Fig. 1. Schematic of experimental setup

Fig. 2. NO₂ absorption cross section and spectrum of blue laser diode

Fig. 3. Measured photoacoustic signals. (a) Measured photoacoustic signals at different pressures; (b) measured photoacoustic signals at different volume fractions and noise level in pure N₂

Fig. 4. Effects of optical power, chopper and relative humidity. (a) Signal-to-noise ratio versus power; (b) influence of chopper position on photoacoustic signal; (c) effect of relative humidity on photoacoustic signal in NO_2;(d) effect of relative humidity on photoacoustic signal in synthetic air

Fig. 5. Response time of gas sensor

Fig. 6. Allan variance analysis

Fig. 7. Continuous monitoring results of atmospheric NO₂. (a) Measured result by gas sensor; (b) monitoring data from Jining Environmental Monitoring Center

Fig. 8. NO₂ volume fraction in automobile exhaust with NO₂ volume fractions before and after photolysis shown in inset