Burlington Free Press: Automobile comparison shoppers who want to reduce their contribution to air pollution and global warming will have some help later this year. A California rule adopted by Vermont will require each new car and light truck to display an Environmental Performance sticker with separate ratings of its greenhouse gas emissions and its contribution to smog. The mandate begins with the 2010 model year. Vehicles will be rated on a 1-10 scale; the cleaner the car, the higher the …