Harvard H2O Instrument


 

Instrument:Harvard H2O Instrument
Principal Investigators:James G. Anderson and Elliot Weinstock
Organization:Department of Chemistry and Chemical Biology
Department of Earth and Planetary Science
Harvard University
12 Oxford Street
Cambridge, MA 02138

 

Measurement Description: The Harvard hygrometer was originally designed to be an integral part of the HOx instrument, sitting in the third axis position, downstream from the two OH/HO2 axes. It measures water vapor by the technique of photofragment fluorescence. Ambient air is brought to the detection axis along a 3" square duct mounted in the nose of the ER-2, at velocities of 40-80 m/sec, for very fast time response and to prevent contamination from the walls. An rf-driven Lyman-alpha hydrogen lamp dissociates water vapor, forming electronically excited OH. Fluorescence from the OH A state is detected at right angles by a photomultiplier tube with a filter to select light near 313 nm. Because most of the fluorescence is quenched by collisions with air molecules at a rate proportional to the number density of air, at ER-2 altitudes the instrument essentially measures water vapor mixing ratio directly. A vacuum photodiode is mounted across the duct facing the lamp and is used to normalize lamp intensity. A mirror mounted next to it reflects the Lyman-alpha radiation back across the duct to a second photodiode to verify the calibrated sensitivity in the mid- to upper troposphere, where water vapor concentrations are sufficient for direct absorption measurements.

A more detailed description of the instrument can be found in Weinstock et al. [1994]. More recently, based on laboratory calibrations, in-flight comparisons with absorption measurements during stairstep flights, and comparisons with the JPL diode laser hygrometer, Hintsa et al. [1999] demonstrated that the instrument accuracy is ±5%, with an additional potential offset of at most 0.1 ppmv.

The water vapor instrument has flown successfully on the ER-2 in the SPADE, CEPEX, STRAT, and POLARIS campaigns. During the SPADE mission, the seasonal cycle of water vapor in the midlatitude stratosphere was observed using correlation plots of water and N2O [Hintsa et al., 1994]. During the CEPEX mission measurements of water vapor in the lower tropical stratosphere were shown to illustrate the seasonal cycle of water vapor by utilizing simultaneous ozone measurements and the radiation code from the NCAR Community Climate Model in a simple 1-D framework [Weinstock et al., 1995]. During the STRAT campaign the instrument detected dehydration at the edge of the Arctic polar vortex during the 1995-1996 winter [Hintsa et al., 1998a]. Additionally, using water vapor along with simultaneous measurements of CO2, N2O, and O3, tropospheric to stratospheric transport in the lowermost stratosphere was identified. [Hintsa et al., 1998b]. More recently, water vapor measurements in the lower tropical stratosphere during STRAT have been used along with simultaneous CO and CO2 measurements and a simple photochemical model to evaluate the use of average saturation mixing ratios derived from radiosonde temperature measurements at the tropopause as the boundary condition for controlling stratospheric water vapor [Weinstock et al., 1999].

Accuracy:5%
Precision:±0.1 ppmv (1 sigma) for a 10-second average
Response time:<1 sec
Location on ER-2:Nose

 

References:

Dessler, A. E., E. M. Weinstock, E. J. Hintsa, J. G. Anderson, C. R. Webster, R. D. May, J.W. Elkins, and G. S. Dutton, An examination of the total hydrogen budget of the lower stratosphere, Geophys. Res. Lett., 21, 2563-2566, 1994.
Hintsa, E. J., E. M. Weinstock, A. E. Dessler, J. G. Anderson, M. Loewenstein, and J. R. Podolske, SPADE H2O measurements and the seasonal cycle of stratospheric water vapor, Geophys. Res. Lett., 21, 2559-2662, 1994.
Hintsa, E. J., P. A. Newman, H. H. Jonsson, C. R. Webster, R. D. May, R. L. Herman, L. R. Lait, M. R. Schoeberl, J. W. Elkins, P. R. Wamsley, G. S. Dutton, T. P. Bui, D. W. Kohn, and J. G. Anderson, Dehydration and denitrification in the Arctic polar vortex during the 1995-1996 winter, Geophys. Res. Lett., 25, 501-504, 1998a.
Hintsa, E. J., K. A. Boeing, E. M. Weinstock, J. G. Anderson, B. L. Gary, L. Pfister, B. C. Daube, S. C. Wofsy, M. Loewenstein, J. R. Podolske, J. J. Margitan, and T. P. Bui, Troposphere-to-stratosphere transport in the lowermost stratosphere from measurements of H2O, CO2, N2O, and O3, Geophys. Res. Lett., 25, 2655-2658, 1998b.
Weinstock, E. M., E. J. Hintsa, A. E. Dessler, J. F. Oliver, N. L. Hazen, J. N. Demusz, N. T. Allen, L. B. Lapson, and J. G. Anderson, A new fast response photofragment fluorescence hygrometer for use on the NASA ER-2 and Perseus remotely piloted aircraft, Rev. Sci. Instrum., 65, 3544-3554, 1994.
Weinstock, E. M., E. J. Hintsa, A. E. Dessler, and J. G. Anderson, Measurements of water vapor in the tropical lower stratosphere during the CEPEX campaign: Results and interpretation, Geophys. Res. Lett., 22, 3231-3234, 1995.
Weinstock, E. M., E. J. Hintsa, J. G. Anderson, A. E. Andrews, R. L. Herman, R. D. May, C. R. Webster, and T. P. Bui, Evaluation of the seasonal cycle of water vapor in the stratosphere derived from tropical tropopause temperatures using a CO photochemical clock, J. Geophys. Res., submitted.