Since 1975, the State of Colorado has mandated the use of oxygenated gasoline during the winter months. There have been technical improvements on fixed-source emissions have been strengthened h improved, and controls in automobile engines, public transportation has beer 88E 15 Standard 1970 1975 1985 1995 2000 2005 Year Figure 9.5 The second 8-hour maximum concentration of Co observed each year in Denver from 1970 to 2004. There have been no violations of the NAaQS carbon monoxide standard for the past decade, owing to the success of air quality control programsFigure 9.5 The second 8-hour maximum concentration of CO observed each year in Denver from 1970 to 2004. There have been no violations of the NAAQS carbon monoxide standard for the past decade, owing to the success of air quality control programs. Since 1975, the State of Colorado has mandated the use of oxygenated gasoline during the winter months. There have been technical improvements in automobile engines, public transportation has been improved, and controls on fixed-source emissions have been strengthened