Global Intercomparison of Tropospheric Oxidant Chemistry in a Common Earth System Model Environment using GEOS-Chem (v14.1.1) and CAM-chem Chemistry within the Community Earth System Model version 2 (CESM2)

Ozone is a major air pollutant and a central species in tropospheric chemistry. Global atmospheric chemistry models tend to agree relatively well with ozone observations but often show large disagreements in their representations of oxidant chemistry, suggesting that ozone and OH would respond differently to perturbations. This is a problem for Earth system models that aim to quantify chemical responses and feedbacks to climate change. Here we introduce an updated version of the GEOS-Chem chemical module as an alternative chemistry option to CAM-chem in the Community Earth System Model version 2 (CESM2). We isolate and evaluate the impact of specific factors affecting differences in the simulation of tropospheric oxidant chemistry between GEOS-Chem and CAM-chem in a common model environment. Results indicate a strong sensitivity to different treatments of tropospheric halogen chemistry, aerosol nitrate photolysis, photolysis, and scavenging in convective updrafts. We use targeted evaluation with observations to arbitrate these differences.