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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
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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)
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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
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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
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5 Temporal trend of Lwn 412 and chlorophyll change Chlor-a Lwn_412 Trend in Pacific Ocean at 40 South
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6 Ratio shift Temporal trend of Lwn 412 and chlorophyll change Chlor-a Lwn_412 Trend in Pacific Ocean at 40 South
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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
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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
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9 Simple linear fit Total NO 2 TroposphericStratospheric OMI SCIAMACHY
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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
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11 Latitudinal dependence Fit form: cos( 4 * latitude ) + linear
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12 Latitude dependence, removed Note that this technique worked well in other ranges
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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
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14 Results with both modifications applied Un-corrected Corrected TotalStratosphericTropospheric
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15 Temporal trend results: before correction
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16 Temporal trend results: after correction
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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
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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
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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
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20 Extra Slides
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21 Aqua Trends of Lwn_412, chlor-a with NO 2 (solid) and without (dashed)
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