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. Nitric oxide (NO) and nitrogen dioxide ( NO2 ) are relevant to airquality due to their roles in tropospheric ozone ( O3 ) production. InChina, NOx emissions are very high and NOx emissions exhausted from on-roadvehicles make up 20 % of total NOx emissions. In order to detect the NOand NO2 emissions on road, a dual-channel cavity ring-down spectroscopy(CRDS) system for NO2 and NO detection has been developed. In thesystem, NO is converted to NO2 by its reaction with excess O3 in the NOx channel, such that NO can be determined through the differencebetween two channels. The detection limits of NO2 and NOx for thesystem are estimated to be about 0.030 ( 1σ , 1 s) and 0.040 ppb (1 σ , 1 s), respectively. Considering the error sources of NO2 absorption cross section and RL determination, the total uncertainty of NO2 measurements is about 5%. The performance of the system wasvalidated against a chemiluminescence (CL) analyser (42i, Thermo Scientific,Inc.) by measuring the NO2 standard mixtures. The measurement resultsof NO2 showed a linear correction factor ( R2 ) of 0.99 in a slopeof 1.031±0.006 , with an offset of ( - 0.940 ± 0.323 ) ppb. Anintercomparison between the system and a cavity-enhanced absorptionspectroscopy (CEAS) instrument was also conducted separately for NO2 measurement in an ambient environment. Least-squares analysis showed that theslope and intercept of the regression line are 1.042±0.002 and( - 0.393 ± 0.040 ) ppb, respectively, with a linear correlation factorof R2=0.99 . Another intercomparison conducted between the system andthe CL analyser for NO detection also showed a good agreement within theiruncertainties, with an absolute shift of ( 0.352±0.013 ) ppb, a slopeof 0.957±0.007 and a correlation coefficient of R2=0.99 . Thesystem was deployed on the measurements of on-road vehicle emission plumesin Hefei, and the different emission characteristics were observed in thedifferent areas of the city. The successful deployment of the system hasdemonstrated that the instrument can provide a new method for retrievingfast variations in NO and NO2 plumes.

Simultaneous measurement of NO and NO&lt;sub&gt;2&lt;/sub&gt; by a dual-channel cavity ring-down spectroscopy technique

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