The number of electric mass-transit buses in Pierce County, King County and the rest of Washington will more than double as the state Department of Ecology spends $13.3 million to help local transit agencies buy 50 of the zero-emission, battery-powered vehicles.

Heavy-duty diesel engines in buses and trucks account for about a third of all diesel emissions in Washington. Replacing 50 such buses with electric ones will eliminate 68,000 tons of carbon dioxide emissions, and cut nitrogen oxide emissions by 70 tons over the lifetime of the vehicles, according to officials.

Transit agencies in King, Snohomish, Pierce, Clark, Benton and Spokane counties will receive up to $300,000 per bus from Washington’s $112 million share of the federal Volkswagen settlement. The grants are intended to help cover the additional costs of purchasing an electric bus, compared to a conventional diesel bus. Transit agencies can also use some of the funding to pay for charging stations, said Maia Bellon, director of the Department of Ecology.

Volkswagen illegally installed software on its diesel cars that activated emission controls only when it detected the emissions were being tested, leading to legal action by federal and state authorities. In ordinary driving, the software allowed the engines to emit as much as 40 times the legal limit for nitrogen oxides, violating the state and federal Clean Air Acts.