1. 1 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)Wayne.Robinson@nasa.gov Tropospheric NO2 measured by satellites Workshop, KNMI, DeBilt, The Netherlands September 10 – 12, 2007

2. 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 NO 2 in ocean color retrievals (Ahmad et. al., 2007, Robinson et. al., 2007)

3. 3 NO 2 Effects on Ocean retrievals Stratospheric NO 2 Tropospheric NO 2 NO2 absorption 412443488530550670 Chlorophyll 400500600700 Wavelength (nm) -5 0 5 10 15 % Change (for 5 X 10 15 molecule NO2 change) Sensitivity of Water-leaving radiance and Chlorophyll retrievals to NO 2 Water-leaving radiance

4. 4 Tests of NO 2 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 NO 2 use, and their percentage change Percentage change trends showed a significant shift around Sep 2004

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

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

7. 7 Temporal trend of Lwn 412 and chlorophyll change and monthly total NO 2 average NO 2 source change Total NO2 for: GOME SCIAMACHY OMI Chlor-a Lwn_412 Trend in Pacific Ocean at 40 South

8. 8 Tests of NO 2 effect in MODIS Aqua retrievals (continued) Percentage trends showed a significant shift around Sep 2004 NO 2 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 NO 2 record by modifying one data source to match the other

9. 9 Simple linear fit Total NO 2 TroposphericStratospheric OMI SCIAMACHY

10. 10 Problems with linear fit Significant biases at some latitudes (esp. 40 S) look for a latitudinal correction Non-linear relation for total NO 2 determine a non-linear relation Low correlation in tropospheric NO 2 derive tropospheric NO 2 from total - stratospheric Address these in the correction

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

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

13. 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 NO 2 (except constant) –Consistency between stratospheric and total –Fit for total worked relatively well for limited stratospheric data range

14. 14 Results with both modifications applied Un-corrected Corrected TotalStratosphericTropospheric

15. 15 Temporal trend results: before correction

16. 16 Temporal trend results: after correction

17. 17 Aqua scene median values (when using OMI, SCIMACHY, or corrected SCIAMACHY NO 2 ) ChlorLwn_412 237% OMI Un-corrected SCIA corrected SCIA OMI Un-corrected SCIA corrected SCIA 50N Atl Atl coast 40N Atl 50S Atl 0.00.20.40.60.81.01.21.40.00.20.40.61.00.8 Median Lwn_412 (mW cm -2 um -1 sr -1 )Median chlorophyll-a (mg m -2 ) 50N Atl Atl coast 40N Atl 50S Atl

18. 18 Summary Modifications to the NO 2 data are possible to create more consistent NO 2 record for ocean work –Latitudinal modification –Non-linear fit Some differences remain and seasonal changes not examined

19. 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 NO 2 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. 20 Extra Slides

21. 21 Aqua Trends of Lwn_412, chlor-a with NO 2 (solid) and without (dashed)