Cass, who received his Ph.D. from Caltech in 1978, taught at the Institute for 24 years. In January 2000 he joined the faculty of the Georgia Institute of Technology as chair and professor of the earth and atmospheric sciences department. He maintained a joint appointment with Caltech.
Cass initiated a global ozone study at 500 sites around the world in 1999, which continues today. His research group takes airborne particle measurements in many parts of the world, including four sites in India and the Maldives. Seven sites in mainland China were monitored for "Operation Blue Sky," which identified pollution sources in Beijing and other cities, and whose results factored into China's 2008 Olympic bid.
A prolific scientist with more than 200 published articles, conference proceedings, book chapters, and technical reports to his credit, Cass's research focused on air pollution, with a particular emphasis on the control of airborne particles, photochemical oxidants, and improved visibility. He was instrumental in identifying the complex mix of airborne chemicals that pollute urban areas like Los Angeles and the Northeastern United States. Of special concern were very fine particles that can be inhaled and stay in the lungs, and that contribute to haze and poor visibility. He once described haze as a "problem of worldwide note and local disgust."
Cass was equally interested in the protection of museum collections and archaeological sites from damage due to air pollution. He and colleagues modeled air quality both within and just outside several museums throughout Southern California, which was useful in evaluating the effectiveness of various measures to protect works of art.
He did extensive research projects for the Getty Conservation Institute, including studies to determine which artist colorants are subject to fading by gaseous pollutants like ozone, as well as many studies to determine air pollutant intrusion into museums and other facilities that house artwork, such as the new Getty Center in Los Angeles.
In China, he helped design computer-based models that simulated the air flow into the Yungang Grottoes, a collection of man-made cave temples dating from the 5th century A.D. that hold more than 50,000 stone carvings. The grottoes are situated in the middle of one of China's largest coal-mining regions. Cass's work contributed to the design of particle filtration systems and appropriate ventilation rates for reducing air pollution within the grottoes.
And in Poland, that country's monarchs had for centuries enhanced their wealth by trading in salt extracted from the huge Wieliczka mine. Over centuries mine workers were encouraged to decorate the mine's interior. The resulting freestanding statues, bas-relief carvings, and immense chandeliers – all carved from salt – have gained worldwide artistic and cultural renown. In the last century, however, the earliest and some of the most valuable of the carvings have melted into featureless blobs, thanks to pollution. Cass's work contributed to the finding that lowering the relative humidity in the mine would protect the salt from further deterioration.
Cass was a member of the U.S. Environmental Protection Agency's advisory committee on Ozone, Particulate Matter and Regional Haze Implementation Programs and formerly served on the EPA's Clean Air Scientific Advisory Committee. He served on the editorial boards of the journals Aerosol Science and Technology, Environmental Science and Technology and served as a member of the research advisory committee and editorial board of the Health Effects Institute.
His sponsored research included work for the Environmental Protection Agency, the California Air Resources Board, NASA, the Department of Defense, Exxon, and the Ford Foundation.
He obtained a patent for systems reducing the deposition of fluid-borne particles.
He served on advisory panels for the National Research Council, the Lovelace Respiratory Research Institute, the Center on Environmental Health Sciences at MIT, the Universities Corporation for Atmospheric Research, and the South Coast Air Quality Management District.
Cass's wife, Jeanie, and son, Rob, were with him at the time of his death. Cass will be cremated and his ashes taken to Maine, where the family has a home, to be scattered over the ocean.
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