We have designed a very compact, low cost lidar system designed for profiling of aerosols in the planetary boundary layer. Our design emphasizes portability, reliability, ease-of-use, and the lowest possible cost. Our goal is an instrument which can provide easy and reliable characterization of the boundary layer for users in operational meteorology or air quality management. The lidar transmitter is a diode laser array. As compared with more conventional laser transmitters, the diode array offers overwhelming advantages in compactness, reliability, and cost. The emission wavelength of the AlGaAs diode array is well matched to the peak sensitivity of silicon avalanche photodiodes. The transmitter and receiver are polarization multiplexed through a common aperture. A relatively large (37 cm) optical aperture compensates for the low transmitter peak power. Since the main mission of this particular lidar is measurement in and around major urban areas where aerosol loadings are generally high, the modest sensitivity is not a severe limitation. The transmitter beam is eye- safe at all ranges. Control and data acquisition are managed by a portable computer. To demonstrate the capabilities of the design, we show simulated results under a wide variety of atmospheric conditions.