The following air emission charts show the emissions from our fossil fuel fired power plants over the prior several years. The trend indicates that the Nitrous Oxide (NOx) and Sulfur Dioxide (SO2) rates have decreased as a result of the voluntary installation of additional pollution controls at our coal-fired power plants, as well as the retirement of Four Corners Units 1, 2 and 3 in 2013 and Cholla Unit 2 in 2015.