Which two countries signed the 'Air Quality Agreement' also known as ("Acid Rain Treaty") in 1991?

The Air Quality Agreement is an environmental treaty between Canada and the United States. It was signed on 13 March 1991 by Canadian prime minister Brian Mulroney and American President George H. W. Bush and entered into force immediately. It was popularly referred to during its negotiations as the "Acid Rain Treaty", especially in Canada. Negotiations began in 1986 when Mulroney first discussed the issue with then-president Reagan. Mulroney repeatedly pressed the issue in public meetings with Reagan in 1987 and 1988.

The fight against acid rain is considered to be an environmental success. Fifteen years after Prime Minister Brian Mulroney and American President George Bush signed the 1991 acid rain accord, acid rain has been reduced by over 40 per cent. In 2006, it's one of the reasons why Mulroney was named the "greenest" prime minister in Canadian history.