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ACE Retrievals for the Atmospheric Chemistry Experiment Chris Boone, Ray Nassar, Sean McLeod, Kaley Walker, and Peter Bernath ASSFTS 12 May, 2005

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ACE Introduction n SCISAT-1 / ACE developed by the CSA n Launched August 12, 2003 n Routine measurements began February No apparent deterioration in performance thus far. n Primary instrument is a Fourier transform spectrometer, operating between 750 and 4400 cm -1 with a resolution of 0.02 cm -1.

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ACE Retrieval Version History n Version 1.0: Initial retrievals for testing software, used to identify problems n Version 2.0: Improved the low-altitude P/T retrievals, but the VMR retrievals sometimes suffered from unphysical oscillations, and so was not widely distributed

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ACE Version History (continued) n Version 2.1: introduced an empirical function for CO 2 at high altitudes during P/T retrievals (described later). Employed only for limited “analysis campaigns,” and was not applied to the entire ACE data set.

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ACE Version History (continued) n Version 2.2: Scales MSIS-calculated P and T above the highest analyzed measurement during the P/T retrieval. Previous versions simply fixed P and T in this region to MSIS values. n This version (now underway) slated as official ACE release.

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ACE P/T Retrieval process n Significant timing uncertainty for ACE-FTS measurements. We must work on a relative altitude scale rather than an absolute altitude scale. n World Geodetic System 1984 n Acceleration due to gravity: n CO 2 vmr (ppm) (t-t o ), t is time in years and t o = January 1, 1977

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ACE Step 1: P and T first guess n Low altitude data (below ~ 30 km) from the Canadian Meteorological Center (CMC) n One or two day delay for analysis results rather than forecast results n High altitude data from MSIS n 40 day delay for best estimate, but one can calculate results before then with possible reduced accuracy

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ACE Step 2: First guess tangent heights n At high altitudes (taken to be above 43 km), instrument pointing information calculated from pure geometry (via STK). n Must allow for an offset (FTS FOV and suntracker axis not aligned) + timing errors n Below 43 km, refraction and clouds complicate tangent height determination. n For poor pointing knowledge, use tangent heights as parameters in P/T retrieval

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ACE Step 2 (continued) n Between 9 and ~25 km, a good first guess for altitude derived from the ratio of the baseline (R b ) at two locations ( and 2502 cm -1 ) in the N 2 /CO 2 continua region Estimate density of the measurement From the CMC data, determine what altitude m corresponds to. Typically good to better than.5 km for tangent heights above 6 km.

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ACE Step 3: Establish tangent heights between 12 & 20 km n With P and T fixed to CMC data, fit for tangent heights between 12 and 20 km. These are for REGISTRATION ONLY. n Use a set of 16 O 12 C 18 O lines near 2620 cm -1 n These same lines will be used later to determine tangent heights below 12 km. n Scale the strengths up by 3.5% to achieve consistency with CO 2 isotopologue 1. n Physical difference between vmrs?

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ACE Step 4: First estimate for reference pressure Using a reduced microwindow set, determine reference pressure P c. All pressures in this region calculated from hydrostatic equilibrium and P c.

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ACE Step 5: Refine reference pressure molecules/cm 3 Below 25 km:

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ACE Step 5 continued: Calculating tangent heights n Use P and T to calculate tangent height separations from the constraint of hydrostatic equilibrium [ (z c -z c+1 ), (z c -z c+2 )] n Propagate downwards (z c+3, z c+4, etc)

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ACE Step 5 continued: Scaling reference pressure The first calculated tangent height absorbs the effect of an error in the reference pressure P c. Determine a refined value for P c and fix during subsequent analysis Note: this is the result after step 6.

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ACE Step 6: Final Fitting n Fix P c to refined value. n Redo high altitude retrieval with the full microwindow set and P c fixed. n Redo low altitude retrieval with the new value for P c.

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ACE Step 7: Altitude Registration n Recall that we are working on a relative altitude grid. n Compare the retrieved tangent heights to the “registration tangent heights” between 12 and 20 km determined earlier. Shift the retrieved profiles to align.

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ACE Step 8: Below 12 km n Below 12 km, we likely can’t improve upon CMC pressures and temperatures, but we need tangent heights for VMR retrievals n Fit for tangent heights using the 16 O 12 C 18 O lines described previously (again scaling the line strengths by 3.5%) n Works down to 5 km or below. n Note that no seasonal or geographical variation is assumed for CO 2, which is something we need to address.

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ACE Molecules being retrieved n H 2 O, O 3, CH 4, N 2 O, NO 2, NO, HNO 3, HCl, HF, CO, CFC-11, CFC-12, N 2 O 5, ClONO 2 n COF 2, SF 6, HCFC-22, HCN, CF4, C 2 H 2, C 2 H 6, OCS, CH 3 Cl, N 2 n Testing out ClO, HOCl, H 2 O 2, HO 2 NO 2 n Starting on isotopologues: HDO, H 2 18 O, H 2 17 O, CH 3 D, 13 CH4. Others to be added. n CCl 4 requires line mixing. HCOOH needs interferences sorted out. H 2 CO.

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ACE Temperature Comparisons

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ACE VMR comparisons L. Froidevaux et al, “Early Validation of Atmospheric Profiles from EOS MLS on the Aura Satellite,” IEEE Transactions on Geoscience and Remote Sensing. MLS-ACEMLS-HALOE Implications for the chlorine budget diurnal corrections for O3?

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ACE HNO 3 -HITRAN2000 vs HITRAN2004

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ACE Instrumental Lineshape (ILS) The nominal ILS did not fit well with the spectra. There were significant self-apodization effects beyond the normal field of view effects. Sample fitting results spanning the ACE-FTS wavenumber range

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ACE Modeling the ILS The modulation function was scaled by the factor: exp[a*x 2 + b*|x| 3 + c*x 4 ] where x is optical path difference. The empirical parameters (a, b, and c) vary linearly with wavenumber. No ILS asymmetry was observed.

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ACE After ILS Characterization

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ACE On the “raw” grid

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ACE No apodization n Higher resolution sidelobes don’t extend as far n Inherent self-apodization effects reduce sidelobes without a need to alter the measured spectrum n Strong saturation or pressure broadening apodizes the sidelobes (resolution dependent) n In “busy” spectral regions, sidelobes tend to destructively interfere (but increase effective noise) n P/T retrieval between 60 and 90 km requires an increased extent of the ILS.

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ACE Conclusions n More data is now being collected, thanks to increased downlink capacity n Not a lot of margin now for computing power. It will take some time for version 2.2 processing to catch up.

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