ULAN BATOR, Mongolia — In the fast-growing shantytowns surrounding Mongolia's capital, coal-burning stoves are the only source of heat during the country's frigid winters. The result is some of the world's dirtiest air. The World Bank estimates that air pollution is responsible for nearly a third of all deaths here, and costs the city an estimated $463 million each year. The government has vowed to clean up the air, but so far progress is slow.