Simultaneous measurement of NO and NOsub2/sub by a dual-channel cavity ring-down spectroscopy technique

摘要

Nitric oxide (NO) and nitrogen dioxide ( NOsub2/sub ) are relevant to air quality due to their roles in tropospheric ozone ( Osub3/sub ) production. In China, NOsubx/sub emissions are very high and NOsubx/sub emissions exhausted from on-road vehicles make up 20?% of total NOsubx/sub emissions. In order to detect the NO and NOsub2/sub emissions on road, a dual-channel cavity ring-down spectroscopy (CRDS) system for NOsub2/sub and NO detection has been developed. In the system, NO is converted to NOsub2/sub by its reaction with excess Osub3/sub in the NOsubx/sub channel, such that NO can be determined through the difference between two channels. The detection limits of NOsub2/sub and NOsubx/sub for the system are estimated to be about 0.030 ( 1σ , 1?s) and 0.040?ppb (1 σ , 1?s), respectively. Considering the error sources of NOsub2/sub absorption cross section and RsubL/sub determination, the total uncertainty of NOsub2/sub measurements is about 5%. The performance of the system was validated against a chemiluminescence (CL) analyser (42i, Thermo Scientific, Inc.) by measuring the NOsub2/sub standard mixtures. The measurement results of NOsub2/sub showed a linear correction factor ( Rsup2/sup ) of 0.99 in a slope of 1.031±0.006 , with an offset of ( - 0.940 ± 0.323 )?ppb. An intercomparison between the system and a cavity-enhanced absorption spectroscopy (CEAS) instrument was also conducted separately for NOsub2/sub measurement in an ambient environment. Least-squares analysis showed that the slope and intercept of the regression line are 1.042±0.002 and ( - 0.393 ± 0.040 )?ppb, respectively, with a linear correlation factor of Rsup2/sup=0.99 . Another intercomparison conducted between the system and the CL analyser for NO detection also showed a good agreement within their uncertainties, with an absolute shift of ( 0.352±0.013 )?ppb, a slope of 0.957±0.007 and a correlation coefficient of Rsup2/sup=0.99 . The system was deployed on the measurements of on-road vehicle emission plumes in Hefei, and the different emission characteristics were observed in the different areas of the city. The successful deployment of the system has demonstrated that the instrument can provide a new method for retrieving fast variations in NO and NOsub2/sub plumes.