Downwind: Three Phases of an Aerosol Form

Once a week, Dr. Matthew Swensen, a geochemist at Lawrence Berkeley National Labs, makes the drive to the top of Mount Tamalpais, just across the Golden Gate Bridge from San Francisco, to collect Chinese particulates. “It’s become kind of a hobby for me,” he says, turning an imaginary wheel to hug a tight curve. “I have a Mini now, and it’s a lot of fun to drive on the mountain roads.” Mount Tam, without immediate pollution sources between the peak and thousands of miles of open Pacific, is what his team calls a “relatively pristine” site, where they can mea