A nitrogen dioxide (NO₂) photoacoustic sensor for environmental monitoring was developed using a low-cost high-power laser diode emitting at 450 nm. A compact low-noise photoacoustic detection module was designed to reduce the sensor size and to suppress noise. A LabVIEW-based control system was employed for the sensor. The parameters of the sensor were studied in detail in terms of laser power and operating pressure. The linearity of the sensor response with laser power and NO₂ concentration confirms that saturation does not occur. At atmospheric pressure, a 3σ detection limit of 250 ppt (part per trillion by volume) was achieved with a 1-s averaging time, which corresponds to the specific detectivity of 3.173 × 10⁻⁹ W cm⁻¹ Hz^−1/2. A 72 h outdoor continuous on-line monitoring of environmental NO₂ was implemented to demonstrate the reliability and validity of the developed NO₂ sensor.