1Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

WP210: Spectral Aliassing Effects on Slant Column Retrieval

R. De Beek and M. Weber

I) Statistics on radiance error:

albedo sequences were analysed to identify worst case scenario from LANDSAT and synthetic albedo perturbation provided by RAL (138 spectra in Spectra_011024.tar, Siddans and Latter)

worst case scenario was defined by square root of maximum variance of differential albedo error spectra (StDev)

definition of differential albedo error spectra:

I() relative intensity perturbation due to albedo variation P() fitted quadratic (cubic) polynomial Wa() albedo weighting function (SCIATRAN) a() albedo sequence with mean albedo subtracted a mean albedo of sequence

( ) ( ) ( ) ( ) / ( )aI P W a a P

2Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

albedo weighting function calculated for a 55N April scenario (SZA=49.5°, RAZ=57°, LOS=0°, albedo=0.5)

categories of albedo sequences:

1m_box_80 40X80 km2 GOME pixel (IT=0.1875s), no IFOV convolution 1m_box_40 40X40 km2 GOME pixel (reduced swath width), no IFOV convolution

gome2_box_80 40X80 km2 GOME pixel, IFOV GOME2 boxcar convolution gome2_box_40 40X40 km2 GOME pixel, IFOV GOME2 boxcar convolution

gome2_80 40X80 km2 GOME pixel, IFOV GOME2 slit function convolution gome2_40 40X40 km2 GOME pixel, IFOV GOME2 slit function convolution

error perturbation calculated for all possible albedo sequences in each scan contained in the 138 selected images (Landsat, synthetic images).

3Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Worst Case Statistics: Ozone (O3)

Landsat Images (black ROT=46ms, blue ROT=6ms)

Position Scan StDev mean albedo sequence file albedo

3585 1 3.87e-04 0.124 img6/l71075090_09020000413_10_1m_box_40 3575 1 3.57e-04 0.127 img6/l71075090_09020000413_10_gome2_box_40 3578 1 3.57e-04 0.127 img6/l71075090_09020000413_10_gome2_40 3318 1 2.55e-04 0.135 img6/l71075090_09020000413_10_1m_box_80 3344 1 2.33e-04 0.132 img6/l71075090_09020000413_10_gome2_box_80 3345 1 2.33e-04 0.132 img6/l71075090_09020000413_10_gome2_80 3360 1 0.29e-04 0.135 img6/l71075090_09020000413_10_gome2_80 Synthetic Images

Position Scan StDev mean albedo sequence file albedo

536 0 4.98e-04 0.466 syn3/pix80_nd3_p1_10_1m_box_80 1131 0 1.38e-04 0.403 syn1/pix80_nd1_p1_10_gome2_box_80 1133 0 1.37e-04 0.403 syn1/pix80_nd1_p1_10_gome2_80

4Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Worst Case Statistics: Nitrogen Oxide (NO2)

Landsat Images (black ROT=46ms, blue ROT=6ms)

Position Scan StDev mean albedo sequence file albedo

3852 7 1.44e-03 0.079 img5/l71075091_09120000413_10_1m_box_40 3978 6 4.77e-04 0.071 img5/l71075091_09120000413_10_gome2_box_40 3980 6 4.75e-04 0.071 img5/l71075091_09120000413_10_gome2_40 1805 7 7.56e-04 0.074 img5/l71075091_09120000413_10_1m_box_80 1930 6 2.49e-04 0.070 img5/l71075091_09120000413_10_gome2_box_80 1933 6 2.48e-04 0.070 img5/l71075091_09120000413_10_gome2_80 3618 1 0.23e-04 0.101 img6/l71075090_09020000413_10_gome2_80 Synthetic Images

Position Scan StDev mean albedo sequence file albedo

509 0 2.67e-03 0.514 syn10/pix80_nd10_p0_10_1m_box_80 664 0 7.70e-04 0.501 syn5/pix80_nd5_p1_10_gome2_box_80 666 0 7.64e-04 0.501 syn5/pix80_nd5_p1_10_gome2_80

5Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Worst Case Statistics: Bromíne oxide (BrO)

Landsat Images (black ROT=46ms, blue ROT=6ms)

Position Scan StDev mean albedo sequence file albedo

275 3 2.50e-04 0.062 img5/l71075091_09120000413_10_1m_box_40 3608 0 5.51e-05 0.206 img6/l71075090_09020000413_10_gome2_box_40 3134 2 5.75e-05 0.186 img6/l71075090_09020000413_10_gome2_40 1221 3 1.88e-04 0.103 img15/p18r32_19991122_10_1m_box_80 3348 1 4.60e-05 0.127 img6/l71075090_09020000413_10_gome2_box_80 3352 1 4.60e-05 0.127 img6/l71075090_09020000413_10_gome2_80 3338 1 0.54e-05 0.137 img6/l71075090_09020000413_10_gome2_80 Synthetic Images

Position Scan StDev mean albedo sequence file albedo

529 0 5.85e-04 0.467 syn3/pix80_nd3_p1_10_1m_box_80 728 0 7.61e-05 0.497 syn5/pix80_nd5_p1_10_gome2_box_80 730 0 7.25e-05 0.497 syn5/pix80_nd5_p1_10_gome2_80

6Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Worst Case Statistics: Chlorine dioxide (OClO)

Landsat Images (black ROT=46ms, blue ROT=6ms)

Position Scan StDev mean albedo sequence file albedo

3876 7 8.21e-04 0.081 img5/l71075091_09120000413_10_1m_box_40 7029 7 1.70e-04 0.105 img5/l71075091_09120000413_10_gome2_box_40 7031 7 1.66e-04 0.105 img5/l71075091_09120000413_10_gome2_40 1829 7 4.36e-04 0.075 img5/l71075091_09120000413_10_1m_box_80 464 2 9.45e-05 0.118 img15/p18r32_19991122_10_gome2_box_80 466 2 9.33e-05 0.118 img15/p18r32_19991122_10_gome2_80 3332 1 0.24e-05 0.137 img6/l71075090_09020000413_10_gome2_80 Synthetic Images

Position Scan StDev mean albedo sequence file albedo

437 0 1.72e-03 0.512 syn10/pix80_nd10_p0_10_1m_box_80 391 0 2.82e-04 0.497 syn5/pix80_nd5_p1_10_gome2_80 801 0 2.87e-04 0.498 syn5/pix80_nd5_p1_10_gome2_box_80

7Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Worst case differential albedo error spectra

•Landsat images •40X80 km2 pixel•ROT=46ms

O3

blue: albedo sequence (w/o units)

red: albedo error spectra

BrO

8Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Worst case differential albedo error spectra (2) •Landsat images•40X80 km2 pixel•ROT=46ms

NO2

blue: albedo sequence (w/o units)

red: albedo error spectra

OClO

9Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Summary:

largest StDev of albedo error spectrum observed without convolution of slit function

maximum error in synthetic spectra about a factor of 2-5 higher than maximum error observed in Landsat images.

basically no differences in StDev due to slit function shape (boxcar vs measured bell like shape)

smaller pixel size (40X40 km2) leads to higher albedo error StDev by a factor of up to 2.

Reduction in ROT (read-out-time) to 6ms reduces albedo error statistics by a factor of about 10

10Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

II.) Error analysis

analysis of 40X80 km2 GOME2 pixel with ROT of 46 ms and 6ms (gome2_80 cases)

only Landsat images considered

since Landsat images are not statistically representative (few images, low fractional cloud cover), analysis has been limited to the worst case scenarios identified

intensity perturbation recalculated for proper mean albedo of corresponding worst case scenario

no instrument noise four trace gas scenarios considered: (1) January 55N, (2) April 55N (free

tropospheric BrO case), (3) July 5N (bio mass burning), (4) October 75S (ozone hole scenario, BrO plume)

3 LOS: east (-44°, pixel 1), nadir (0°, pixel 12), west (+44°, pixel 24)

11Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Ozone:

Legend:

solid: Jan 55N (1)dotted: Apr 55N (2)dashed: Jul 5N (3)dash-dotted: Oct 75S (4)

blue: 49ms, red: 6mstriangles: maximum SC errorsquares: minimum SC error

maximum SC error: 0.02% (0.002% 6ms)

12Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Nitrogen Dioxide:

Legend:

solid: Jan 55N (1)dotted: Apr 55N (2)dashed: Jul 5N (3)dash-dotted: Oct 75S (4)

blue: 49ms, red: 6mstriangles: maximum SC errorsquares: minimum SC error

maximum SC error: 2% (0.2% 6ms)

13Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Bromine Oxide:

Legend:

solid: Jan 55N (1)dotted: Apr 55N (2)dashed: Jul 5N (3)dash-dotted: Oct 75S (4)

blue: 49ms, red: 6mstriangles: maximum SC errorsquares: minimum SC error

maximum SC error: 1% (1% 6ms)

14Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Chlorine dioxide:

Legend:

solid: Jan 55N (1)dotted: Apr 55N (2)dashed: Jul 5N (3)dash-dotted: Oct 75S (4)

blue: 49ms, red: 6mstriangles: maximum SC errorsquares: minimum SC error

maximum SC error (Oct 75S only ): 10% (2% 6ms)

15Mark.Weber@uni-bremen.de

GOME2 Error Study

WP 210: Spectral Aliassing

PM3 12/2001

Conclusion:

SC Error due to spatial aliassing is small maximum error of 0.02% for ozone, 2% for NO2, 1% for BrO, and

10% for OClO in ozone hole scenarioexplanations:

small DOAS windows (<<1024 detector pixels) finite slit width (IFOV of 0.29° ~ 4 km on ground)

errors generally larger at low SZA (low SC in tropics)

spatial aliassing errors below errors due to instrumental noise

Detector read-out of 6ms reduces error by about a factor of 8, but is probably not needed for GOME2