We propose herein a new principle to measure atmospheric NO using an NO₂ detector. We tested this method to determine whether precise measurements are achievable or not when using a laser induced fluorescence detector for NO₂. To convert NO to NO₂, O₃ is added to the sample air. When adding O₃, not all of the NO is used in the generation of NO₂: some of the NO does not react with the O₃ and some of the formed NO₂ reacts with O₃.Using a box model we simulated the time evolution of the NO₂ concentration after the addition of the O₃. We found the optimum condition of reaction time and O₃ concentration by both calculation and experiment. To verify the usefulness of ambient air measurements using this technique, observations were conducted. NO concentrations obtained by this system were compared with those of the chemiluminescence technique and resulted in excellent agreement.