GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 SNR Basic Column Retrieval R. De Beek, M. Weber, R. Siddans (RAL), B. Latter (RAL) SNR output from RAL noise model program (PR1, Section 1.3). SNR calculation for sec GOME pixels (corresponding to integration of 10 successive LOS in CDI simulations) and 30 sec IT solar irradiance. Gaussian random noise spectra created assuming model noise level corresponds to 2  (95% percentile). Outliers above 2  are removed (no spurious spikes). Slant column fit results from noise free spectra are used as reference for percent error estimate 24 scenes (Version 3) times 24 GOME ground pixel (across track) fits carried out for the 4 fitting windows (O3, NO2, BrO, OClO)

GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 Ozone slant column fit nm NO2, BrO, O3 (221 K), O3 (240 K) Quadratic polynomial subtracted Summary errors all below 0.7% largest error observed in ozone hole scenario (151 DU) error not dependent on scan angle average max. error ~0.2% (high albedo) and ~0.3% (low albedo) approximating 2  standard deviation.

GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 NO2 slant column fit nm O3, O4, H2O Quadratic polynomial subtracted Summary errors all below 60% error not dependent on scan angle average max error ~20% (high albedo) and ~30% (low albedo) for stratospheric NO2 columns

GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 BrO slant column fit nm O3(T), NO2, O4, BrO, H2CO Cubic polynomial subtracted Summary errors all below 80% average max error ~30% (high albedo) and ~50% (low albedo) for stratospheric BrO columns error smaller for enhanced PBL BrO scenes (<15%) free tropospheric BrO (1ppt) scene only distinguishable from other scenarios at high albedo

GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 OClO slant column fit nm nm O3, NO2, BrO, O4 Cubic polynomial subtracted Summary OClO only observable under twilight condition and after chlorine activation due to PSC formation (ozone hole scenario) ozone hole scenario: error below 150% (low albedo) and below 80% (high albedo)

GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 SZA dependence of slant column error: O3 and NO2 O3  higher errors at large SZA and low ozone NO2  higher errors at at large SZA (high and low albedo) and in tropics (low albedo)

GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 SZA dependence of slant column error: BrO and OClO BrO  lowest errors in presence of tropospheric BrO (49° and 76° SZA) OClO  only retrievable under ozone hole condition (76° SZA)

GOME2 Error Study WP 150: Basic SNR column retrieval PM2 10/2001 First Conclusion for IT= sec: Slant column errors larger than 20% for minor trace gases other than ozone Ozone slant column errors are well below 1% Errors are very much reduced in regions of enhanced tropospheric column amounts (local events) Co-adding might be necessary to reduce error