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_ 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  W a ( ) albedo weighting function (SCIATRAN)   a( ) albedo sequence with mean albedo subtracted  amean albedo of sequence

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).

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 e img6/l _ _10_1m_box_ e img6/l _ _10_gome2_box_ e img6/l _ _10_gome2_ e img6/l _ _10_1m_box_ e img6/l _ _10_gome2_box_ e img6/l _ _10_gome2_ e img6/l _ _10_gome2_80 Synthetic Images Position Scan StDev mean albedo sequence file albedo e syn3/pix80_nd3_p1_10_1m_box_ e syn1/pix80_nd1_p1_10_gome2_box_ e syn1/pix80_nd1_p1_10_gome2_80

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 e img5/l _ _10_1m_box_ e img5/l _ _10_gome2_box_ e img5/l _ _10_gome2_ e img5/l _ _10_1m_box_ e img5/l _ _10_gome2_box_ e img5/l _ _10_gome2_ e img6/l _ _10_gome2_80 Synthetic Images Position Scan StDev mean albedo sequence file albedo e syn10/pix80_nd10_p0_10_1m_box_ e syn5/pix80_nd5_p1_10_gome2_box_ e syn5/pix80_nd5_p1_10_gome2_80

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 e img5/l _ _10_1m_box_ e img6/l _ _10_gome2_box_ e img6/l _ _10_gome2_ e img15/p18r32_ _10_1m_box_ e img6/l _ _10_gome2_box_ e img6/l _ _10_gome2_ e img6/l _ _10_gome2_80 Synthetic Images Position Scan StDev mean albedo sequence file albedo e syn3/pix80_nd3_p1_10_1m_box_ e syn5/pix80_nd5_p1_10_gome2_box_ e syn5/pix80_nd5_p1_10_gome2_80

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 e img5/l _ _10_1m_box_ e img5/l _ _10_gome2_box_ e img5/l _ _10_gome2_ e img5/l _ _10_1m_box_ e img15/p18r32_ _10_gome2_box_ e img15/p18r32_ _10_gome2_ e img6/l _ _10_gome2_80 Synthetic Images Position Scan StDev mean albedo sequence file albedo e syn10/pix80_nd10_p0_10_1m_box_ e syn5/pix80_nd5_p1_10_gome2_ e syn5/pix80_nd5_p1_10_gome2_box_80

GOME2 Error Study WP 210: Spectral Aliassing PM3 12/2001 Worst case differential albedo error spectra Landsat images 40X80 km 2 pixel ROT=46ms  O3 blue: albedo sequence (w/o units) red: albedo error spectra  BrO

GOME2 Error Study WP 210: Spectral Aliassing PM3 12/2001 Worst case differential albedo error spectra (2) Landsat images 40X80 km 2 pixel ROT=46ms NO2  blue: albedo sequence (w/o units) red: albedo error spectra OClO 

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

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)

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: 6ms triangles: maximum SC error squares: minimum SC error  maximum SC error: 0.02% (0.002% 6ms)

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: 6ms triangles: maximum SC error squares: minimum SC error  maximum SC error: 2% (0.2% 6ms)

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: 6ms triangles: maximum SC error squares: minimum SC error  maximum SC error: 1% (1% 6ms)

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: 6ms triangles: maximum SC error squares: minimum SC error  maximum SC error (Oct 75S only ): 10% (2% 6ms)

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 scenario  explanations:  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