Tropospheric-stratospheric chemistry in the CNRM climate model ARPEGE-climat 7.0.1

Abstract. This study presents the evaluation of chemistry schemes implemented in ARPEGE-Climat 7.0.1, the latest version of the atmospheric model component of the CNRM climate model (CNRM-ESM) which will be used for future research. Three chemistry schemes of increasing complexity are considered: the stratospheric-upper-tropospheric REPROBUS scheme, and the stratospheric-tropospheric RELACS and RACMOBUS schemes. Atmosphere-only (AMIP) simulations (1979–2014) have been evaluated in terms of key chemical species against the previous generation of AMIP CNRM simulations (CNRM-ESM2-1), which ran with REPROBUS only, and against the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis of atmospheric composition in the troposphere. In the stratosphere, ARPEGE-Climat 7.0.1 provides improved total column ozone representation, reducing biases previously identified in CNRM-ESM2-1. Stratospheric water vapour is also better simulated due to increased moistening, though differences in methane oxidation between RELACS and RACMOBUS require further investigation. Additionally, issues remain with nitric acid in both RELACS and RACMOBUS, and for the RACMOBUS hydrogen chloride and nitrous oxide distributions. In the troposphere, RELACS and RACMOBUS show significant differences in carbon monoxide, with RACMOBUS simulating too low carbon monoxide throughout the entire troposphere, and in peroxy acetyl nitrate (PAN) with RELACS simulating among the highest PAN levels typically modelled. Nitrogen species concentrations are excessive at high northern latitudes in winter, partially due to missing nitrogen pentoxide heterogeneous loss processes. Tropospheric ozone is generally well represented, closely following reanalysis data. Overall, ARPEGE-Climat 7.0.1 marks significant progress in CNRM atmospheric chemistry modelling, supporting future climate-chemistry interaction studies.