Abstract: The concentration of nitric oxide in a gas is determined by oxidizing NO to NO2 and then measuring the concentration of NO2 in the gas, which is proportional to the concentration of NO. Preferably, gaseous NO2 molecules diffuse through a plurality of capillary membrane fibers and undergo a chemiluminescent reaction with a reagent flowing within; the light from the reaction is measured to determine NO2 concentration. In another aspect of a preferred embodiment, gas is passed through a scrubber before the concentration of NO2 is measured, in order to substantially remove carbon dioxide and ambient NO2 from the gas without substantially affecting the concentration of nitric oxide therein.