Compact, Fast Cavity Ring-Down Spectroscopy Monitor for Simultaneous Measurement of Ozone and Nitrogen Dioxide in the Atmosphere

A sensitive, compact detector for the simultaneous measurement of O-3 and NO2 is presented in this work. There are two channels in the detector, namely the O-x channel and the NO2 channel. In the presence of excess NO, ambient O-3 is converted to NO2 in the O-x measurement channel. In both channels, NO2 is directly detected via cavity ring-down spectroscopy (CRDS) at 409 nm. At a 10 s integration time, the O-x and NO2 channels have a 1 sigma precision of 14.5 and 13.5 pptv, respectively. The Allan deviation plot shows that the optimal sensitivity of O-3 and NO2 occurs at an integration time of similar to 60 s, with values of 10.2 and 8.5 pptv, respectively. The accuracy is 6% for the O-3 channel and 5% for the NO2 channel, and the largest uncertainty comes from the effective NO2 absorption cross-section. Intercomparison of the NO2 detection between the NO2 and O-x channels shows good agreement within their uncertainties, with an absolute shift of 0.31 ppbv, a correlation coefficient of R-2 = 0.99 and a slope of 0.98. Further intercomparison for ambient O-3 measurement between the O-3/NO2-CRDS developed in this work and a commercial UV O-3 monitor also shows excellent agreement, with linear regression slopes close to unity and an R-2 value of 0.99 for 1 min averaged data. The system was deployed to measure O-3 and NO2 concentrations in Hefei, China, and the observation results show obvious diurnal variation characteristics. The successful deployment of the system has demonstrated that the instrument can provide a new method for retrieving fast variations in ambient O-3 and NO2.