The organic constituents of atmospheric aerosols collected at Niwot Ridge, Colorado, along with various physical and meteorological data, were measured during a collaborative field study. Temporal changes in the composition of samples collected during summertime at Niwot Ridge, a rural site in the Rocky Mountains west of Denver, were studied. Volatile organic compounds were thermally desorbed from aerosol particles, separated by gas chromatography, and identified by mass spectrometry. For each of 48 2- to 5-hour samples, 18 organic compounds in aerosol particles, 11 organic and inorganic compounds in the vapor phase, wind direction, and time of day were measured. Relationships among the variables were analyzed by principal component analysis in order to examine the covariations within the data set. The 31 variables are grouped into seven factors, and individual compounds, as well as the factors, serve as molecular markers for biologic and anthropogenic emission sources. Elucidation of several distinct associations within both rural and urban air masses was possible because of the high time resolution achieved in the collection and analysis of aerosols. In addition, factor scores were used to illustrate how several organic compounds vary with respect to local atmospheric dynamics. ¿ American Geophysical Union 1996