Cavity Ring Down Spectroscopy_腔衰荡光谱.ppt

Applications of cavity ring-down spectroscopy CRDS in atmospheric chemistry Bristol, United Kingdom University of Bristol Bristol Overview Absorption spectroscopy Principles of cavity ring-down spectroscopy Applications in atmospheric chemistry Sensing of trace constituents C2H2, NO2, IO Aerosol particles Molecular photochemistry 1. Absorption spectroscopy Absorption spectroscopy 2. Cavity ring-down spectroscopy Applications of CRDS Atmospheric Chemistry Trace gas sensing Isotopologue ratios Aerosol optical properties Photochemistry Other research fields Chemistry of plasmas and flames Breath analysis medical diagnosis Astrochemistry etc. Analysis of breath samples Volatile components of human breath contain signatures of a variety of medical conditions. Non-invasive method of diagnosis. Current medical practice involves bag collection, pre-concentration of samples and analysis by gas chromatography. Laser spectrometers offer cheaper, faster alternatives. Examples: NO, ethane, 13CO2 CO2 isotopologue analysis Cavity ring-down spectroscopy Cavity modes 3. The Earth’s atmosphere Chemistry in the troposphere Chemistry studied by CRDS 4. Diode-laser trace gas sensors Distributed feedback DFB and external cavity diode lasers available in near-IR and visible Output powers ~ 1 – 10 mW Bandwidths ~ 1 MHz Measurement of atmospheric CH4 Detection of atmospheric C2H2 Atmospheric C2H2 has mostly anthropogenic sources Atmospheric lifetimes ~ days to weeks Tracer for polluted air masses Mixing ratios 0.8 – 2.5 ppbv in rural areas Monitor via P 17 line of n1 + n3 band at 1535.393 nm CRDS detection limit in 1 atm air ~ 2.5 ppbv t 14 ms CRDS detection limits Allan variance analysis cw CRDS apparatus Tests of cw CRDS measurements Apel Reimer standard mixture of 75 VOCs C2 – C11 Monitoring C2H2 in lab air Monitoring atmospheric C2H2 Isotopologue measurements Measure d13C for C2H2 in cigarette smoke a test source Use H13C12CH and H12C12CH absorption lines R 13