It would appear that we Americans are pumping even more shit into the air each day than moderate pessimists had thought. From the AIP's latest Physics News Update (#696, possibly not on the web yet):

THE MASSIVE NORTHEAST BLACKOUT of a year ago not only shut off electricity for 50 million people in the US and Canada, but also shut off the pollution coming from fossil-fired turbogenerators in the Ohio Valley. In effect, the power outage was an inadvertent experiment for gauging atmospheric repose with the grid gone for the better part of the day. And the results were impressive. On 15 August 2003, only 24 hours after the blackout, air was cleaner by this amount: SO2 was down 90%, O3 down 50%, and light-scattering particles down 70% over "normal" conditions in the same area. The haze reductions were made by University of Maryland scientists scooping air samples with a light aircraft. The observed pollutant reductions exceeded expectations, causing the authors to suggest that the spectacular overnight improvements in air quality "may result from underestimation of emission from power plants, inaccurate representation of power plant effluent in emission models or unaccounted-for atomospheric chemical reactions." (Marufu et al., Geophysical Research Letters, vol 31, L13106, 2004.)