The impact of copper and iron distribution on reactive oxygen species concentrations in the atmospheric multiphase system

Redox reactions transition metal ions (TMI), such as iron and copper, affect the concentrations of reactive oxygen species (ROS) in atmospheric cloud droplets and aqueous aerosol particles. Copper and iron have distinct emission sources resulting in only a small number fraction of cloud condensation nuclei and droplets that contain these metals. The fact that TMI reactions only occur in a small subset of particles and droplets is not taken into account in current multiphase chemistry models that are usually initialized with TMI concentrations derived from bulk sampling.

Our previous model studies have shown that model predictions based on bulk iron concentrations may significantly underestimate total OH and HO₂ budgets if iron is assumed in all cloud (Ervens, 2022; Khaled et al., 2022). We extend this approach to copper reactions and to reactions between copper and iron ions. We use a multiphase chemistry box model to investigate the importance of the number fraction of TMI-containing particles and droplets and show under which atmospheric conditions detailed information on this parameter is most important. The aim of our study is to identify the impacts of the copper and iron distributions in cloud droplets and aqueous aerosol particles on the total gas and aqueous budgets of OH, HO₂, H₂O₂ and O₃ in the multiphase system. Our model results give guidance for measurement needs to further constrain the ROS budgets in the atmosphere.