A differential absorption lidar (DIAL) system, based on a tunable solid-state Ti:sapphire laser, was developed for nitrogen dioxide monitoring in the blue region. Efficient generation of blue light was realized by sum-frequency mixing between a Ti:sapphire laser, pumped by the second harmonic (SH) of a Nd:YAG laser, and a second Nd:YAG laser. A maximum output pulse energy of 20 mJ at 453 nm was achieved. In order to reduce the error from the change of atmospheric conditions, the laser wavelength was switched on and off resonance in every shot, and the data were averaged over 500 pulse pairs. As an example of measurement, a spatial distribution of nitrogen dioxide emission from a diesel engine was measured by this DIAL system. The detection limit achieved was 0.2 ppm, with a spatial resolution of 12 m.