Collett is a University Distinguished Professor in the Department of Atmospheric Science studying air quality. Particular topics of interest include pollution processing by clouds and fogs, acid rain, nitrogen deposition, fine particles and their role in regional haze formation, and air quality impacts of oil and gas development.

Collett was appointed by Governor Polis to the Regional Air Quality Council and previously served on the USDA Agricultural Air Quality Task Force. He is a Fellow of the Cooperative Institute for Research in the Atmosphere and the American Meteorological Society. Since 2022, he has served as Technical Editor-in-Chief of the Journal of the Air & Waste Management Association.

Collett earned his undergraduate degree in chemical engineering from MIT. He then studied environmental engineering science at the California Institute of Technology, earning both his M.S. and Ph.D. He was a postdoctoral fellow in atmospheric physics at ETH-Zurich.