The black soot that big rig trucks belch from their chugging diesel engines may soon become a thing of the past. In one of the more far-reaching smog regulations that California has ever proposed, state air regulators are considering a first-in-the-nation plan that would require nearly every privately owned, heavy diesel truck in the state to install a filter that would reduce emissions of soot from their rigs by 85 percent. The new regulation would affect 1 million truckers, half of them registered out of state who regularly drive on California freeways. If approved by the California Air Resources Board...