Summer in the city: Pollution acts as sunscreen

There probably aren't many people in the world who think as quantitatively about the grimy particulates floating around in our atmosphere -- such as car exhaust, soot and smoke -- as John Frederick and Jeff Gaffney.

Frederick, Professor and Chairman of Geophysical Sciences, and Gaffney, a chemist at Argonne National Laboratory, are collaborating on a project to measure the properties of these atmospheric aerosols -- the particles big and small, dark and light -- that are floating around in the lower atmosphere. The project is funded by the University of Chicago/Argonne National Laboratory Collaborative Grants program.

Gaffney is conducting a study to comprehensively measure the distribution of particles of various sizes, as well as how these particles reflect or absorb sunlight. Frederick, with graduate student Carynelisa Erlick, is modeling the effects of the density and the properties of the particles on the transmission of ultraviolet radiation and visible light to the Earth's surface.

"It turns out that the grime in cities, mostly from car exhaust, has an SPF of around 2 -- it cuts the amount of UV radiation in half," Frederick said. "That means under the same conditions, you can stay outside twice as long in the city before getting a sunburn as you could in relatively clear country air. No one had any idea, until we started looking at this, that it had such a big effect. It's not to say, at all, that air pollution is good for you, but it's surprising there are these other effects of particulate air pollution that no one has thought about before."

Frederick said he has gotten several calls recently from the White House Office of Science & Technology Policy. Since the Environmental Protection Agency is considering tightening the standards for air pollution, particularly ozone in the lower atmosphere, OSTP is interested in the costs and benefits of cleaning up ozone. (Ozone in the stratosphere protects us from harmful UV radiation, whereas much closer to Earth it is a harmful pollutant produced in part by gases released from automobiles.) One possible cost of attempting to reduce the amount of ozone in the lower atmosphere, OSTP figured, is that it might increase the transmission of UV light.

Frederick said he plugged in the numbers and found about a 5 percent attenuation of UV light by ozone in a polluted boundary layer. "So the effect of cleaning up ozone, which has all kinds of other deleterious effects on human health, would be a small increase in the amount of sunburning, DNA-damaging radiation reaching the Earth," he said. "But that's very small in comparison to the 50 percent attenuation in UV transmission we see with atmospheric aerosols."

Frederick, who has long studied the effects of stratospheric (upper atmosphere) ozone on the transmission of ultraviolet light to the earth's surface, began thinking about particles in the lower atmosphere when he mused about sunlight while looking out his office window. "I can see the Sears Tower from my window," he said, "and I began noticing that on some days, there is a brownish haze around it. One day, while taking off from Midway Airport, I measured the height of the haze and found it to be about two or three Sears Towers tall."

Some time later, when Frederick was on a research trip to South America, he found himself descending through a similar brownish haze en route to Buenos Aires. He began to wonder about the effects of this haze, mostly composed of atmospheric aerosols, on the transmission of ultraviolet light. "And fortunately, I had a smart student who could figure out how to include this in our models," he said.

Argonne's Gaffney is collecting air samples near Chicago and at a Department of Energy site in Oklahoma called the Atmospheric Radiation Measurement site. He is measuring the sizes of the particles collected, which determines how much they scatter light, as well their composition and optical properties, or how well they absorb or reflect light. Frederick and Erlick plug the data Gaffney collects into their theoretical models, which help predict how much the aerosols affect light transmission among other effects.

Frederick said that darker particles, such as soot from industrial pollution and the particles in car exhaust, mostly absorb sunlight. Other particulate matter, such as smoke from a forest fire, reflects sunlight, which makes it look whitish. The particles not only attenuate the visible and ultraviolet light reaching the Earth's surface, but they may affect heating and cooling as well.

"Since the dark particles absorb sunlight they will also have an effect on the temperature in urban areas, but we don't know yet how much," he said. "Whether you get a cooling or a heating effect depends on the optical properties. There is evidence of a cooling trend in some industrial cities in China, and this may be due to the reflective nature of the particles produced there."

While the effect of airborne particles on the transmission of UV light is a hot topic right now, Frederick said, what's even more interesting is that particulate air pollution might have consequences for long-term climate change. Models for predicting climate change are only beginning to be sophisticated enough to take clouds into effect. "Now we know they'll have to think about air pollution, too," Frederick said.

Frederick and Gaffney presented the preliminary findings of their collaboration at a May 2 UC/ANL Collaborative Grants symposium. Frederick said the results of the UV work, which forms the basis of Erlick's Ph.D. thesis, will be published sometime in the next year.

-- Diana Steele