Vertical variations of the tropospheric aerosol concentration and size distribution, for sizes between 0.12-μm and 8-μm diameter, in the Southwest Pacific have been examined. The data were obtained from flights by an instrumented aircraft over a period of more than 3 years. The data show that 44% of the variance in the concentration of the accumulation mode aerosol (0.12 μm to 0.5 μm) can be accounted for when it is modeled by an exponential decrease with height to 2400 m and a constant concentration above. The coarse mode aerosol concentration (greater than 0.5 μm) also decreases exponentially with height to 2400 m (46% of the variance accounted for) at a rate only slightly greater than for the accumulation mode aerosol. More of the variance can be accounted for by fitting the exponential decrease multiplied by a different constant term to each vertical profile separately. This implies an improvement in the predictability of the aerosol concentration in the vertical if the column amount or the concentration at some height is known. Examination of the shape of the size distributions in the free troposphere shows no coarse mode, although the tail of the accumulation mode distribution extends to sizes greater than 0.5 μm. At low altitudes the concentration of accumulation model particles is greater and a coarse mode is apparent. The shapes of the distributions, particularly above the boundary layer, are similar and independent of the particle concentration. ¿American Geophysical Union 1993