In 2000 Status and Trends, the EPA reports an increasing focus on tracking and controlling ground-level ozone and fine particles, key components of smog and haze. Progress has been slowest for ground-level ozone. In some regions of the United States, ozone levels have actually increased in the past ten years. The ozone increase correlates to the increase in NO x emissions from power plants and other sources. NO x emissions also contribute to acid rain, haze and particulate matter. Sulfates, formed mainly from coal-fired power plant emissions, are the main source of particles in the eastern United States. The emissions also contribute to the formation of acid rain. The EPA's emissions trading program successfully reduced these air pollutants, resulting in improved visibility in the eastern United States.