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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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Simple linear fit

Total NO2 Tropospheric Stratospheric

OM

I

SCIAMACHY

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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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Latitudinal dependence

Fit form:

cos( 4 * latitude ) + linear

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Latitude dependence, removed

• Note that this technique worked well in other ranges

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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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Temporal trend results: before correction

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Temporal trend results: after correction

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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

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Extra Slides

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