In this work, a compact sub-ppb-level NOx chemiluminescence sensor, ideal for outdoor monitoring, was developed based on a well-designed air-fed ozoniser and capillary flow stabiliser. A strategy for the air-fed discharge and the optimal injected air flow rate were investigated with the broadband absorption technique to ensure a stable and sufficient ozone production without NOx by-products. A stable sampling flow rate, which is critical to the relative chemiluminescence method, was achieved by controlling the pressure drop over a capillary tube with a diameter of similar to 100 mu m. The critical point of P-in/P-out >2.5 was revealed with experiments. The performance of the proposed NOx sensor was assessed with the standard gas in the concentration ranges of 60.3 ppb to 12.6 ppm. By analysing the Allan variance, it can be determined that a detection limit of 25 ppt is obtained at an integration time of 285 s. Finally, a three-day continuous monitor of the atmospheric NO/NOx locally in Beijing was conducted, which suggests the great potential of the proposed sensor for environmental measurement.