The influence of clouds on radical concentrations: observations and modelling studies of HO_x during the Hill Cap Cloud Thuringia (HCCT) campaign in 2010

Abstract. The potential for chemistry occurring in cloud droplets to impact atmospheric composition has been known for some time. However, the lack of direct observations and uncertainty in the magnitude of these reactions led to this area being overlooked in most chemistry transport models. Here we present observations from Mt Schmücke, Germany, of the HO₂ radical made alongside a suite of cloud measurements. HO₂ concentrations were depleted in-cloud by up to 90% with the rate of heterogeneous loss of HO₂ to clouds necessary to bring model and measurements into agreement, demonstrating a dependence on droplet surface area and pH. This provides the first observationally derived assessment for the uptake coefficient of HO₂ to cloud droplets and was found to be in good agreement with theoretically derived parameterisations. Global model simulations, including this cloud uptake, showed impacts on the oxidising capacity of the troposphere that depended critically on whether the HO₂ uptake leads to production of H₂O₂ or H₂O.