Simultaneous measurement of NO and NO₂ by a dual-channel cavity ring-down spectroscopy technique

• Main Authors: Z. Li, R. Hu, P. Xie, H. Chen, X. Liu, S. Liang, D. Wang, F. Wang, Y. Wang, C. Lin, J. Liu, W. Liu
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-06-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://www.atmos-meas-tech.net/12/3223/2019/amt-12-3223-2019.pdf

<p>Nitric oxide (NO) and nitrogen dioxide (<span class="inline-formula">NO₂</span>) are relevant to air quality due to their roles in tropospheric ozone (<span class="inline-formula">O₃</span>) production. In China, <span class="inline-formula">NO_<i>x</i></span> emissions are very high and <span class="inline-formula">NO_<i>x</i></span> emissions exhausted from on-road vehicles make up 20&thinsp;% of total <span class="inline-formula">NO_<i>x</i></span> emissions. In order to detect the NO and <span class="inline-formula">NO₂</span> emissions on road, a dual-channel cavity ring-down spectroscopy (CRDS) system for <span class="inline-formula">NO₂</span> and NO detection has been developed. In the system, NO is converted to <span class="inline-formula">NO₂</span> by its reaction with excess <span class="inline-formula">O₃</span> in the <span class="inline-formula">NO_<i>x</i></span> channel, such that NO can be determined through the difference between two channels. The detection limits of <span class="inline-formula">NO₂</span> and <span class="inline-formula">NO_<i>x</i></span> for the system are estimated to be about 0.030 (<span class="inline-formula">1<i>σ</i></span>, 1&thinsp;s) and 0.040&thinsp;ppb (1<span class="inline-formula"><i>σ</i></span>, 1&thinsp;s), respectively. Considering the error sources of <span class="inline-formula">NO₂</span> absorption cross section and <span class="inline-formula"><i>R</i>_L</span> determination, the total uncertainty of <span class="inline-formula">NO₂</span> measurements is about 5%. The performance of the system was validated against a chemiluminescence (CL) analyser (42i, Thermo Scientific, Inc.) by measuring the <span class="inline-formula">NO₂</span> standard mixtures. The measurement results of <span class="inline-formula">NO₂</span> showed a linear correction factor (<span class="inline-formula"><i>R</i>²</span>) of 0.99 in a slope of <span class="inline-formula">1.031±0.006</span>, with an offset of (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.940</mn><mo>±</mo><mn mathvariant="normal">0.323</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="76pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="7c1517a6eedf42a18e21f870565c7e4e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-3223-2019-ie00001.svg" width="76pt" height="10pt" src="amt-12-3223-2019-ie00001.png"/></svg:svg></span></span>)&thinsp;ppb. An intercomparison between the system and a cavity-enhanced absorption spectroscopy (CEAS) instrument was also conducted separately for <span class="inline-formula">NO₂</span> measurement in an ambient environment. Least-squares analysis showed that the slope and intercept of the regression line are <span class="inline-formula">1.042±0.002</span> and (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.393</mn><mo>±</mo><mn mathvariant="normal">0.040</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="76pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="40cda32cc76768ef1404b596f4420b73"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-3223-2019-ie00002.svg" width="76pt" height="10pt" src="amt-12-3223-2019-ie00002.png"/></svg:svg></span></span>)&thinsp;ppb, respectively, with a linear correlation factor of <span class="inline-formula"><i>R</i>²=0.99</span>. 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 (<span class="inline-formula">0.352±0.013</span>)&thinsp;ppb, a slope of <span class="inline-formula">0.957±0.007</span> and a correlation coefficient of <span class="inline-formula"><i>R</i>²=0.99</span>. 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 <span class="inline-formula">NO₂</span> plumes.</p>