Photoacoustic NO 2 traces detection based on continuous and pulsed lasers emitting at 532 nm is presented. Both systems were calibrated with known NO 2-pure air mixtures. A minimum detectable volume concentration of 20 parts in 10 9 (S/N=1) and 15 parts in 10 9 for the resonant and pulsed regime, respectively, were obtained. These values were limited by electrical and acoustical noise of the microphone and electronics. Measurements of NO 2 concentrations in urban air are compared with home-manufactured mixtures. Traces of nitrogen dioxide from two different car exhausts are also detected and quantitatively assigned from the calibration curves.