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

Nitric oxide (NO) and nitrogen dioxide (inline-formulaNO2) are relevant to air quality due to their roles in tropospheric ozone (inline-formulaO3) production. In China, inline-formulaNOx emissions are very high and inline-formulaNOx emissions exhausted from on-road vehicles make up 20 % of total inline-formulaNOx emissions. In order to detect the NO and inline-formulaNO2 emissions on road, a dual-channel cavity ring-down spectroscopy (CRDS) system for inline-formulaNO2 and NO detection has been developed. In the system, NO is converted to inline-formulaNO2 by its reaction with excess inline-formulaO3 in the inline-formulaNOx channel, such that NO can be determined through the difference between two channels. The detection limits of inline-formulaNO2 and inline-formulaNOx for the system are estimated to be about 0.030 (inline-formula1σ, 1 s) and 0.040 ppb (1inline-formulaσ, 1 s), respectively. Considering the error sources of inline-formulaNO2 absorption cross section and inline-formulaRL determination, the total uncertainty of inline-formulaNO2 measurements is about 5%. The performance of the system was validated against a chemiluminescence (CL) analyser (42i, Thermo Scientific, Inc.) by measuring the inline-formulaNO2 standard mixtures. The measurement results of inline-formulaNO2 showed a linear correction factor (inline-formulaR2) of 0.99 in a slope of inline-formula1.031±0.006, with an offset of (inline-formula $M24inlinescrollmathml-normal 0.940±normal 0.323$ 76pt10ptsvg-formulamathimg7c1517a6eedf42a18e21f870565c7e4e amt-12-3223-2019-ie00001.svg76pt10ptamt-12-3223-2019-ie00001.png ) ppb. An intercomparison between the system and a cavity-enhanced absorption spectroscopy (CEAS) instrument was also conducted separately for inline-formulaNO2 measurement in an ambient environment. Least-squares analysis showed that the slope and intercept of the regression line are inline-formula1.042±0.002 and (inline-formula $M27inlinescrollmathml-normal 0.393±normal 0.040$ 76pt10ptsvg-formulamathimg40cda32cc76768ef1404b596f4420b73 amt-12-3223-2019-ie00002.svg76pt10ptamt-12-3223-2019-ie00002.png ) ppb, respectively, with a linear correlation factor of inline-formulaR2=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 (inline-formula0.352±0.013) ppb, a slope of inline-formula0.957±0.007 and a correlation coefficient of inline-formulaR2=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 inline-formulaNO2 plumes.

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Li, Zhiyan / Hu, Renzhi / Xie, Pinhua / et al: Simultaneous measurement of NO and NO2 by a dual-channel cavity ring-down spectroscopy technique. 2019. Copernicus Publications.

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