The Vulcan Project has done extensive research into US CO2 emissions. They've broken down the US into small grid squares (10 km by 10 km) and estimated *hourly* emissions for an entire year (2002)!
Now that data set is crazy huge (5 GB)! Fortunately, they've released the results for the entire year by county.
We have converted this data into KML layers which you can download and add to your own maps.