We present a functional real-time NO2 sensor based on a ZnO field-effect transistor. The dynamic response of the sensor is calculated using a phenomenological charge trapping model, using only experimentally determined parameters. This analytical model is implemented in the sensor protocol to create a hardware demonstrator sensor. We show that the partial NO2 pressure in ambient air can be monitored in real-time for concentrations as low as 40 ppb. The response is verified by simultaneously measuring the NO2 content with a calibrated reference sensor. A perfect agreement between the measured and reference data is obtained, which validates the methodology. The sensor is fabricated using standard IC technology, which can easily be miniaturized and used in handheld applications.