# Retrieval and Comparison of Multi-Satellite Polar Ozone Data from the EMI Series Instruments
## 1 Introduction  
As an important component of Earth's atmosphere, ozone is a trace gas that can absorb the vast majority of shortwave ultraviolet (UV) radiation from the sun, thus protecting organisms from UV radiation damage [1, 2]. In addition, ozone exhibits strong absorption in the infrared band centered on 9.6 \(\upmu\)m; therefore, it is one of the greenhouse gases that produce a warming effect in the troposphere [3, 4]. About 90% of ozone is distributed in the stratosphere at altitudes of 10-50 km and 10% is distributed in the troposphere below 10 km [5, 6]; thus, ozone plays a crucial role in climate systems. Ever since [PERSON], [PERSON], and their colleagues [7, 8] identified the Antarctic ozone hole caused by significant ozone loss in spring in Antarctica, ozone has been the focus of the scientific community in the polar regions, which are relatively less affected by human activities and whose climate change better reflects global climate change trends [9]. The chlorinated fluorocarbons (CFCs) generated by anthropogenic emissions cause significant ozone loss in the Antarctic spring through the catalytic cycling of polar stratospheric clouds within polar vortices [10, 11]. In 1987, the Montreal Protocol was signed to systematically eliminate CFCs [12, 13, 14, 15]. As a result, the ozone levels over the Antarctic Pole are rebounding, signaling the beginning of ozone layer recovery [16, 17]. Based on the long-term observation demand for the recovery of stratospheric ozone holes, satellite observations have the advantages of large-scale and continuous observation and can be used to estimate the global longwave radiation effects of ozone and quantify radiation forcing [18; 19]. Satellite-based measurements of total ozone columns (TOCs) have promoted atmospheric research, such as exploring stratospheric dynamics, monitoring the ozone hole, and measuring the vertical profile on a global scale [20; 21; 22; 23]. In addition, these measurements have played a crucial role in verifying the simulation results of the chemical models [24; 25]. With improvements in instrument spectral resolution, radiometric performance, and spatiotemporal coverage, the payload of the same observation method needs to be continuously upgraded [26]. Satellite-based observation of ozone began with the Solar Back Ultraviolet (SBUV) and the Total Ozone Mapping Spectrometer (TOMS) in the 1970s [27; 28]. The Global Ozone Monitoring Experiment-2 (GOME-2) series instruments on Metop-A, Metop-B, and Metop-C have a coarse spatial resolution of 80 \(\times\) 40 km\({}^{2}\) and can provide global coverage within three days [29; 30; 31; 32]. The Sentinel 5 precursor (SSP) satellite was launched in October 2017, facilitating research on atmospheric chemistry and regional pollutant monitoring [33; 34]. The first-generation Environmental Trace Gases Monitoring Instrument (EMI) features a coarse spatial resolution of 13 \(\times\) 48 km\({}^{2}\); it was launched in May 2018 [35; 36]. After that, with the launch of the second-generation EMI series instruments in 2021 and 2022 [37; 38], multi-satellite comparison and fusion is meaningful for achieving the goal of the long-term stability of satellite global observations. The second-generation EMI series instruments feature a spatial resolution of 13 \(\times\) 48 km\({}^{2}\). As a crucial component of the EMI series instruments' observations, precise TOC products are obtained using the Differential Optical Absorption Spectroscopy (DOAS) technique [38; 39]. This classical technique, widely employed to retrieve the column density of trace gases, utilizes nadir-viewing satellite sensors [31; 34]. The algorithm for retrieving the vertical columns of ozone mainly consists of three steps: derive the so-called slant column densities (SCDs), air mass factors (AMFs) calculations, and vertical column densities (VCDs) calculations [36; 37].