Optimized OClO Retrieval Algorithm for TROPOMI: Evaluation and Validation in Preparation of the Operational AC-SAF Sentinel-5 OClO Product

Abstract

Understanding stratospheric ozone depletion and recovery is crucial for atmospheric science. Ozone shields Earth from harmful UV radiation and influences atmospheric chemistry and global climate. Ozone depletion rates are driven by chlorine, bromine, nitrogen, and hydrogen oxides, with polar regions being particularly affected due to low temperatures that enhance chlorine activation. Chlorine dioxide (OClO) serves as a critical indicator of chlorine activation, a precursor to ozone destruction, although it does not directly participate in ozone depletion. The ClO-BrO catalytic cycle is central to ozone destruction, contributing to the formation of the polar ozone hole. OClO is predominantly observed in the polar vortex during winter and spring, yet current models often underestimate its abundance under specific conditions, emphasizing the need for more accurate measurements and modeling. OClO columns are retrieved using the Differential Optical Absorption Spectroscopy (DOAS) method, a highly sensitive technique applied in both ground-based and satellite observations. Satellite missions such as SCIAMACHY, GOME-2, OMI, and TROPOMI have facilitated global OClO monitoring. Despite advancements, retrieval challenges persist, including spectral interferences with other atmospheric species. This study presents an enhanced TROPOMI OClO retrieval product, developed through rigorous optimization of DOAS retrieval parameters. Key improvements include the refinement of the spectral fit window, the adjustment of polynomial degree, and the mitigation of interferences from other atmospheric species. This optimized approach facilitates reliable retrievals and lays the groundwork for time-series analysis, seasonal climatological studies of OClO, and comprehensive validation efforts. The results contribute to a deeper understanding of chlorine activation dynamics and stratospheric ozone chemistry. Importantly, this study provides a basis for future satellite missions, including the operational OClO Sentinel-5 products to be provided by DLR in the framework of the AC-SAF.