The scientists monitored air pollution near Fairbanks, Alaska, in winter 1973. They found that levels of pollutants like hydrocarbons, carbon monoxide, and nitric oxide were higher during periods of atmospheric inversion and traffic. Vehicle emissions were the main source of these pollutants. The highest concentrations recorded were 9.7 ppm for hydrocarbons, 22.1 ppm for carbon monoxide, 482 ppm for carbon dioxide, and over 10,000 N/cu cm for condensation nuclei. These levels were much higher than normal background levels.