Continuous wave cavity ring-down spectroscopy applied to in situ detection of dinitrogen pentoxide (N2O5)

Abstract

We describe an instrument using cavity ring-down spectroscopy (CRDS), an ultrasensitive detection method, to detect NO3 and N2O5 (via thermal dissociation to NO3 at 80 °C). We use continuous-wave (cw) cavity ring-down spectroscopy, allowing a highly reliable diode laser to act as the light source. This instrument uses all solid-state laser and optical components, and it is compact, portable, and power efficient. Advantages and disadvantages of cw CRDS compared to pulsed CRDS are discussed. Inlet losses and possible interferences were extensively investigated. We show field observations of N2O5 mixing ratios in ambient air. Dinitrogen pentoxide was measured because low ambient temperatures shift the equilibrium between NO3, NO2, and N2O5 strongly towards N2O5. Therefore, only N2O5 is assumed to be present in significant quantities in Fairbanks, AK, during winter. From these data, we identify times where the N2O5 mixing ratio must be zero (due to the absence of ozone indicating presence of NO) and use these time periods to measure the system’s operational detection limit. The 2σ detection limit for N2O5 is 2.4 parts per trillion by volume (pptv) in a 25 s average. Prediction of the N2O5 detection limit from estimates of ring-down signal noise indicates a detection limit of 1.6 pptv, in the same averaging period. The observed detection limit is about 50% larger than the predicted detection limit, indicating that other noise sources affect the true detection limit. A similar instrument using pulsed CRDS was described in the last year, and we compare our instrument to this instrument for detection of N2O5.