Ever wonder what's in the black cloud that emits from some semi trucks that you pass on the freeway? Lawrence Berkeley National Laboratory (Berkeley Lab) scientist Thomas Kirchstetter knows very precisely what's in there, having conducted detailed measurements of thousands of heavy-duty trucks over months at a time at two San Francisco Bay Area locations.​With a specially outfitted research van equipped with sophisticated monitors for several pollutant types, he and his team are studying emissions levels from diesel trucks to understand and analyze the impact of new control technologies and California air pollution regulations.