Cavity enhanced spectroscopic techniques for in situ measurement: Pushing the limits of sensitivity.

摘要

New technology is needed to address questions of midlatitude ozone loss, transport across the tropopause, and coupling between chemistry and climate change. Cavity enhanced absorption spectroscopy (LEAS) offers the necessary sensitivity while maintaining a robust design capable of withstanding the rigors of flight. Moreover, LEAS measurements can be tied directly to reliable, traceable standards and physical constants; yet, the measurement technology is sufficiently general to observe a broad range of chemical species. The challenge to implementing such systems becomes entirely focussed on identifying a feasible light source. Fortunately, developments in nonlinear photonics and fiber technology have yielded many new options---appropriate for both CEAS systems and others.; Passive cavity enhanced systems demonstrate noise equivalent absorption of 2.4 x 10-11 cm-1 / Hz for cavity ringdown spectroscopy (CRDS), while integrated cavity output spectroscopy (ICOS) systems demonstrate a noise equivalent absorption of 1.9 x 10-12 cm-1/ Hz . Such ultrasensitive instruments are sufficient to offer 50 ppt sensitivity of NO2 in one second at 585 nm, or signal-to-noise of 10 to 1 for depletion ratios (deltaD) for water in the lower stratosphere. A new CEAS technique, the cavity output autocorrelation spectroscopic technique (COAST), is developed which is far more robust than other cavity based techniques while requiring far fewer supporting electronics. This technique will provide an excellent option for low maintenance, routine monitoring applications.; By combining these techniques with new light sources, miniaturized detectors, and distributed processing electronics, miniature instrumentation can be developed for NASA's Altair unmanned aerial vehicle (UAV). The combined capabilities of this flight platform and the new instrumentation offers the ability to address climate change, the ramifications of pollution, and atmospheric transport in entirely new ways.