A broadband cavity ringdown spectrometer for in-situ measurements of atmospheric trace gases

[摘要] This paper describes a broadband cavity ringdown spectrometer andits deployment during the 2002 North Atlantic Marine BoundaryLayer Experiment (NAMBLEX) to measure ambient concentrations ofNO₃, N₂O₅, I₂ and OIO at the Mace HeadAtmospheric Research Station, Co. Galway, Ireland. The effectiveabsorption path lengths accessible with the spectrometer generallyexceeded 10 km, enabling sensitive localised ``point" measurementsof atmospheric absorbers to be made adjacent to the otherinstruments monitoring chemically related species at the samesite. For the majority of observations, the spectrometer was usedin an open path configuration thereby avoiding surface losses ofreactive species. A subset of observations targeted theN₂O₅ molecule by detecting the additional NO₃formed by the thermal dissociation of N₂O₅. In all casesthe concentrations of the atmospheric absorbers were retrieved byfitting the differential structure in the broadband cavityringdown spectra using a methodology adapted from long pathdifferential optical absorption spectroscopy. The uncertainty ofthe retrieval depends crucially on the correct treatment andfitting of the absorption bands due to water vapour, a topic thatis discussed in the context of analysing broadband cavity ringdownspectra. The quality of the measurements and the retrieval methodare illustrated with representative spectra acquired duringNAMBLEX in spectral regions around 660 nm (NO₃ andN₂O₅) and 570 nm (I₂ and OIO). Typical detectionlimits were 1 pptv for NO₃ in an integration time of 100 s, 4 pptvfor OIO and 20 pptv for I₂ in an integration time of 10 min.Additionally, the concentrations of atmospheric watervapour and the aerosol optical extinction were retrieved in bothspectral regions. A companion paper in this issue presents thetime series of the measurements and discusses their significancefor understanding the variability of short lived nitrogen andiodine compounds in the marine boundary layer.

[学科分类] 大气科学