Diode Laser Hygrometer (DLH) Data and Instrument Status Glenn Diskin, Tom Slate, Mario Rana NASA LaRC Jim Podolske NASA ARC ATTREX Science Team Meeting Boulder, Colorado October 23-25, 2013

Diode Laser Hygrometer (DLH) - description Open-Path tunable diode laser spectrometer operating in the 1.4 µm spectral region – Uses commercial telecom fiber-coupled laser – Similar to previous versions of DLH, but built specifically for ATTREX, with new features Wavelength modulation at ~2 kHz – Data analysis based on 2F, 4F demodulation, normalized by signal power Line-locked to absorption line in low-pressure reference cell – Laser wavelength scanned periodically to determine laser-generated harmonic offsets Uses one of three absorption lines, depending on conditions (primarily concentration), plus an altitude-dependent modulation Double-pass external path configuration – “Mirror” is panel of retroreflecting roadsign material on wing-mounted fin – Sample volume is primarily outside of aircraft boundary layer – Internal optical path is pressurized and purged with dry air – No inlet effects, such as condensation, evaporation, hysteresis, etc. – Long path-length, combined with line-locked, multi-harmonic detection provide excellent sensitivity and rapid time response – Normalization by return power allows measurements to be made within clouds Cloud extinction can be assessed from return power signal (not done yet for ATTREX) Real-time mixing ratio determination using onboard pressure, temperature – During ATTREX-2 (2013), internal correction applied to real-time data

DLH Installation on Global Hawk U 46 Zone External Path m RT Internal Path 0.55 m AcronymWeight (lb) Power (W) Measurement Quantity Sampling Rate Precision (1σ) Accuracy DLH50280 H 2 O vapor mixing ratio 100 Hz 1%, or 50 ppbv 10%

Internal Path larger than on previous installations - Accommodates offset to Global Hawk lower fairing - Includes actuated folding mirror to allow for in-flight adjustment of beam path DLH Optical Housing Humidity Assessment - Internal Optical Path Measurement - Vaisala DMT152 DLH Optical Housing - Internal Path New During ATTREX-2 / Coupling to Global Hawk fairing rigid, leak-free - Optical Housing pressurized during flight * Purge/Refill cycling on demand * Reduced sensitivity to internal moisture - Mixing fans added to improve internal homogeneity and aid in moisture removal

DLH Data Status – ATTREX-2 R0 data in ATTREX archive for all six science flights, R1 processing underway – R1 data use improved modulation assessment based on data acquired in AIDA chamber during AquaVIT2 – Data from ATTREX-1 will be reprocessed as well Data cover entire range of altitudes and concentrations encountered Data coverage is generally complete, with short calibration events every 30 minutes used to determine laser-induced harmonic offsets Instrument signal to noise was very good; signal to internal background was much improved from ATTREX-1, and effect of internal humidity is measured and largely corrected – For R0 and R1 data, direct subtraction is used; still see what looks like residual early in each flight – Will investigate correcting using 2F/4F sensitivity differences, as was done for MACPEX

DLH Data Examples -- comparison with NOAA WV Science Flight 5 (2013/02/26) Science Flight 6 (2013/03/01) Thanks to NOAA WV group for pre-release copy of R1 data

Performance Issues Found in ATTREX-2, and Plans for Mitigation Effects of internal humidity are still seen, early in flights, although magnitude is much improved from ATTREX-1 – More aggressive pre-flight purging of Optical Housing Including adding / repositioning stirring fans – Purging of Electronics / Laser Chassis – Utilization of in-fiber sensor to separate effects from the two volumes – Utilization of multiple harmonics to further reduce impact on signal – Expect moisture control to be even more important flying from Guam Will need larger supply of UZ Air than in ATTREX-2 Internal Path optical alignment degraded in flight, reducing signal to noise of that component – Redesigned mounting of breadboard to decouple from optical housing flexure Insufficient time to implement redesign; will use simpler mounting strategy instead for ATTREX-3