Over the period 1971 to 1990, 265 high-altitude balloons with optical particle counters were launched at Laramie, Wyoming, in a long-term study of the stratospheric sulfate aerosol layer. The tropospheric aerosol record obtained at this background continental site is examined here. All aerosol particle size ranges display pronounced seasonal variations, with the condensation nuclei concentration and the optically active (r≥0.15 μm) component showing a summer maximum throughout the troposphere. The dominant source of this aerosol is believed to be photochemically driven gas to particle conversion. Larger particles (r>1 μm) peak in spring and are probably the result of long range transport of Asian desert dust although there is evidence that sulfate aerosol is also present in these air masses. Mass estimates, assuming spherical sulfate particles, indicate an average column mass between altitudes of 2.5 km (33 m above the surface) and 10 km of about 4 and 16 mg m-2 in winter and summer, respectively. The annual average of about 10 mg m-2 is somewhat larger than estimated with model calculations for sulfate aerosol in this region but is within the uncertainties of model and measurements. Calculated optical depths vary between 0.01 and 0.04 from winter to summer; the estimated mass scattering cross section is about 3 m2 g-1 throughout the troposphere. A distinct anticorrelation exists between the optically active and the condensation nuclei components, resulting in a maximum in the mixing ratio of the latter just below the tropopause where the larger particles generally show a minimum. This relation is due to the coagulation of the small, newly nucleated particles with the existing larger particles and to the competition for available condensable vapors presented by the large particles, resulting in an effective new particle source and reservoir region occurring in the upper troposphere. There is evidence for a decreasing trend of 1.6--1.8% per year in the optically active tropospheric aerosol over the past 20 years which may be related to a similar reduction in SO2 emissions in the United States over this period. ¿ American Geophysical Union 1992