# Preliminary Global NO2 Retrieval from EMI-II Onboard GF5B/DQ1 and Comparison to Tropopmi
## 2 Data and Methods
### EMI-II No\({}_{2}\) Retrieval Algorithm
(3) The tropospheric VCD is calculated using the ratio of the tropospheric SCD to the tropospheric AMF, with the method for calculating the tropospheric AMF following the approach of [PERSON] et al. (2004) [27]:  
\[\text{AMF}=\frac{\sum\limits_{l}m_{l}x_{aJ}\cdot c_{l}}{\sum\limits_{l}x_{aJ}} \tag{2}\]  
\[c_{l}=1-0.003[T(p)-T_{0}] \tag{3}\] \begin{table}
\begin{tabular}{c c c c c} \hline
**Parameters** & **GF5B EMI-II** & **DQ1 EMI-II** & **TROPOMI** \\ \hline Fitting window & & 405–465 nm & 405–465 nm & 405–465 nm \\ Reference & Irradiance measured on & Irradiance measured on & Annual mean (2005) \\ spectrum I\({}_{0}\) & & 8 October 2021 & 23 May 2022 & solar reference \\ Polynomial & & 5 th-order & 5 th-order & 5 th-order \\  & O\({}_{3}\) & \(\surd\) (223 K) & \(\surd\) (223 K) & \(\surd\) (243 K) \\  & NO\({}_{2}\) & \(\surd\) (220 K) & \(\surd\) (220 K) & \(\surd\) (220 K) \\  & O\({}_{4}\) & \(\surd\) (293 K) & \(\surd\) (293 K) & \(\surd\) (293 K) \\  & H\({}_{2}\)O vapour & \(\surd\) (280 K) & \(\surd\) (280 K) & \(\surd\) (280 K) \\  & H\({}_{2}\)O (liquid) & \(\surd\) (295 K) & \(\surd\) (295 K) & \(\surd\) (295 K) \\  & Ring effect & \(\surd\) & \(\surd\) & \(\surd\) \\ Offset correction & & \(\surd\) & \(\surd\) & \(\times\) \\ \hline \end{tabular}
\end{table} 
Table 2: Spectral fitting parameter settings for GF5B EMI-II, DQ1 EMI-II, and TROPOMI. where \(m_{l}\) represents the scattering weight AMF (box-AMF), which describes the vertical sensitivity of the retrieved parameters. \(x_{aJ}\) denotes the NO\({}_{2}\) sub-column in layer l. \(c_{l}\) is the temperature correction factor used to adjust the absorption cross-section spectrum according to the effective temperature at the specific layer \(p\). \(T_{0}\)(220 K) refers to the temperature of the NO\({}_{2}\) absorption cross-section used in the DOAS fitting. The calculation of the AMF for partially cloudy scenes follows the methodology established by [PERSON] et al. (2011) [PERSON] et al. (2011), utilizing the Independent Pixel Approximation (IPA). This approach involves representing the scene as a linear combination of cloudy (\(M_{cl}\)) and clear (\(M_{cr}\)) components:  
\[\text{AMF}=\omega\text{AMF}_{cl}+(1-\omega)\text{AMF}_{cr} \tag{4}\]  
where \(\omega\) denotes the effective cloud fraction. In this paper, we use TROPOMI cloud products instead of EMI-II due to the lack of mature and public cloud products derived from EMI-II. The calculation of box-AMF varies with three angles (solar zenith angle, viewing zenith angle, and relative azimuth angle) and three surface parameters (surface albedo, surface elevation, and surface pressure). In this study, the box-AMF lookup tables were computed using the SCIATRAN RTM [28], and AMF values were interpolated based on the actual conditions of these six parameters [12]. For the a priori NO\({}_{2}\) profiles and surface pressure parameters, we utilized data from the GEOS-cf CTM [29]. This CTM divides the atmosphere into 72 layers and provides outputs with a temporal resolution of one hour, a spatial resolution of 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\), and global coverage. The output information includes NO\({}_{2}\) profiles (in volume mixing ratio), temperature profiles (in K), hybrid pressure level profiles (in Pa), and the tropopause layer index (to differentiate between the stratosphere and troposphere). Surface albedo data and surface elevation data were obtained from the OMLER [30] and GMTED2010 products, respectively.