The team will also be running simultaneous flights in California, to see how far away they can detect the plume. "We want to see what the global impact of this one city is," he says. Ramanathan is especially interested in unraveling the relationship between air pollution and climate change, since prior research from his lab has shown that airborne particles in emissions can mask up to half of the greenhouse effect by reflecting sunlight back into space.

The researchers plan to combine the AUAV measurements with those from NASA satellites and back-calculated wind trajectories. The results should give them a clearer picture of what the air looks like and whether more sunlight is reaching the earth's surface as a result of the decreased emissions.

Ramanathan is concerned that worldwide efforts to reduce air pollution over the next few decades could as much as double the rate of global warming. With a huge number of unknowns in this equation, he's hoping that their work can help him better understand the problem.

In order to accurately measure the impact of Beijing's emissions reductions, the scientists must also know what normal air pollution conditions would be. Greg Carmichael, an environmental and chemical engineer at the University of Iowa, has been modeling Beijing's emissions and creating estimates for what the pollution levels would have been pre-cutbacks, as well as estimates for what they should be now.

Carmichael can't give specifics until the final numbers are in, but he can say that the air quality in Beijing is somewhere between 10 and 50 percent better than it would have been without the strict controls. This is a wide margin to be sure, he says, "but it's a very complex system. And to take Beijing and be able to reduce your emissions by 50 percent is a huge success. It's a difficult thing to do." Los Angeles and other cities have spent 20 years or more trying to improve their air quality, he notes, and they still have a ways to go.