A remote emissions sensing system is provided with NOx detection capability. First, a reading of the ambient NOx concentration is taken just prior to each vehicle passing through the system. This measurement accounts for any ambient NOx concentration that may be lingering from the exhaust of a previous vehicle. Next, the system takes a blocked reading when the vehicle is located in the path of the beam. This reading accounts for any ambient or system noise that may exist. Finally, the system takes a reading of the exhaust plume as the beam passes through the plume. A processor determines the portion of the reading due to the exhaust plume by subtracting the ambient and blocked readings from the exhaust plume reading. As a result, a more accurate exhaust concentration reading is provided. Additionally, the system may process exhaust plume readings only in a predetermined wavelength band associated with the known absorption spectrum of NOx so that data points for which there is no significant absorption of NOx may be eliminated. Thus, any noise or other interference in the non-absorptive wavelengths are minimized. Also, changes in the intensity of the radiation are compensated by subtracting a baseline intensity from each signal.