1 Xiong Liu Harvard-Smithsonian Center for Astrophysics K.V. Chance, C.E. Sioris, R.J.D. Spurr, T.P. Kurosu, R.V. Martin, M.J. Newchurch, P.K. Bhartia August 3, 2004 Direct Tropospheric Ozone Retrieval from GOME

2 Outline n Introduction n Retrieval Approach n Validation with DOBSON, Ozonesonde, and TOMS n Global Distribution of Tropospheric Ozone n Future Directions

3 Satellite-based Tropospheric Ozone Retrieval n Satellite observations are crucial for studying the global distributions, spatial and temporal variability, sources and sinks, transport, seasonal behavior of tropospheric ozone. n Challenge: only about 10% of the total ozone, difficult to accurately separate tropospheric ozone and stratospheric ozone n Methods u Residual-based approaches: Total ozone – Stratospheric Ozone F Coarse temporal resolution (i.e., monthly) F Subject to large uncertainties in the assumption made about stratospheric ozone F Limited area coverage (e.g., most of tropospheric ozone retrievals from TOMS are limited in the tropics) u Direct ozone profile retrieval (e.g., from GOME, OMI, SCIAMACHY, TES): forward model simulation + a priori knowledge + spectral fitting

4 Retrieval Approach n Optimal Estimation n Measurements  GOME Channel 1a: nm  Spatial resolution: 960 km x 80 km or 240 km x 40 km n Wavelength and radiometric calibrations  Derive variable slit widths and shifts between radiances/irradiances  Fitting shifts between trace gas absorption cross-sections and radiances  On-line correction of Ring filling in of the solar and telluric absorption features  Improved polarization correction using GOMECAL  Undersampling correction MIN MisfitSmooth and Regularization

5 Retrieval Approach n Improve forward model simulation  LIDORT + look-up table correction of errors due to neglecting polarization  Cloud-top height and cloud fraction from GOMECAT  Monthly-mean SAGE stratospheric aerosols + GEOS-CHEM tropospheric aerosols  Daily ECMWF temperature profiles and NCAR/NCEP surface pressure  Initial surface albedo derived from 370 nm, where has minimal absorption n A priori  TOMS V8 climatology + TOMS EP monthly mean total ozone  Assume a correlation length of 5 km to construct a priori covariance matrix from climatological variances n Retrieval Grid  Almost the same as 11-layer Umkehr grid except the bottom 2 or 3 layers are modified by the NCAR/NCEP reanalysis tropopause pressure

6 Validation and Intercomparison n Comparison with TOMSV8 and DOBSON Total Ozone, Ozonesonde Observation  GOME observations are 8 hours and 600 km within ozonesonde/DOBSON observation  TOMS total ozone columns within GOME footprint are averaged GOME-TOMS= 0.3 ± 5.9 DU Hohenpeisenberg (48N, 11E)

7 Validation and Intercomparison GOME-TOMS= -2.1 ± 7.1 DU Lauder (45S, 170E) n The average biases are within 3 DU with standard deviation within the range of ozone variability, retrieval and measurement uncertainties.

8 Validation and Intercomparison Hohenpeisenberg (48N, 11E)

9 Validation and Intercomparison Lauder (45S, 170E)

10 Global Distribution of Tropospheric Ozone

11 Tropospheric Ozone ( 09/04/97-09/15/97 )

12 Summary and Future Plans n Ozone profiles and tropospheric ozone columns 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 well with TOMS and DOBSON total ozone. n The profiles, stratospheric ozone, and tropospheric ozone compare well with ozonesonde observation. The average bias is within 3 DU with standard deviation within the uncertainties of measurements and ozone variability. n Future Plan: u Tropospheric ozone retrieval from SCIAMACHY during the INTEX period and area. u Improve retrieval including channel 2 u Tropospheric ozone assimilation and simulation

13 Cross Section of Retrieved Ozone Profiles

14 Global Distribution of Stratospheric Ozone

15 Stratospheric Ozone ( 09/04/97-09/15/97 )

16 Global Distribution of Total Ozone

17 Total Ozone ( 09/04/97-09/15/97 )

18 What do we expect from GOME? Siddans, 2003 Assume GOME measurement errors and no other systematic errors, there are about 7-9 DFS from nm region.

19 What can we get now? Average GOME/SHADOZ Bias DFS Soebijanta, 2003 For most retrievals, the resolving length is about km in km, > 30 km in 0-20 km [Meijer et al., 2003]