Denver had a severe problem in meeting the national standards, the automotive emission controls were primarily set for sea level, and they were not effective at 5000 ft altitude 40 National ambient air quality one-hour standard 30 main commuter traffic 20 10 Eight-hour standard 0 4681012141618202224 Hour(MST) Figure 9.4 CO concentrations averaged over a one-hour period during a weekday several years ago at a downtown location in Denver Colorado. The standard shown of 35 ppm is also for a one-hour averaging timeFigure 9.4 CO concentrations averaged over a one-hour period during a weekday several years ago at a downtown location in Denver, Colorado. The standard shown of 35 ppm is also for a one-hour averaging time. main commuter traffic Denver had a severe problem in meeting the national standards; the automotive emission controls were primarily set for sea level, and they were not effective at 5000 ft altitude