# Preliminary Global NO2 Retrieval from EMI-II Onboard GF5B/DQ1 and Comparison to Tropopmi
## 2 Data and Methods
### EMI-II No\({}_{2}\) Retrieval Algorithm  
The tropospheric NO\({}_{2}\) retrieval algorithm for GF5B and DQ1 EMI-II data is similar to that for GF5 EMI data [12] and involves three key steps. (1) Spectral fitting to obtain the total NO\({}_{2}\) slant column density (SCD). The NO\({}_{2}\) SCD was derived using the conventional differential optical absorption spectrometry (DOAS) method, which integrates the measurements along the effective optical path from the sun, through the atmosphere, to the instrument. In this study, Table 2 provides the parameters used for fitting the NO\({}_{2}\) spectrum. For GF5B and DQ1 EMI-II spectra, the reference spectra were obtained on 8 October 2021 and 23 May 2022, respectively. Within the fitting window of 405-465 nm, a fifth-order polynomial was applied. The fitting process incorporated the absorption cross-sections for NO\({}_{2}\) [220 K], O\({}_{3}\) [223 K], H\({}_{2}\)O vapour, H\({}_{2}\)O liquid, O\({}_{4}\), and the ring effect, all of which were convolved with the slit function to match the EMI-II's resolution. Additionally, a first-order intensity offset correction was applied. Prior to retrieving the SCD, a wavelength calibration was performed using solar Fraunhofer lines from a highly accurate reference solar atlas to minimize discrepancies between the radiance wavelength and the absorption cross-section [26]. Figure 1: FWHM variation with the number of rows in the VIS band for (**a**) DQ1 EMI, (**b**) GF5B EMI, (**c**) GF5 EMI, and (**d**) TROPOMI. The initial SCD fitting results for EMI NO\({}_{2}\) display uneven stripes along the along-track direction. This phenomenon is attributed to the instability of the FWHM and calibration errors in the irradiance data. Such issues are common in payloads employing two-dimensional CCD push broom imaging, including similar instruments like OMI [25]. These biases need to be removed by spatial filtering before subsequent procedures. We updated the de-stripe correction scheme based on the method in [PERSON] et al. (2019) [12]. Its main features are as follows: (1) pixels with SCDs exceeding 1.5\(\sigma\) are excluded when the mean SCD of each row is calculated; (2) after the preliminary correction, the mean VCD of each row is compared with the climatology value of a clean ocean area to obtain the revised SCD; (3) the revised SCD is used to correct the SCD correction value; (4) the revised SCD correction value is used to correct the initial SCD. The de-stripe scheme can preserve the variation characteristics of the SCD with the VZA in the cross-track direction, which is more realistic. (2) Stratosphere-tropospheric separation (STS) was used to estimate the concentration of NO\({}_{2}\) in the stratosphere and remove it from the total SCD to obtain the tropospheric SCD. The STREAM STS method was used in this study to estimate the stratospheric VCD. This method is less dependent on external input data and directly uses satellite observations to calculate a series of weights for each pixel, based on which the stratospheric concentrations are obtained via weight convolution. Among them, the \"pollution weight\" is used to reduce the contribution of potential pollution areas, the \"cloud weight\" is used to increase the contribution of cloud observation pixels, and the \"tropospheric residual weight\" is used to adjust the total weight to avoid abnormal tropospheric residuals. The tropospheric residual is the difference between the total VCD and stratospheric VCD, which can be directly converted to tropospheric VCD as follows:  
\[V_{trop}=R_{\text{trop}}\times\frac{A_{strat}}{A_{trop}} \tag{1}\]  
where \(R_{\text{trop}}\) is the tropospheric residual; \(A_{strat}\) is a geometric air mass factor (AMF) (\(AMF_{\mathcal{O}}\)) and is defined as \(AMF_{G}=\sec\theta_{s}+\sec\theta_{v}\), which is a function of the solar zenith angle \(\theta_{s}\) and satellite viewing angle \(\theta_{v}\) in the absence of atmospheric scattering. \(A_{trop}\) is the tropospheric AMF.