Xiong Liu Harvard-Smithsonian Center for Astrophysics December 20, 2004 Direct Tropospheric Ozone Retrieval from GOME

2 Outline n Introduction n Algorithm description n Retrieval characterization n Intercomparison with Ozonesonde, TOMS, and Dobson n Global distribution of tropospheric ozone and comparison with GEOS-CHEM model results n Summary and future work

3 Introduction n Current tropospheric ozone retrievals are mainly based on the residual approaches: limited to certain latitude ranges and to monthly level n GOME: first nadir-viewing satellite instrument that allows direct tropospheric ozone retrieval from the space. n Several groups [Munro et al., 1998; Hoogen et al., 1999; Hasekamp et al., 2001; van der A et al, 2002; Muller et al., 2003; Liu et al., 2004] have developed ozone profile retrieval algorithms from GOME: each of them demonstrates that limited tropospheric ozone information can be derived. n However, tropospheric ozone retrieval remains very challenging from GOME: u Require accurate and consistent calibrations. u Need to fit the Huggins bands to high precision. u Tropospheric ozone is only ~10% of total column ozone.

4 Algorithm Description n Inversion technique: Optimal Estimation n Measurements: nm, nm; Spatial resolution: 960×80 km 2 n Perform detailed wavelength and radiometric calibrations:  Derive variable slit widths and shifts between radiances/irradiances  Fit shifts between trace gas absorption cross-sections and radiances  Co-add adjacent pixels from nm to reduce noise  Improve polarization correction using GOMECAL (  Perform undersampling correction with a high-resolution solar reference  Fit degradation for nm on line in the retrieval n Use LIDORT to simulate radiances and weighting functions n Improve forward model simulation:  On-line correction of Ring filling in of the solar and telluric absorption feature with first-order single scattering RRS model [Sioris and Evans, 2002]  Look-up table correction of polarization errors [van Oss, personal comm.]  Monthly-mean SAGE stratospheric aerosols [Bauman et al., 2003]  GEOS-CHEM tropospheric aerosols [Martin et al., 2002]

5 Variable Slit Widths and Shifts

6 Algorithm Description n Improve forward model simulation (continue):  Brion’s ozone absorption cross-sections [Brion et al., 1993]  Daily ECMWF temperature profiles (  Daily NCEP/NCAR surface pressure (  Cloud-top height from GOMECAT [Kurosu et al., 1999]  Cloud fraction derived at nm with albedo database [Kolemeijer et al.,2003]  Wavelength dependent albedo (2-order polynomial) from nm n A priori: latitude and monthly dependent TOMS V8 climatology (a priori and its variance) [McPeters et al., 2003, AGU] n Retrieval Grid: 11 layers, almost the same as the Umkehr grid  Bottom 2-3 layers are modified by tropopause/surface pressure  Tropospheric column ozone is directly retrieved n State Vector: 47 parameters  11 O albedo (1 for ch1a & 3 for ch2b) + 4 Ring (1 for ch1a & 3 for ch2b) + 8 O 3 shift + 8 rad./irrad. shift + 3 degradation correction (ch1a only) + 2 undersampling + 2 NO BrO + 2 SO internal scattering n Fitting residual: 0.40% for band 1a, 0.17% for band 2b, 0.3% for both n Speed : ~17 hours on a 2GHz processor for one day, could be operational

7 Retrieval Characterization n Averaging Kernel: characterize the retrieval n DFS: diagonal elements of averaging kernels n A priori influence:

8 Examples of Averaging Kernels

9

10 A Priori Influence (06/7-9/1997) TOMS V8 A Priori GEOS-CHEM A Priori Retrieval with TOMS V8 A Priori Retrieval with GEOS-CHEM A Priori

11 Retrieval Errors

12 Validation and Intercomparison n GOME data are collocated at 25 ozonesonde stations during n Validate retrievals against TOMS V8, Dobson/Brewer total ozone, and ozonesonde. n Ozonesonde data mostly from WOUDC, and some from CMDL, SHADOZ, and NDSC. n Collocation criteria:  Within ~8 hours, 1.5° latitude and ~500 km in longitude  Average all TOMS points within GOME footprint n Number of comparisons: 4429, 952, and 1937 with TOMS, Dobson, and ozonesonde, respectively

13 n GOME-Dobson: within retrieval uncertainties and ozone variability.  Biases: <5 DU, and <8 DU at two high-latitude stations  1  : 3-6 DU in the tropics, 6-19 DU at higher latitudes. Total Column Ozone Comparison n GOME-TOMS: within retrieval uncertainties and saptiotemporal variability.  Biases: <3 DU except 3-8 DU at a few high-latitude stations  1  : 2-4 DU in the tropics, 4-11 DU at higher latitudes. A Priori Retrieval Dobson TOMS

14 n GOME-Ozonesonde:  Systematic differences exists at Carbon Iodine, CMDL, SHADOZ stations  Bias: <3 DU (2%), except at Ny Ålesund and Neumayer (-3.3% and 4.5%)  1  : 4-9 DU (4-6%) in the tropics and DU (5-10%) at higher latitudes. Stratospheric Column Ozone Comparison A Priori Retrieval Ozonesonde Column ozone between tropopause to~30-35 km 1%-KI buffered 2%-KI unbuffered

15 A Priori Retrieval Ozonesonde n GOME-SONDE within retrieval uncertainties.  Biases: <4 DU (15%) except –5.5, 4.4, 5.6 DU (16-33%) at NyÅlesund, Naha, Tahiti  1  : 3-7 DU (13-28%) A Priori Retrieval Ozonesonde n GOME-SONDE within retrieval uncertainties.  Biases: <4 DU (15%) except –5.5, 4.4, 5.6 DU (16-33%) at NyÅlesund, Naha, Tahiti  1  : 3-7 DU (13-28%) Tropospheric Column Ozone Comparison

16 n The degradation is well handled n GOME retrievals agree well with ozonesonde  Biases: <2 DU (10%)  1  : <10 DU (25%) Profile: Hohenpeißenberg (48N,11E), A Priori Retrieval Ozonesonde

17 n Positive bias in the middle, negative bias at two ends, probably due to some systematic bias in radiance spectra and the wavelength dependent correction is not perfect.  Biases: < 4 DU (40%)  1  : <4 DU (30%) n Bias in tropospheric/stratospheric column ozone is reduced due to canceling errors. Profile: America Samoa (14S,171W), A Priori Retrieval Ozonesonde

18 Examples of Daily Global Tropospheric Ozone High ozone over biomass burning South Atlantic Paradox Low tropospheric ozone in tropical Pacific Bands of high ozone at mid-latitudes High ozone at high-latitudes during late winter and early spring

19 Monthly Mean Tropospheric Ozone (09/96-11/97)

20 GOME vs. GEOS-CHEM Tropospheric Ozone SON,96 R= ±6.8DU DJF,96-97 R= ±5.3DU MAM,97 R= ±4.5DU JJA,97 R= ±5.7DU

21 Summary n Ozone profiles and tropospheric column ozone are derived from GOME using the optimal estimation approach after detailed treatments of wavelength and radiometric calibrations and improvement of forward model inputs. n Retrieved total ozone compares very well with TOMS and Dobson/Brewer total ozone. n The profiles, stratospheric ozone, and tropospheric ozone compare well with ozonesonde observations except some stratospheric bias at Carbon Iodine stations, CMDL, and SHADOZ stations. n Global distribution of tropospheric ozone is presented. It clearly shows the signals due to biomass burning, air pollution, stratospheric- troposphere exchange, transport and convection. n The overall structures of retrieved tropospheric ozone are similar to those of GEOS-CHEM, but significant differences exist.

22 Future Work n Complete tropospheric ozone retrieval for the 8-year GOME data record and apply the algorithm to SCIMACHY and OMI data n With the aid of GEOS-CHEM, other observations, or model fields, understand the GOME/GEOS-CHEM similarities and differences, and investigate global/regional distribution of tropospheric ozone. n Tropospheric ozone radiative forcing n Tropospheric/stratospheric ozone variability

23 GOME vs. GEOS-CHEM Tropospheric Ozone

24 GOME vs. GEOS-CHEM Tropospheric Ozone

25 GOME vs. GEOS-CHEM Tropospheric Ozone

26 GOME vs. GEOS-CHEM Tropospheric Ozone

27 GOME vs. GEOS-CHEM Tropospheric Ozone