<P> Vehicle emissions control reduces emissions of nitrogen oxides from motor vehicles . </P> <P> A number of international treaties on the long - range transport of atmospheric pollutants have been agreed for example, the 1985 Helsinki Protocol on the Reduction of Sulphur Emissions under the Convention on Long - Range Transboundary Air Pollution . Canada and the US signed the Air Quality Agreement in 1991 . Most European countries and Canada have signed the treaties . </P> <P> In this regulatory scheme, every current polluting facility is given or may purchase on an open market an emissions allowance for each unit of a designated pollutant it emits . Operators can then install pollution control equipment, and sell portions of their emissions allowances they no longer need for their own operations, thereby recovering some of the capital cost of their investment in such equipment . The intention is to give operators economic incentives to install pollution controls . </P> <P> The first emissions trading market was established in the United States by enactment of the Clean Air Act Amendments of 1990 . The overall goal of the Acid Rain Program established by the Act is to achieve significant environmental and public health benefits through reductions in emissions of sulfur dioxide (SO) and nitrogen oxides (NO), the primary causes of acid rain . To achieve this goal at the lowest cost to society, the program employs both regulatory and market based approaches for controlling air pollution . </P>

Where does sulfuric acid in acid rain come from