Although auto tailpipes have historically been a major source of smog, the auto industry said that it had largely dealt with that issue.
It would reduce urban pollution, however, by shifting the source of smog from car exhaust pipes to power plants, which are often located outside cities.
He announced that he would impose the nation's toughest restrictions on power plant emissions, a major source of smog, acid rain and greenhouse gases.
It cannot come too soon for the choking residents of China's cities, but the country's government may finally be turning its attention to its primary and deadliest source of smog.
The agency, the Justice Department and various state pollution authorities are investigating whether the engines, which use computerized controls to manage fuel consumption and emissions, were illegally designed to skirt the Clean Air Act's controls on a leading source of smog, officials said.
The affected vehicles ― sold under the Wolfsburg, Germany-based Volkswagen Group brands Audi, Skoda and Seat, as well as VW ― drove roughly 220 billion miles between 2008 and 2015, spewing 240,000 tons of nitrogen oxide, the source of smog and acid rain, into the air.
This information, it stated has "provided important underlying data to help understand the sources of smog".
The smokestack controls will reduce both soot and nitrogen oxides, major sources of smog and acid rain.
The main sources of smog, or ground-level ozone, are emissions from automobiles, power plants and factories.
SCAQMD says that heavy-duty trucks are the number one source of smog-forming emissions in Southern California, and given the population density near the ports, eliminating tailpipe emissions from the local area was a key goal.
