SNR Basic Column Retrieval R. De Beek, M. Weber, R. Siddans (RAL), B. Latter (RAL)

1. 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 0.1875 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)

2. Ozone slant column fit • 325-335 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.

3. NO2 slant column fit • 425-450 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

4. BrO slant column fit • 344-360 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

5. OClO slant column fit • 363-393 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)

6. 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)

7. 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)

8. First Conclusion for IT=0.1875 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