Short-lived halogen sources and chemistry in the Community Earth System Model v2 (CESM2-SLH)

The implementation of short-lived halogen (SLH) sources and atmospheric chemistry in the Community Earth System Model v1 (CESM1) allowed to study the influence of SLH chemistry on the oxidative capacity of the atmosphere and, consequently, the Earth's climate. In this manuscript, we summarize 15 years of research on SLH chemistry and present a complete revision of the porting of the original developments into the latest version of CESM (v2), hereafter CESM2-SLH. This includes a detailed description of all offline and online sources of organic and inorganic SLH, as well as the gas-phase and heterogeneous recycling of chlorine, bromine and iodine in the troposphere and stratosphere, including their species-independent atmospheric sinks. In doing so, we provide a comprehensive evaluation of how changes in model parameters and coupled dynamics within the Community Atmosphere Model v6 (CAM6) affect SLH abundances and their implications. The new CESM2-SLH implementation offers various model configurations and resolutions, all of which result in equivalent global budgets and zonal distributions of organic and inorganic chlorine, bromine and iodine, which in turn, lead to SLH impacts on atmospheric composition that are consistent with previous CESM1 results. The released CESM2-SLH version includes specific namelist options, input files and technical notes detailing the most relevant SLH updates implemented in main CESM2/CAM6 routines. Our results show that the tropospheric halogen budget and tropical stratospheric injection of organic and inorganic chlorine, bromine and iodine species in CESM2-SLH are in agreement with observational assessments, resulting in significant global ozone reductions (−21 % to −28 % at the surface, −17 % to −22 % in the troposphere and up to −3 % in the stratosphere). Equivalent changes in OH and NO 2 abundance are also found, respectively ranging between −2 % to −9 % and −1 % to −10 %), depending on the specific model configuration and resolution. Based on this, we encourage the wider CESM community to consider the released CESM2-SLH scheme to obtain a more realistic representation of the background influence of natural and anthropogenic short-lived halogen sources and chemistry in air quality and Earth's climate studies.