Status
The Harvard Herriott Hygrometer (HHH) is a multipass Herriott cell that measures water vapor via direct detection. Predicted accuracy and precision are ± 3–5% and ± 0.05 ppmv H2O, in the lower stratosphere, for a 10-s integration time, respectively. The theory and application of HHH as a water vapor instrument are laid out in the context of making accurate measurements traceable to laboratory standards. In conjunction with the Harvard Water Vapor (HWV) instrument, HHH will establish ultimate credibility via three, independent detection methods in-flight and five for laboratory and in-field calibration. A multi-detection, calibration system of this nature is beyond the scope of any in existence today. Because HHH promises such high reliability and slight margins of error, the data acquired by this instrument should minimize the uncertainty associated with natural and anthropogenic climate forcing. HHH may serve as a prototype instrument for the use of miniaturized, TDL systems as in situ quantifiers of atmospheric gases via the straightforward method of direct detection, thus extending the scientific payback of this new system.