The first remote measurements of O3 and aerosols across a tropopause fold are presented in this paper. An airborne differential absorption lidar (DIAL) system was used to obtain profiles of O3 and aerosols along a cross section or a fold on April 20, 1984, over southern Nevada and California. The DIAL measurements across the tropopause fold show a 2.0-km-deep layer, with high O3 concentrations and enhanced aerosol backscattering, that slopes downward from north of Las Vegas, Nevada, to the top or the planetary boundary layer (PBL) over Yuma, Arizona. This is the first continuous mapping of a tropopause fold from the upper troposphere to the PBL. Mixing ratios of O3 in excess of 200 parts per billion by volume (ppbv) were measured remotely in the fold, and these values were corroborated by in situ measurements of O3 through the fold at an aircraft altitude of 6.7 km. Enhanced aerosol backscattering was observed in the tropopause fold and attributed to stratospheric aerosols from the El Chichon eruption. An analysis of the potential vorticity distribution along the DIAL flight track was performed using radiosonde data. A high positive correlation was found between the DIAL O3 mixing ratios and the potential vorticity values in the fold. The average ratio between O3 and potential vorticity was found to be 50.2 ppbv/10-5 cm2 deg g-1 s-1 in the fold. The decrease in layer thickness, potential vorticity, ozone mixing ratio, and aerosol backscatter down the axis of the fold is consistent with a convergent entrainment of tropospheric air across both boundaries of the fold and subsequent irreversible mixing by small-scale turbulent motions. ¿American Geophysical Union 1987