount of these particles,” says Anderson. Unfortunately, the scientists could not find their source.

Traffic contributes a lot of particles by spewing exhaust and kicking up dirt. In the mornings, lots of these particles appear just downwind of the interchange between Interstate-10 and Interstate-17 in Phoenix.

With help from the Arizona Department of Transportation, ASU scientists are studying how traffic contributes to air pollution. They are taking air samples and measuring wind direction at various places along Phoenix-area freeways. They are separating particles that come from different directions to find out where those particles are created.

“I don’t think anybody’s ever done this before. I’m interested in whether the speed and number of large trucks affects the emissions,” says Anderson, who leads the study.

Ultimately, the goal of studying pollution is to figure out how to reduce the amount of pollution and to minimize its health effects. By figuring out where pollutants are created, and how they move through the area, the ASU scientists are moving closer to answers.