31st Annual Meeting of the DPS, October 1999
Session 52. Outer Planet Chemistry Posters
Poster Group II, Thursday-Friday, October 14, 1999, , Kursaal Center

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[52.01] Error analysis of Galileo Probe Mass Spectrometer atmospheric mixing ratio measurements

M.H. Wong, S.K. Atreya (Univ. Michigan), P.R. Mahaffy (NASA GSFC)

Ratios of Galileo Probe Mass Spectrometer (GPMS) counts are required to derive mixing ratios of atmospheric gases. To obtain the mixing ratio of a gas X (q\rm X), we divide counts at the atomic mass of X (C\rm X) by counts at the atomic mass of a chosen reference gas (C\rm ref). Detector deadtime has a large effect at high counting rates. A constant, \tau\rm DT, characterizes the deadtime correction, but we solve a transcendental equation to obtain the corrected counts, so propagation of error from \tau\rm DT to error in mixing ratios is not straightforward. We discuss selection of the best values of \tau\rm DT, and estimate the uncertainty in \tau\rm DT. We also discuss the selection of reference gases to minimize deadtime and other errors, and we estimate the effect of choice of reference gas and error in \tau\rm DT on the error in the count ratio C\rm X/C\rm ref.

A laboratory-derived calibration factor (cc\rm X) is needed to convert GPMS C\rm X/C\rm ref into mixing ratio q\rm X. Deadtime corrections are also used in the calibration experiments, introducing a source of error in cc\rm X. In addition, the pressure dependence of cc\rm X for some gases is another source of error. We discuss difficulties with parametrizing the pressure dependence of cc\rm X. We examine sources of error in calibration constants for hydrogen sulfide and methane as an example, and finally derive Error Bars for q\rm H_2S and q\rm CH_4 on Jupiter as measured by the GPMS.

The author(s) of this abstract have provided an email address for comments about the abstract: mike.wong@umich.edu

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