介绍了在409 nm处采用二极管激光腔衰荡光谱(CRDS)技术探测大气中NO₂的浓度(体积分数，下同)。通过调制二极管激光器参数，优化其输出光谱，获得NO₂的有效吸收截面。探讨水蒸气、臭氧及其他痕量气体可能引起的测量干扰。时间分辨率为1 s时，系统探测限为6.6×10^-11。考虑到NO₂气体吸收截面和系统RL(衰荡腔长与腔内气体单次吸收光程长的比值)的不确定性，该系统的测量误差为±4.5% (1σ)。在实验室条件下，该系统和非相干宽带腔增强(IBBCEAS)系统同时测量NO₂样气，将二者测量结果进行了对比，测量结果具有很好的一致性，线性拟合斜率为0.988±0.005，相关性为0.99。同时该系统被应用于实际环境大气中NO₂浓度的测量，测得NO₂浓度在1×10^-8~5×10^-8范围内，平均浓度为3.5×10^-8。为了证实CRDS系统的测量结果，将其与长光程差分吸收光谱(LP-DOAS)系统测量NO₂浓度的结果进行对比，二者获得了较好的一致性。实验结果表明，CRDS技术可实现大气中NO₂高灵敏度、高时间分辨率的在线探测。

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.