Post on 15-Dec-2015
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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
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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)
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NO2 Effects on Ocean retrievals
Stratospheric NO2
Tropospheric NO2
NO2 absorption
412
443
488
530550
670
Chlorophyll
400 500 600 700Wavelength (nm)
-5
0
5
10
15%
Ch
ang
e (f
or
5 X
10
15
mo
lec
ule
NO
2 c
ha
ng
e)
Sensitivity of Water-leaving radiance
and Chlorophyll retrievals to NO2
Water-leaving radiance
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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
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Temporal trend of Lwn 412 and chlorophyll change
Chlor-a
Lwn_412
Trend in Pacific Ocean at 40 South
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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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Temporal trend of Lwn 412 and chlorophyll change and monthly total NO2 average
NO2 source changeTotal NO2 for:
GOME SCIAMACHY OMI
Chlor-a
Lwn_412
Trend in Pacific Ocean at 40 South
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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
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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
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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
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Results with both modifications appliedU
n-co
rrec
ted
Cor
rect
ed
Total StratosphericTropospheric
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Aqua scene median values(when using OMI, SCIMACHY,or corrected SCIAMACHY NO2)
ChlorLwn_412
237%
OMI
Un-corrected SCIA
corrected SCIA
OMI
Un-corrected SCIA
corrected SCIA50N Atl
Atl coast
40N Atl
50S Atl
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 1.00.8Median 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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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
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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