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A new method has been described and evaluated which will detect ppt levels of hydrocarbons in the atmosphere. An automatic GC was used with a concentrator arid a photoionization detector to measure benzene, toluene and xylene. The values obtained in the Boston, MA area were in good agreement with the literature value. The BTX data were compared with simultaneous CO2 data and some interesting features were observed. Good correlation was obtained with simultaneous CO2 data during rush hour traffic and no correlation during the early morning hours when the BTX values would rise while the CO2 values decreased to their ambient levels of 350 ppm.