We describe a nitrogen dioxide (NO(2)) detecting technique based on external modulation and correlation detection using a diode laser operating at 445 nm. The technique is suitable for real-time monitoring of nitrogen dioxide concentration because of its straightforward data evaluation, restraining noises, and a low degree of complexity. Measurements of concentrations of nitrogen dioxide have been carried out at room temperature and atmospheric pressure. The absorption signals have been extracted through the correlation detection technique and evaluated by a least-squares method. The results suggest a detection limit of 5 ppm using a 20 cm long gas cell with 100 ms integration time.