Observations, Modeling, and Laboratory Studies
(Seinfeld, Flagan, Cass, Wennberg, Okumura, Sander)

The Earth's atmosphere is both fragile and chemically complex. Its composition has changed continuously since the formation of the Earth some 4.5 billion years ago. Changes have accelerated over the last century as the result of human activities. Release to the atmosphere of reactive compounds such as nitrogen oxides, carbon monoxide, and a variety of organic species has affected the chemistry of the atmosphere from heavily anthropogenically influenced urban areas to the global troposphere itself. Such emissions affect the oxidizing capacity of the atmosphere (i.e., the ability of the atmosphere to cleanse itself by converting chemical species into compounds more readily removed from the atmosphere). These emissions have also led to steadily increasing amounts of ozone in the troposphere. Formation of aerosols associated with anthropogenic emissions lead to health hazards in urban areas. They also alter the transmission of solar radiation through the atmosphere, and influence cloud properties, thereby affecting the climate system. Air quality is of major societal importance and will continue to be a prominent environmental issue of the 21st century.

    Caltech has one of the country's major efforts in tropospheric research. Profs. Seinfeld and Flagan conduct laboratory studies on photochemistry and atmospheric aerosol formation and are also principal investigators on the new research aircraft Pelican, which is heavily instrumented to make measurements on aerosol-climate effects. Prof. Seinfeld conducts a comprehensive program on modeling of the troposphere, from the urban to global scale. Prof. Cass has a major program of atmospheric measurements of gas and aerosol composition as well as modeling the effects of emission changes on urban ozone and aerosol levels. Prof. Wennberg's in situ measurements of the OH radical in the upper troposphere aboard the NASA ER-2 are the basis for the current understanding of the chemistry of the upper troposphere. He is developing a major program on state-of-the-art instrumentation for detection of tropospheric trace gases. Prof. Okumura and Dr. Sander are undertaking laboratory studies of reaction rates and spectroscopy of species such as alkoxy and alkyl peroxy radicals that play a critical role in tropospheric ozone formation.