A three-channel thermal dissociation cavity ring-down spectrometer for the continuous measurement of ambient NO2, total peroxy nitrates and total alkyl nitrates

A newly constructed thermal dissociation cavity ring-down spectrometer (TD-CRDS) for simultaneous measuring NO₂, total peroxy nitrates (ΣPNs) and total alkyl nitrates (ΣANs) was presented. NO₂ is detected directly at around 405.46 nm, ΣPNs and ΣANs are detected as NO₂ after thermal decomposition at 180℃ and 360℃. The influences of the recombination reaction of RO₂ radicals in two different types of heated inlets were discussed and compared, and the thermal decomposition efficiency of PNs was found to be higher with the value of 96% at the heated inlet filled with glass beads than the other (72%). Possible interferences, mainly O₃ (including reactions of O₃ via NO and O₃ via NO₂) and NOx (such as the recombination reactions of NOx and peroxy radicals at different thermal temperatures), were quantitatively characterised. The effects were found to be much weaker in the heated inlet filled with glass beads. Thus, a calibration method for measuring ΣPNs and ΣANs was established, especially to solve the accurate measurement of ΣPNs and ΣANs under high amounts of ambient NOx and O₃ in China. At the time resolution of 20 s, the detection limits of the TD-CRDS instrument for NO₂, ΣPNs and ΣANs are 6 pptv (1σ), 15 pptv (1σ) and 15 pptv (1σ), respectively. Finally, we applied the instrument to the Hefei field campaign, obtaining the concentration distribution and variation characteristics of ΣPNs and ΣANs.