Comparing total inorganic chlorine climatologies from CMAM39 and ACE-FTS

Abstract:

Inorganic chlorine radicals are major catalysts for ozone depletion. Industrial chlorine sources have been phased out since the 1987 Montreal Protocol and its subsequent amendments, allowing ozone recovery to begin, but a significant amount of inorganic chlorine remains in the atmosphere in the form of inert reservoir species (e.g. HCl, ClONO₂, and HOCl) and active radical species (e.g. ClO and Cl). These gases make up the total inorganic chlorine chemical family, Cl_y. The extent of Cl_y’s impact on ozone in a given year is heavily determined by the partitioning between radical and reservoir species. Therefore, studying chlorine partitioning in climate models is important for evaluating their ability to simulate ozone recovery. Here, we focus on the four main components of Cl_y: HCl, ClONO₂ , ClO, and HOCl. The specified dynamics version of the Canadian Middle Atmosphere Model (CMAM39) now provides the 6-hourly fields for each of these gases from 1980 up to the end of 2018. We compare model-based global upper tropospheric and stratospheric climatologies for these gases with climatologies based on satellite measurements taken from 2004-2018. For HCl, ClONO₂, and ClO, we use measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). The ClO measurements are supplemented with colocated measurements from the Microwave Limb Sounder (MLS) that have been interpolated onto the ACE-FTS measurement grid and scaled to ACE-FTS local solar times using a chemical box model. The HOCl values are derived from similarly interpolated and scaled MLS measurements. The upper and lower ends of the ClO and HOCl profiles are extended with a climatology from the Superconducting Submillimeter-Wave Limb-Emission Sounder. To ensure a valid comparison, the CMAM39 model fields are also interpolated onto the ACE-FTS locations and times. Comparing the model-based climatologies to the measurement-based climatologies allows us to evaluate how well CMAM39 captures total inorganic chlorine behaviour over a 15-year time span and to examine trends in chlorine partitioning during this time period. These are both crucial to predicting ozone recovery in the coming decades.