 Sulfate aerosols have declined significantly since 1970 with the Clean Air Act in the United States and similar policies in Europe. The Clean Air Act was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants, including PM ’ s, dropped by 53 % in the US. In 1975, the masked effects of trapped greenhouse gases finally started to emerge and have dominated ever since.
