The 2.4 million people who live along Utah's Wasatch Front experience some of the most severe winter particulate matter air pollution in the nation. Now, analysis of measurements taken during National Oceanic and Atmospheric Administration (NOAA) research flights in 2017 indicates that emissions from a single source, a magnesium refinery, may be responsible for a significant fraction of the fine particles that form the dense winter brown clouds that hang over Salt Lake City.