Simultaneous measurement of NO and NO 2 by a dual-channel cavity ring-down spectroscopy technique

Abstract

Abstract. Nitric oxide (NO) and nitrogen dioxide ( NO2 ) are relevant to air\nquality due to their roles in tropospheric ozone ( O3 ) production. In\nChina, NOx emissions are very high and NOx emissions exhausted from on-road\nvehicles make up 20\u2009% of total NOx emissions. In order to detect the NO\nand NO2 emissions on road, a dual-channel cavity ring-down spectroscopy\n(CRDS) system for NO2 and NO detection has been developed. In the\nsystem, NO is converted to NO2 by its reaction with excess O3 \nin the\n NOx channel, such that NO can be determined through the difference\nbetween two channels. The detection limits of NO2 and NOx for the\nsystem are estimated to be about 0.030 ( 1σ , 1\u2009s) and 0.040\u2009ppb (1 σ , 1\u2009s), respectively. Considering the error sources of NO2 \nabsorption cross section and RL determination, the total uncertainty of\n NO2 measurements is about 5%. The performance of the system was\nvalidated against a chemiluminescence (CL) analyser (42i, Thermo Scientific,\nInc.) by measuring the NO2 standard mixtures. The measurement results\nof NO2 showed a linear correction factor ( R2 ) of 0.99 in a slope\nof 1.031±0.006 , with an offset of ( - 0.940 ± 0.323 )\u2009ppb. An\nintercomparison between the system and a cavity-enhanced absorption\nspectroscopy (CEAS) instrument was also conducted separately for NO2 \nmeasurement in an ambient environment. Least-squares analysis showed that the\nslope and intercept of the regression line are 1.042±0.002 and\n( - 0.393 ± 0.040 )\u2009ppb, respectively, with a linear correlation factor\nof R2=0.99 . Another intercomparison conducted between the system and\nthe CL analyser for NO detection also showed a good agreement within their\nuncertainties, with an absolute shift of ( 0.352±0.013 )\u2009ppb, a slope\nof 0.957±0.007 and a correlation coefficient of R2=0.99 . The\nsystem was deployed on the measurements of on-road vehicle emission plumes\nin Hefei, and the different emission characteristics were observed in the\ndifferent areas of the city. The successful deployment of the system has\ndemonstrated that the instrument can provide a new method for retrieving\nfast variations in NO and NO2 plumes.