# Retrieval and Comparison of Multi-Satellite Polar Ozone Data from the EMI Series Instruments
## 3 Methods  
### Deriving the Ozone Slant Column Density  
The SCDs of atmospheric composition were retrieved using the DOAS technique, which is based on the Lambert-Beer law [40; 41]. This method involves initially fitting the measured optical depths within a carefully selected spectral range to calculate the SCDs (integrated atmospheric concentrations along the effective light path) of the absorbers [42]. The basic equation describing the DOAS technique is [39]:  
\[\ln\frac{I(\lambda)}{I_{o}(\lambda)}=\sum\left(\sigma_{i}^{*}(\lambda)\cdot c _{i}\cdot L\right)=\sum\left(\sigma_{i}^{*}(\lambda)\cdot SCD_{i}\right) \tag{1}\]  
where \(I(\lambda)\) denotes the measured back-scattered earthshine intensity at wavelength \(\lambda\), \(I_{o}(\lambda)\) denotes the original solar intensity, \(\ln\frac{I(\lambda)}{I_{o}(\lambda)}\) denotes the optical density, \(\sigma_{i}^{*}\) denotes the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **EMI-GF5(01A)** & **EMI-GF5(02)** & **EMI-DQ01** \\ \hline \multirow{4}{*}{Spectral range (nm)} & UV1: 240–290 & UV1: 240–290 & UV1: 240–290 \\  & UV2: 290–380 & UV2: 290–380 & UV2: 290–380 & UV2: 290–380 \\  & VIS1: 390–530 & VIS1: 390–530 & VIS1: 390–530 \\  & VIS2: 550–710 & VIS2: 550–710 & VIS2: 550–710 \\  & 0.3–0.6 nm & 0.3–0.6 nm & 0.3–0.6 nm \\ Spatial resolution & \(13\times 24\) km\({}^{2}\) & \(13\times 24\) km\({}^{2}\) & \(13\times 24\) km\({}^{2}\) \\  & Field of view & \(114^{\circ}\) & \(114^{\circ}\) & \(114^{\circ}\) \\ Reference spectrum & Monthly averaged & Monthly averaged & Monthly averaged \\  & solar spectrum & solar spectrum & solar spectrum \\  & 13:30 p.m. & 10:30 a.m. & 13:30 p.m. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical specifications of the EMI-GF5(01A), EMI-GF5(02), and EMI-DQ01 instruments. cross section of trace gas \(i\), \(c_{i}\) denotes the concentration of trace gas \(i\), \(L\) denotes the optical path length, and the product of \(c_{i}\) and \(L\) defines the \(SCD\). In this study, we present the ozone SCD retrieval results of the EMI series, which were realized using version 3.2 of the QDOAS software program [43]. Through the DOAS retrieval residual fitting of the respective instruments, the wavelength intervals and polynomial fitting order with relatively low fitting root mean square (RMS) values are selected. Then, the retrieval parameters of the EMI series instruments were finally determined. The fitting intervals of the EMI-GF5 (01A), EMI-GF5 (02), and EMI-DQ01 were 320-340 nm, 326-334 nm, and 325-335 nm, respectively, and the polynomial fits were 4 th, 5 th, and 5 th order, respectively. The absorption cross sections were considered in the retrieval process. Table 2 lists the parameter settings for the DOAS fit of the EMI series TOCs. A typical ozone DOAS fit for the EMI-DQ01 dataset is shown in Figure 1. The retrieved ozone SCDs and associated errors were approximately 7.157 \(\times\) 10\({}^{18}\) molecules/cm\({}^{2}\) and 2.841 \(\times\) 10\({}^{17}\) molecules/cm\({}^{2}\), respectively. The RMS values for the residuals of the spectral fit were 1.08 \(\times\) 10\({}^{-3}\). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & **Source** & **EMI-GF5 (01A)** & **EMI-GF5 (02)** & **EMI-DQ01** \\ \hline Fitting interval & & 320–340 nm & 326–334 nm & 325–335 nm \\ Polynomial & & Order 4 & Order 5 & Order 5 \\ order & & 223 K, 243 K & & \\ O\({}_{3}\) & (orthogonality) [44] & \(\surd\) & \(\surd\) & \(\surd\) \\ NO\({}_{2}\) & 298 K [45] & \(\surd\) & \(\surd\) & \(\surd\) \\ SO\({}_{2}\) & 298 K [46] & \(\surd\) & \(\surd\) & \(\surd\) \\ BrO & 223 K [47] & \(\surd\) & \(\surd\) & \(\surd\) \\ HCHO & 297 K [48] & \(\surd\) & \(\surd\) & \(\surd\) \\ Ring & Calculated using & \(\surd\) & \(\surd\) & \(\surd\) \\  & QDOAS & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detailed parameters of the ozone DOAS fitting for EMI-GF5(01A), EMI-GF5 (02), and EMI-DQ01. Figure 1: A selected example of ozone DOAS fitting from EMI-DQ01 data on 10 December 2023.