Challenges in simulating ozone depletion events in the Arctic boundary layer: A case study using ECHAM/MESSy for spring 2019/2020

Abstract

Ozone depletion events (ODEs) and bromine explosions (BEs) occur regularly in the springtime polar boundary layer. ODEs alter the oxidation capacity of the polar boundary layer and promote formation of toxic mercury. We investigated Arctic ODEs and BEs in 2019/2020 using the chemistry-climate model ECHAM/MESSy v2.55.2, nudged with ERA5 reanalysis data. Model results were evaluated against surface ozone measurements, satellite-derived tropospheric BrO vertical column densities (VCDs), and in situ data from the MOSAiC expedition. The model underestimated boundary layer (BL) height during shallow BL conditions, coinciding with a warm surface temperature bias (2-10 K), particularly below-10 °C, likely inherited from ERA5. An updated model configuration, incorporating more realistic multi-year sea ice and relaxed bromine release thresholds, improved agreement with coastal ozone observations (Eureka, Utqiavik) but still failed to reproduce strong ODEs observed during MOSAiC. Consequently, modeled surface BrO mixing ratios were overestimated, while BrO VCDs were underestimated, suggesting that simply increasing Br2 emissions does not resolve discrepancies. A weaker colocation between modeled BrO VCDs and ODEs aligns with prior airborne studies and may reflect tropospheric chemical and transport processes rather than stratospheric contributions. Despite decreasing Arctic sea ice extent and increasing BrO VCDs, long-Term records from Alert, Utqiavik, and Zeppelin show a decline in strong ODE frequency since 2008. This suggests that bromine emissions from first-year sea ice alone may not fully account for observed ODE variability, and that additional climate-sensitive mechanisms may modulate Arctic ozone chemistry. Long-Term model integrations are recommended to better understand these trends.