With Every Breath You Take

ASU scientists are busy following the flow of air pollution in the Phoenix metro area.

The problem with studying air pollution is that it just won’t stay put. It blows in the wind, rises with the sun’s warmth, skirts 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 creating the stuff might never even notice its ill effects. But their neighbors downwind could face some nasty results.

For example, ozone originating on Tempe freeways plagues residents living up in the northeastern foothills. Particles formed in a power plant miles out of town slink southward into the Phoenix metropolitan area.

Researchers at Arizona State University are picking apart this intricate choreography as it occurs throughout the Valley of the Sun. The scientists study the results of both field and laboratory experiments to learn about the composition and flow of pollutants in urban areas. They also use mathematical models and build computer simulations.

“This is an area where ASU can really excel,” explains Joe Fernando, a professor of engineering and director of the Environmental Fluid Dynamics Program at ASU. “We live in complex terrain. We have a pollution problem because of that terrain. But we also have a group of people here, experts in geography, mathematics, chemistry, and engineering, who are able and willing to study the problem.”

Fernando is the lead investigator for two interdisciplinary pollution studies in the Phoenix area called PAFEX I and PAFEX II. The experiments were originally funded by a grant from the National Science Foundation. The intent was to study the flow of air pollution in cities in what is called “complex terrain,” or terrain with mountains and other irregular features. The U.S. Department of Energy is providing additional funding to further analyze PAFEX data.

To date, the ASU researchers have measured particulate matter and ozone, the primary pollutants affecting the Phoenix area. Because pollution patterns vary according to season, they conducted field work in both summer and winter. The field component of PAFEX I took place at Grand Canyon University in Phoenix in the winter of 1998. PAFEX II fieldwork was conducted at Falcon Field airport in Mesa during the following summer.

Ozone: Peaks and Valleys
Summertime is ozone season. The Phoenix area climate makes it a perfect home for this lung-irritating gas.

“Ozone formation is a photochemical process,” explains Andrew Ellis, an ASU climatologist and PAFEX researcher. “If you have a very dry atmosphere, without a lot of water vapor, then more solar radiation gets into the lower part of the atmosphere.”

Once formed, ozone has little chance to escape the area. Barriers include low wind, few weather systems, and mountainous surroundings.

PAFEX II data shows that ozone hits the northeast portion of the Phoenix area particularly hard.

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 central valley, it pulls more air in from the west.

The ASU scientists figured out what was really happening on high ozone days.

“In central Phoenix, air containing a lot of ozone is produced,” Ellis says. “That air drifted east toward the Superstitions every afternoon. By about 4 p.m. each day, an ozone plume was located right over Mesa. The plume continued to the northeast up the Salt River Valley toward the Superstitions.”

Ozone forms when volatile organic compounds (such as gasoline) combine with oxides of nitrogen (NOx) in a chemical reaction. NOx is produced by combustion engines like those found in cars, trucks, and airplanes.

These compounds appear to originate in the Tempe/East Phoenix area.

“During the daytime hours, when that plume formed and headed towards Mesa, it really had a western anchor. The anchor was right here in Tempe at the junction of all the major freeways — Interstate 10, U.S. 60 and the Loop 101. This is the area where you expect to find a lot of morning traffic and a lot of production of nitrogen dioxide. We think that’s the source,” says Ellis.

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

“Ozone is forming all the time. By the time it gets to East Mesa, the ozone concentration is actually higher. The gas plume floats northeast to Fountain Hills. At 6 p.m. each day, Fountain Hills is home to a higher concentration of ozone than at anytime in downtown Phoenix,” he says.

The researchers found that the pollutants traveled east through the afternoon, but could not escape the metro area because the Superstition Mountains blocked their exit. The polluted air would start to rise up the mountain slopes, but when the mountains cooled after sunset, the same air would slide back down. It could not escape.

“The Superstitions cool pretty rapidly after sunset. The air becomes a little bit heavier and starts to slide back down into the Valley,” Ellis says.

The result? “Not only did we see an ozone peak in Mesa at 4 p.m. as the air slid up the Superstitions, we also saw a peak on high ozone days at about 10:30 at night when the same air slid back down the mountains.”

A Matter of Particulates
Particulate matter is the other pollutant of concern in the Phoenix area. Some scientists refer to this pollutant as “aerosols.” Aerosols are tiny airborne particles, either solid or liquid. When inhaled, these particles can be dangerous, especially the smallest ones.

The Environmental Protection Agency records the air concentration of all airborne particles less than 10 microns in diameter. Recently, they also began tracking particles under 2.5 microns.

A single micron is equal to one millionth of a meter. For scale, consider that a single human hair is 25 to 100 microns wide. The smallest bit of beach sand is about 90 microns. Tobacco smoke particles are between 0.01 and 1 micron.

“The new standard is PM 2.5—particulate matter of less than 2.5 microns,” says Fernando. “EPA scientists believe that these are more hazardous than PM 10. These are very small particles.”

The EPA tracks only particle size, not type. However, the health effects of particulates vary widely according to their composition. Unlike the EPA, the ASU researchers did analyze the composition of the particles they found. This information helped them find out what harmful particles are out there. It also helped to identify their sources.

“Where do they come from? Who’s producing the material in the air that we’re inhaling? Some sources emit particles with distinctive compositions or structures, providing a kind of source fingerprint,” says Peter Buseck, an ASU chemist who analyzes particulate matter.

“Another reason to study particle composition is climate. Everyone knows about greenhouse gases. Particles also have climate effects. But they’re more complicated than gases. Some particles produce net warming effects and some produce net cooling effects,” he says.

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

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

A postdoctoral researcher looked at different power plants and modeled the transports that would have happened on the days when the particles appeared. The likely source turned out to be a coal-burning power plant near Joseph City, Arizona, 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 it’s possible.”

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

This down slope flow comes in at about 40 to 50 meters off the ground. It stays at that level all night, when the ground is cold and the air is stable. The situation changes at sunrise.

“When the sun comes up, the area near the ground gets heated. The warmer air starts rising. The cooler air loaded with particulates from the down slope flow goes down,” Fernando explains. “A heavy shear exists between the two layers. This causes instability. The air begins to mix. This is one of the major findings of the study—how the flow takes particles from 50 meters high to ground level at sunrise,” he says.

Of course, local sources contribute 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, their source remains unknown.

Traffic contributes plenty of particles. Cars and trucks and buses spew exhaust and kick up dirt and dust. In the mornings, lots of these particles appear just downwind of the “The Stack,” the huge I-10/I-17 interchange in Phoenix.

With help from the Arizona Department of Transportation, ASU scientists are studying traffic’s contribution to air pollution. Researchers take air samples and measure wind direction at various locations along Phoenix-area freeways. Samples in hand, the scientists separate particles that come from different directions to find out where they originate. ADOT provides data on traffic patterns.

“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 is lead investigator for the study.

The researchers analyze the particles using a scanning electron microscope. Bombarded with high-energy electrons, the samples generate an X-ray signal. Each element gives off a unique X-ray “signature.” A computer connected to the microscope automatically interprets the X-ray signature and also measures the size and shape of each particle.

Unfortunately, this system only identifies inorganic material.

“Inorganic compounds are identified by their elements and their crystal structure,” says Anderson. “Organic compounds are all pretty much made of carbon, hydrogen, oxygen, and nitrogen in different combinations. You can’t tell how they’re combined. If you have a large sample you can do this type of analysis, but not on a particle-by-particle basis. I analyze the particles one at a time.”

Sticky Simulations
The goal of all this research is not just to study what’s in the air, but to understand and, ultimately, predict how it moves. The ASU scientists use both laboratory experiments and mathematical modeling to understand the complex flow of pollutants in three dimensions over time.

Don Boyer is the chair of ASU’s Department of Mechanical and Aerospace Engineering. He developed the laboratory portion of the PAFEX experiments.

“We’re working on using lab experiments to better understand physics and to develop better numeric models. The numeric models will be used to make predictions. The physics [of fluid dynamics] is not well understood, and it’s easier to learn from lab models,” he says.

The lab models use fluids to simulate air flow. Aerospace engineers often use such methods to design aircraft wings.

The fluids in this study contained particles to simulate pollutants. The researchers created containers with slopes and irregular surfaces which they heated and cooled to simulate daily temperature changes. They then used laser beams and high resolution cameras to detect the movements of particles. Unfortunately, they ran into some difficulty trying to simulate air flow in complex terrain.

“You can’t simulate turbulence very well,” says Boyer. “That is a major drawback and one we’re trying to address.”

Ultimately, Boyer says that the field work and mathematical modeling are more useful for this particular study. But the lab experiments did offer valuable information about methodology.

“We did learn some things about lab flows and about the difficulty of simulating such complicated flows in the lab. For example, in this instance we learned that the best approach is the field/numerical approach. It’s somewhat of a negative result, but we did learn something,” Boyer says.

Mathematical Models
The flow of air in complex terrain is a complicated process involving many factors: wind speed, wind direction, temperature, humidity, and landscape, just to name a few. The PAFEX researchers use mathematical formulas to describe this movement over space and time. Using computers, they generate 3D pictures of how air flow patterns should occur.

There are some highly sophisticated models available to describe air flows, but they can only resolve data on a one square kilometer level.

“In complex terrain we have to get smaller—we have to get to urban scale. That gets very difficult,” says Berman.

“We started developing a new model, an urban model,” says Fernando. “Currently, the models we use get about one kilometer resolution. Below that we don’t see anything.”

ASU postdoctoral researcher Sang-Mi Lee has been working to develop a new model that will resolve a much smaller area. Improving resolution means solving equations at a greater number of node points on the grid.

“As the number of node points increase, the computing power needed also increases. As a result, you can’t calculate as large an area at a higher resolution,” says Lee.

To deal with this problem, the researchers use a procedure called “nesting.” First they run the original model with a one-kilometer resolution. Then, they run the new model within that model.

“We now use what we call a neighborhood scale model. The model can provide resolutions up to 10 meters squared,” Fernando explains. “We can resolve buildings. We fly an airplane over Phoenix and map all the buildings and their heights and get a topographic map. With that information in hand, we run the model and try to predict the flow through the downtown area. Then we take some measurements.”

The group is constantly improving its models.

“Models are not perfect. We constantly update them when they don’t work. They keep on changing,” Fernando adds.

In that respect, the models are much like the air flow patterns they represent.—Diane Boudreau