“This is interesting. Not too thick,” said Jim Crawford, an atmospheric chemist wearing a motion-sickness patch behind his ear. It was afternoon in late July 2019, and Crawford was bearing down on a skein of wildfire smoke visible from the cockpit of a former commercial jet that NASA had retrofitted into an airborne laboratory. In the cabin, 35 scientists and engineers were calibrating their instruments. The mood was wired: Would their tools, most designed to measure urban pollutants, work in air thick with particulates? How would the 50-year-old plane respond in a smoke column? The DC-8 shuddered and jumped as it entered a plume lofted 12,000 feet high by a fire outside of Missoula, Mont. “Forty-five seconds, then turn it around,” Crawford directed the pilots. The turbulence was surprisingly mild, and he wanted to go back through it.