Thermal dissociation cavity enhanced absorption spectrometer for detecting ANs and PNs in the atmosphere

We have developed a thermal dissociation cavity enhanced absorption spectroscopy (TD-CEAS) for in situ measurement of NO₂, total peroxy nitrates (PNs) and total alkyl nitrates (ANs) in the atmosphere. The instrument uses one optical cavity for measuring NO₂ absorption at 435 - 455 nm. Three channels with heating modules are set before the detecting cell in parallel, for measuring ANs, PNs and NO₂ by stabilizing the temperature of 653 K, 453 K and normal, respectively. Three-channel cycle measurement is realized by dynamic switching design with the cycle time of 3 min, by assuming the air mass change is negligible when measuring on adjacent channels in each cycle. Therefore, the instrument is feasible in relative stable air masses, such as the chamber studies or field campaigns away from the emission source regions. The limit of detection (LOD) is estimated to be 97 pptv (1σ) at 6 s intervals for NO₂. The measurement uncertainty of NO₂ is estimated to be 8%, which mainly originates from effective cavity length, mirror reflectivity, and NO₂ absorption cross section. The uncertainty of PNs and ANs measurement should be enlarged due to the sampling loss and the thermal dissociation efficiency. This instrument had been successfully applied in a field observation in China. Up to 3 ppbv PNs and ANs were observed during ozone pollution episodes. The inter-comparison of NO₂ with that measured by PL-CLD, as well as PNs with that measured by GC-ECD will be presented.