Madronich should know. He was research mentor for protégé Stephanie Rivale during four summers in the Significant Opportunities in Atmospheric Research and Science program. The goal of SOARS is to increase the number of students from underrepresented groups enrolled in master's and doctoral programs in the atmospheric and related sciences. Each of Rivale's summer projects built on previous work. Her collaboration with Madronich has affected the direction of some of his own research and has led to some interesting findings.
It all began in the summer of 1996, when Rivale examined the production of urban ozone, which is made when three ingredients are present: nitrogen oxides, hydrocarbons, and ultraviolet light. "Scientists have tended to look at how we humans have affected the emissions of nitrogen oxides and hydrocarbons. But we haven't looked as much at what humans are doing to ultraviolet light, and how that affects pollution," Madronich says.
To study this problem, Rivale and Madronich compared ozone production chemistry in two very different environments: Amsterdam and Mexico City. Amsterdam, at sea level and high latitude, has low sun angles and much weaker ultraviolet levels than Mexico City, which is at high altitude and low latitude, with a higher sun angle and much stronger light. "Our model showed that ozone formation was very sensitive to the amount of radiation, and so of course much more ozone was produced in Mexico City. This was our beginning study."
Rivale and Madronich added twists to that original research over the course of three more summers. "One that was really interesting was in 1999," says Madronich. That year, Rivale went to Mexico City under the auspices of SOARS to collaborate with researchers at the National Autonomous University of Mexico (UNAM). "They collected data on the aerosol haze in Mexico City, and Stephanie also looked at optical measurements made on the ground of how much ultraviolet radiation was coming down through the haze."
Rivale and Madronich used computer models and the collected data on the composition and amount of particles in the haze to reconcile the reduction of ultraviolet light that reaches the surface in Mexico City. "We found that because of the hazinessthe air pollutionthere is some 20 to 30 percent less ultraviolet light reaching the ground than there would be in a clean atmosphere," says Madronich. "Therefore, there is less formation of urban ozone. So one type of pollution is helping to offset another type of pollution."
Rivale, Madronich, and their colleagues at UNAM detailed their work in the journal Atmospheric Environment. The Mexico City findings have implications for the United States, although the effects expected would be less dramatic since the levels of pollution are normally lower. "There are tradeoffs," Madronich says. "Our findings are a reminder that we shouldn't just regulate one type of pollution. We need to act on several fronts at the same time."
During another summer, Rivale and Madronich examined the chemical reactions that keep ozone levels low in extremely polluted cities and what happens when that polluted air is transported out to the countryside. They found that when the polluted air moved away from the city and became more diluted, the chemical reactions "reignited" and actually produced ozone in quantities exceeding the amounts that had been limited by the pollution in the city.
These findings pertain to a more extensive and very complicated question: What happens when pollution is exported from an urban area to the region, the continent, and the entire globe? "In my opinionand Stephanie has played a large role in my developing this opinionit's not a simple proportional system where you increase the pollution in the city and you get a proportional increase spreading everywhere," says Madronich. "The translation between local pollution and global pollution is not linear. If you increase pollution locally, you may actually slow local ozone production, but accelerate it downwind. This is, from the chemical point of view, one of the biggest complications in trying to relate what humans do, which is mostly on a very local scale, to consequences on the global scale."
Madronich is currently involved in the Megacity Impact on Regional and Global Environment (MIRAGE) project, an initiative within NCAR to look at the question of what large cities might do to the regional and global atmosphere today and 10 to 100 years in the future. "We anticipate that most of the growth in population and economic throughput will be in the tropics, with continuing rapid growth of urbanization. We don't really have a clear picture of what that's going to do to the global atmosphere.
"Stephanie has been very helpful to me in better understanding some of the complications of urban-to-global export, in connecting the small scale to the large scale," Madronich adds. "In subtle ways, she contributed to our group's evolution of thinking in these directions. Some of the projects she has done have shown us that, one, we can actually tackle some of these questions and get some interesting answers, and two, there are interesting answers, but they're not simple answers."
Rivale, who completed a master's degree in chemical engineering from the University of Colorado at Boulder this spring, has joined Air Sciences, an air-quality consulting firm in Lakewood, Colorado. "SOARS was the experience of a lifetime," says Rivale. "It helped me become unwilling to accept things as they are, but instead constantly strive to improve the world in which we live and work. I truly appreciate all the time and effort of the SOARS staff and the UCAR community to make SOARS such a great experience."
Edited by Bob Henson,
Prepared for the Web by Jacque Marshall
Last revised: Wed Aug 8 17:05:07 MDT 2001