The design and construction of a NO2 trace gas sensor is presented. The gas sensor design is based on Diode Laser Absorption spectroscopy with frequency modulation of the diode current for sensing low (parts per billion per unit volume-ppbv) concentrations of the NO2 gas. Homemade and factory-made calibration gas cells were constructed and used in quantifying the ambient NO2 concentration. The homemade gas cells of 1.87kgm-3 and 187mgm-3 tested the extremes of high and low concentrations. Results show the sensor has ability to measure above 1mgm-3 ambient concentration. Factory-made (THORLABS INC.,USA) gas cells were constructed of 3,6,9 and 12mgm-3 and tested the linearity response of the sensor. The diode laser spectrometer was then used to determine concentration levels for ambient NO2 gas at a site located 150m from Chiromo Road, Nairobi. The ambient NO2 concentration data was obtained by means of standard addition of the factory made gas cells concentration. The concentration levels were measured within a range of 31-316mgm-3. The performance of the sensor was compared to that of an ISO (International Standards Organisation) certified measurement scheme such as The Griess-Saltmann calorimetric method.