A winter temperature inversion traps smog over the Salt Lake Valley. A new University of Utah study found that as much as 13 percent of the water vapor in the smog comes from fossil-fuel combustion. The combustion vapor not only rises and falls in concert with daily traffic rush hours and overnight use of home furnaces, but also correlates with rises and falls in combustion-produced carbon dioxide, the major gas causing global warming. Measuring chemical signatures in water vapor from combustion may provide researchers a new way to trace the sources of pollutants and carbon dioxide.