Abstract: NO_3 and N_2O_5 are important participants in nocturnal atmospheric chemical processes, and their concentrations are of great significance in the study of the mechanism of nocturnal atmospheric chemical reactions. A two-channel diode laser based cavity ring-down spectroscopy (CRDS) instrument was developed to monitor the concentrations of NO_3 and N_2O_5 in the atmosphere. The effective absorption length ratio and the total loss coefficient of the instrument were calibrated using laboratory standard samples. The effective absorption cross section of NO_3 at 662 nm was derived. A detection sensitivity of 1.1 pptv NO_3 in air was obtained at a time resolution of 1 s. N_2O_5 was converted to NO_3 and detected online in the second CRDS channel. The instrument was used to monitor the concentrations of NO_3 and N_2O_5 in the atmosphere of winter in Hefei in real time. By comparing the concentration changes of pollutants such as nitrogen oxides, ozone, PM_2.5 in a rapid air cleaning process, the factors affecting the concentrations of NO_3 and N_2O_5 in the atmosphere were discussed.