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A Temporally Consistent NO 2 data record for Ocean Color Work

A Temporally Consistent NO 2 data record for Ocean Color Work. Wayne Robinson, Ziauddin Ahmad, Charles McClain, Ocean Biology Processing Group (OBPG) NASA, Goddard Space Flight Center ( Wayne.Robinson@nasa.gov ) Tropospheric NO2 measured by satellites Workshop,

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A Temporally Consistent NO 2 data record for Ocean Color Work

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  1. A Temporally Consistent NO2 data record for Ocean Color Work Wayne Robinson, Ziauddin Ahmad, Charles McClain, Ocean Biology Processing Group (OBPG) NASA, Goddard Space Flight Center (Wayne.Robinson@nasa.gov) Tropospheric NO2 measured by satellites Workshop, KNMI, DeBilt, The Netherlands September 10 – 12, 2007

  2. Background • Ocean Color – starting with satellite top-of-atmosphere observations, determine the visible radiances leaving the water surface and from them, the chlorophyll concentration and other in-water quantities • Good ocean color retrievals depend on removing the effects of atmospheric aerosols and gases (about 90% of Lt is atmospheric) • Many effects have been accounted for (air molecules, ozone, oxygen, moisture, pressure change) • Recent work has shown importance of NO2 in ocean color retrievals (Ahmad et. al., 2007, Robinson et. al., 2007)

  3. NO2 Effects on Ocean retrievals Sensitivity of Water-leaving radiance and Chlorophyll retrievals to NO2 15 Stratospheric NO2 Tropospheric NO2 NO2 absorption 10 Chlorophyll 5 % Change(for 5 X 1015 molecule NO2 change) 412 443 488 530 550 670 0 Water-leaving radiance -5 400 500 600 700 Wavelength (nm)

  4. Tests of NO2 effect in MODIS Aqua retrievals • A monthly set of average NO2 was made for the GOME, SCIAMACHY, and OMI data with a preference to highest resolution • Tests include temporal trends of Lwn and Chlor-a before and after NO2 use, and their percentage change • Percentage change trends showed a significant shift around Sep 2004

  5. Temporal trend of Lwn 412 and chlorophyll change Trend in Pacific Ocean at 40 South Lwn_412 Chlor-a

  6. Temporal trend of Lwn 412 and chlorophyll change Trend in Pacific Ocean at 40 South Ratio shift Lwn_412 Chlor-a

  7. Temporal trend of Lwn 412 and chlorophyll change and monthly total NO2 average Trend in Pacific Ocean at 40 South Total NO2 for: GOME SCIAMACHY OMI NO2 source change Lwn_412 Chlor-a

  8. Tests of NO2 effect in MODIS Aqua retrievals(continued) • Percentage trends showed a significant shift around Sep 2004 • NO2 data source change from SCIMACHY to OMI coincided with the ratio shift • Variability was seen in different latitude ranges • The shift needed to be removed to make a consistent NO2 record by modifying one data source to match the other

  9. Simple linear fit Total NO2 Tropospheric Stratospheric OMI SCIAMACHY

  10. Problems with linear fit • Significant biases at some latitudes (esp. 40 S) look for a latitudinal correction • Non-linear relation for total NO2 determine a non-linear relation • Low correlation in tropospheric NO2 derive tropospheric NO2 from total - stratospheric Address these in the correction

  11. Latitudinal dependence Fit form: cos( 4 * latitude ) + linear

  12. Latitude dependence, removed • Note that this technique worked well in other ranges

  13. Non-linear relation • 5 forms of non-linear fit were tested • Best fit was an exponential with constant • Y intercept was relatively low (-0.4 for total) • Same functional form and coefficients used for stratospheric NO2 (except constant) • Consistency between stratospheric and total • Fit for total worked relatively well for limited stratospheric data range

  14. Results with both modifications applied Total Tropospheric Stratospheric Un-corrected Corrected

  15. Temporal trend results: before correction

  16. Temporal trend results: after correction

  17. 50N Atl 50N Atl Atl coast Atl coast 40N Atl 40N Atl 50S Atl 50S Atl Aqua scene median values(when using OMI, SCIMACHY,or corrected SCIAMACHY NO2) Lwn_412 Chlor OMI Un-corrected SCIA corrected SCIA OMI Un-corrected SCIA corrected SCIA 237% 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 Median Lwn_412 (mW cm-2 um-1 sr-1) Median chlorophyll-a (mg m-2)

  18. Summary • Modifications to the NO2 data are possible to create more consistent NO2 record for ocean work • Latitudinal modification • Non-linear fit • Some differences remain and seasonal changes not examined

  19. References • Ahmad, Z., C.R. McClain, J.R. Herman, B. Franz, E.J. Kwiatkowska, W.D. Robinson, E.J. Bucsela, M. Tzortziou, 2007, “Atmospheric Correction for NO2 Absorption in Retrieving Water-Leaving Reflectances from the SeaWiFS and MODIS Measurements”, Appl. Optics 46, pp 6504 – 6512. • Robinson, W.D., Z. Ahmad, B.A. Franz, S.W. Bailey, C.R. McClain, NO2 Data Use for Ocean Color, on http://oceancolor.gsfc.nasa.gov/staff/wayne/no2/ocrt_talk/ocrt_no2_poster_cor_no2.pdf

  20. Extra Slides

  21. Aqua Trends of Lwn_412, chlor-a with NO2 (solid) and without (dashed)

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