Abstract
Abstract. Nitric oxide (NO) and nitrogen dioxide (NO2) are relevant to air quality due to their roles in tropospheric ozone (O3) production. In China, NOx emissions are very high and NOx emissions exhausted from on-road vehicles make up 20 % of total NOx emissions. In order to detect the NO and NO2 emissions on road, a dual-channel cavity ring-down spectroscopy (CRDS) system for NO2 and NO detection has been developed. In the system, NO is converted to NO2 by its reaction with excess O3 in the NOx channel, such that NO can be determined through the difference between two channels. The detection limits of NO2 and NOx for the system are estimated to be about 0.030 (1σ, 1 s) and 0.040 ppb (1σ, 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 was validated against a chemiluminescence (CL) analyser (42i, Thermo Scientific, Inc.) by measuring the NO2 standard mixtures. The measurement results of NO2 showed a linear correction factor (R2) 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 NO2 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 R2=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 R2=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 NO2 plumes.
Highlights
In recent years, with the improvement of people’s living standards, more and more attention is given to the improvement of the living environment
A compact, sensitive and accurate instrument based on diode-laser cavity ring-down spectroscopy with the centre wavelength of 403.64 nm has been demonstrated for detection of trace amounts of NO2 and NOx in ambient air
Contrast measurements between the dual-channel cavity ring-down spectroscopy (CRDS) instrument and a CL analyser on NO2 standard mixtures were performed, which showed a good correlation between the two different techniques
Summary
With the improvement of people’s living standards, more and more attention is given to the improvement of the living environment. It has been demonstrated that these optical methods can achieve a high detection sensitivity and the detection limit is several parts per trillion with a time resolution of several seconds (Li et al, 2004; Wild et al, 2014; Gherman et al, 2008; Kebabian et al, 2008). Osthoff et al (2006) constructed a pulsed cavity ring-down spectrometer where a pulsed (20–100 Hz, up to 25 mJ) frequency-doubled Nd:YAG laser was used for the simultaneous measurements of NO2, nitrate radical (NO3), and dinitrogen pentoxide (N2O5) in the atmosphere, and the detection limit of 40 ppt (1σ ) for 1 s data was achieved for NO2 with an uncertainty within ±4 % under laboratory conditions. The main advantages of this instrument compared with CL instruments are its low detection limit and high sensitivity as well as its potential ability for trace measurements without calibration and interferences
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