Renyi Zhang, an atmospheric chemist, is studying one such substance, isoprene, given off by oak trees and leading to increased ozone in our atmosphere. Working under a $300,000 grant from the National Science Foundation, Zhang and chemistry professor Simon North have taken on the challenge of unraveling the more than 1,000 reactions that transform organically released isoprene into toxic atmospheric pollutants.
"Air pollution is probably one of the most serious problems facing humankind in the 21st century," said Zhang, a professor in the College of Geosciences. "And certainly, much of that pollution results from human activities. But most people are not aware of the role played by chemical reactions which change substances produced by biogenic species into harmful airborne pollutants.
"Isoprene - C5H8 - is released by the respiration of oak trees and is the second-most abundant naturally produced hydrocarbon (after methane) in our atmosphere," he continued.
"After a complicated series of chemical reactions, isoprene facilitates ozone production, so increased isoprene means more ozone in the air."
Ozone in the upper atmosphere blocks out harmful ultraviolet radiation from the sun, Zhang explained, but nearer the ground, it traps infrared radiation reflected back up from Earth and contributes to heating the air near the planet's surface, the so-called "Greenhouse Effect." So, more ozone can mean rising temperatures near ground-level, contributing to global warming.
"Although near-ground ozone has some beneficial effects, providing excited oxygen atoms needed to produce the free OH radicals that help to bind other chemicals like sulfur and cleanse them from the atmosphere, excess ozone proves harmful to the health of humans and plants," Zhang said. "For example, too much ozone can retard tree growth or even kill trees. And if too many trees die, there will be more CO2 in the air, further trapping heat and raising the temperature of the planet."
Zhang and North are studying isoprene oxidation related to oak trees in the Houston area, where ozone is contributing to increasing air pollution. They are seeking to understand the critical reactions out of the 1,000 in the isoprene to ozone chain in order to find ways to abate air pollution and allow trees to continue their life-cycle without increasing environmental damage.
Zhang will be using laboratory apparatus to study isoprene using chemical ionization mass spectrometry, while North will look at the chemical process using laser-induced fluorescence. Both researchers also employ methods of quantum chemical calculation to analyze their experimental results. In addition to the NSF grant, their work is being funded by the Welch Foundation, the Texas Advanced Research Program (Chemistry) and the U.S. Department of Energy (DOE).
"The isoprene chain reaction is very complicated - in fact, it's been studied for over 30 years without significant results with regard to fundamental details," said Zhang. "Dr. North and I seeking to discover the direction in which reaction pathways proceed. If we can fully understand the critical steps in the reaction, maybe we can determine where best to intervene in the process to keep both our oak trees and ourselves healthier."
(See, among others: Zhang, D., and R. Zhang, Unimolecular decomposition of nitrooxyalkyl radicals from isoprene reaction with NO3, J. Chem. Phys., 116, 9721-9728 (2002); and Zhang, D., R. Zhang, C. Church, and S.W. North, Experimental study of hydroxylalkyl peroxy radicals from OH-initiated reactions of isoprene, Chem. Phys. Lett., 343, 49-54 (2001)).