## 6 Sensitivity Analyses for the Retrieval of GOME-2A SiF Using PCs
### Spectral Window  
In a second experiment, we test the effect of the spectral window. We now run the retrieval for the four months using spectral windows 734-758 nm, 712-758 nm and 734-783 nm in addition to the baseline window running from 712 to 783 nm. All other settings are the same as in the baseline retrieval. The four windows differ among each other with respect to the atmospheric absorption bands they cover. The 734-758-nm window contains mainly Fraunhofer lines, the 712-758-nm window covers a water vapour absorption band and the 734-783-nm window covers the oxygen A band. Table 4 summarizes the results and reports differences in retrieved SiF values with respect to the baseline fit window of 712-783 nm.  
Overall, SiF maps are most noisy for the shortest fit window, which is understandable as there are simply less spectral points and because there is no filling-in of atmospheric absorption bands that can provide information. Differences typically get smaller and correlations get better when the spectral window resembles the baseline window more. Mean differences are most meaningful here: Importantly, we see that when the H\({}_{2}\)O band is mostly excluded SiF values generally decrease and when the O\({}_{2}\) A band is excluded SiF values typically increase. We remark that the shorter fit window containing only Fraunhofer lines is used in SiF NASA (v26), but that the retrieval setup also differs in a number of other aspects from the experiment reported here.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Spectral Window** & **Month** & **RMS** & **Mean** & **STD** & **R** & **Slope** & **Intercept** & **N** \\ \hline \multirow{8}{*}{734–758 nm (FH)} & January & 1.16 & \(-\)0.11 & 1.15 & 0.45 & 0.23 & 0.53 & 28,805 \\  & April & 0.41 & \(-\)0.08 & 0.40 & 0.74 & 0.51 & 0.21 & 37,677 \\  & July & 0.43 & \(-\)0.09 & 0.42 & 0.72 & 0.67 & 0.27 & 44,297 \\  & October & 1.58 & \(-\)0.19 & 1.57 & 0.22 & 0.07 & 0.41 & 40,592 \\  & **All** & **1.01** & **\(-\)0.12** & **1.00** & **0.42** & **0.21** & **0.43** & **151,371** \\ \hline \multirow{8}{*}{712–758 nm (H\({}_{2}\)O, FH)} & January & 0.41 & 0.02 & 0.41 & 0.91 & 0.67 & 0.20 & 28,813 \\  & April & 0.57 & 0.25 & 0.51 & 0.85 & 0.43 & 0.09 & 37,700 \\ \cline{1-1}  & July & 0.46 & 0.21 & 0.41 & 0.93 & 0.59 & 0.15 & 44,325 \\ \cline{1-1}  & October & 0.33 & 0.04 & 0.33 & 0.95 & 0.62 & 0.13 & 40,761 \\ \cline{1-1}  & **All** & **0.45** & **0.14** & **0.43** & **0.90** & **0.58** & **0.13** & **151,599** \\ \hline \multirow{8}{*}{734–783 nm (FH, O\({}_{2}\) A)} & January & 0.46 & \(-\)0.28 & 0.37 & 0.88 & 1.48 & 0.10 & 28,844 \\  & April & 0.21 & \(-\)0.03 & 0.21 & 0.86 & 1.06 & 0.01 & 37,706 \\ \cline{1-1}  & July & 0.40 & \(-\)0.30 & 0.26 & 0.90 & 1.34 & 0.18 & 44,328 \\ \cline{1-1}  & October & 0.28 & \(-\)0.14 & 0.24 & 0.90 & 1.25 & 0.07 & 40,786 \\ \cline{1-1}  & **All** & **0.34** & **\(-\)0.19** & **0.29** & **0.87** & **1.31** & **0.09** & **151,664** \\ \hline \hline \end{tabular}
\end{table}
Table 4: Similar to Table 3 but for tests with different fit windows. Spectral windows 734–758 nm, 712–758 nm or 734–783 nm are compared to 712–783 nm (baseline). The windows contain Fraunhofer lines (FH), water vapour and/or oxygen absorption, as indicated in the table. All correlations are highly significant.