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A sensitive, compact detector for the simultaneous measurement of O₃ and NO₂ is presented in this work. There are two channels in the detector, namely the O_x channel and the NO₂ channel. In the presence of excess NO, ambient O₃ is converted to NO₂ in the O_x measurement channel. In both channels, NO₂ is directly detected via cavity ring-down spectroscopy (CRDS) at 409 nm. At a 10 s integration time, the O_x and NO₂ channels have a 1σ precision of 14.5 and 13.5 pptv, respectively. The Allan deviation plot shows that the optimal sensitivity of O₃ and NO₂ occurs at an integration time of ~60 s, with values of 10.2 and 8.5 pptv, respectively. The accuracy is 6% for the O₃ channel and 5% for the NO₂ channel, and the largest uncertainty comes from the effective NO₂ absorption cross-section. Intercomparison of the NO₂ detection between the NO₂ and O_x channels shows good agreement within their uncertainties, with an absolute shift of 0.31 ppbv, a correlation coefficient of R² = 0.99 and a slope of 0.98. Further intercomparison for ambient O₃ measurement between the O₃/NO₂-CRDS developed in this work and a commercial UV O₃ monitor also shows excellent agreement, with linear regression slopes close to unity and an R² value of 0.99 for 1 min averaged data. The system was deployed to measure O₃ and NO₂ concentrations in Hefei, China, and the observation results show obvious diurnal variation characteristics. The successful deployment of the system has demonstrated that the instrument can provide a new method for retrieving fast variations in ambient O₃ and NO₂.