Diode laser cavity ring-down spectroscopy (CRDS) for the measurement of NO₂ concentration (volume fraction, hereinafter the same) in the atmosphere is presented. The output spectrum of the diode laser is optimized by changing the external modulation signal. The effective cross- section of NO₂ is a convolution of the highresolution absorption cross-section and the laser spectrum. The interference of O3, water vapor and other gases has also been investigated. The limit of detection is 6.6×10^-11 for NO₂ at 1 s time resolution. The 1σ accuracy of the measurement is ±4.5%, with the largest uncertainty being the effective NO₂ absorption cross-section and RL (ratio of ring down cavity length and single absorption optical path of the gas in cavity). Comparison of this system to the incoherent broad-band cavity-enhanced absorption spectroscopy (IBB-CEAS) instrument for NO₂ concentration measurement shows linear agreement (R₂=0.99) with slope of 0.988±0.005 in laboratory. The system is employed to measure the ambient NO₂. The NO₂ concentration varies from 1×10^-8 to 5×10^-8 with an average level of 3.5×10^-8. To verify the results of the CRDS instrument, a long path differential optical absorption spectroscopy (LP-DOAS) system is used to measure ambient NO₂ simultaneously. The data sets for NO₂ exhibit high correlation and good agreement within the combined accuracy of the two methods. Linear fits show a slope of 1.051±0.006 in ambient air. The experimental results show that the diode laser cavity ring-down spectroscopy can realize high sensitivity and high temporal resolution in situ detection of NO₂ in the atmosphere.