科研之友
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摘要
Diode laser cavity ring-down spectroscopy (CRDS) for the measurement of NO2concentration (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 NO2is 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 6.6×10-11for NO2at 1 s time resolution. The 1σaccuracy of the measurement is ±4.5%, with the largest uncertainty being the effective NO2absorption 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 NO2concentration 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 NO2concentration varies from 1×10-8to 5×10-8with 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 NO2simultaneously. The data sets for NO2exhibit 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 NO2in the atmosphere.
