HR: 0800h
AN: A21A-0710    [Abstracts]
TI: Laboratory Calibration and Flight Validation of an Aircraft Based Instrument to Measure Water Isotopes in the Upper Troposphere and Lower Stratosphere.
AU: * St. Clair, J M
EM: stclair@huarp.harvard.edu
AF: Harvard Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, MA 02138 United States
AU: Hanisco, T F
EM: tfh@huarp.harvard.edu
AF: Harvard Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, MA 02138 United States
AU: Anderson, J G
EM: anderson@huarp.harvard.edu
AF: Harvard Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, MA 02138 United States
AB: The relative abundance of the hydrogen isotopes of water, H$_2$O and HDO, is a sensitive indicator of the condensation history of air in the near-tropopause region. The observations of the isotopes present a particular challenge because of the very high probability of sampling artifacts in the detection of water vapor itself and the certainty that the isotopes bear those same errors. We have developed a fluorescence-based instrument that has the sensitivity to measure the relative abundance of H$_2$O and HDO without the sampling artifacts associated with large sampling volumes. The instrument combines a new water photolysis system with our pre-existing instrument for laser induced fluorescence detection of OH. Water is photolyzed with an excimer lamp source at $172\;nm$, producing ground state OH and OD radicals that are detected with state selective laser induced fluorescence at $287\;nm$. The experiment has three notable characteristics. The first is the high sensitivity afforded by laser induced fluorescence detection. At stratospheric mixing ratios of H$_2$O ($4\;ppm$ at $50\;mbar$), the relative abundance of H$_2$O and HDO can be measured with a S/N $> 12$ in a $16\;s$ acquisition cycle. The second is a reduction in the exchange of water isotopes on surfaces within the instrument: the OH and OD radicals are removed with near unity efficiency after collisions with walls in the system and are not detected. The third is a rigorous laboratory evaluation of artifacts in the sampling of water vapor and its isotopes, and an empirical demonstration of the instrument's capabilities. The instrument layout and unique detection scheme virtually eliminates the possibility of contamination and resulting measurement hysteresis. These characteristics enable an independent validation of the absorption-based water isotope instruments \emph{in situ}. Laboratory calibration will be discussed in depth, and data will be presented from recent test flights where the laser induced fluorescence instrument was flown alongside an absorption-based water isotope instrument and a Lyman-$\alpha$ water vapor instrument.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting