Short-Lived Halogen Sources and Chemistry in the Community Earth System Model v2 (CESM2-SLH)

Abstract. The implementation of short-lived halogen (SLH) sources and atmospheric chemistry in the Community Earth System Model v1 (CESM1), has extended the capabilities of this state-of-the-art model to study how SLH chemistry alters the oxidative capacity of the atmosphere and, consequently, the Earth’s climate. In this manuscript, we summarize 15 years of research and developments of SLH chemistry in CESM1 and present a complete revision of the porting of the original SLH implementation into the latest released 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 component sets (compsets) 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 important SLH updates implemented over the CESM2/CAM6 routines. Our results show that the global tropospheric burden and tropical stratospheric injection of organic and inorganic chlorine, bromine and iodine species in CESM2-SLH are in agreement with observational assessments and previous modelling studies using CESM1, resulting in significant reductions in global ozone abundance (−21–28 % at the surface, −17–22 % in the troposphere and −2–3 % in the stratosphere) as well as in OH and NO2 (ranging between −2–9 % and −1–10 %, respectively), 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.