Significant Influence of UV-vis Irradiation on Cloud Activation Efficiencies of Ammonium Sulfate Aerosols under Simulated Chamber Conditions

Abstract. In this work, an optimized protocol to generate an expansion-type liquid clouds with and without UV-visible light irradiation conditions for simulation chamber studies is presented. Sensitivity of the process to key parameters such as initial relative humidities, temperature inhomogeneities, droplets lifetime or seed particle number is illustrated. The obtained clouds have shown that not all seeds particles were always activated and so an iterative numerical method has been re-designed to separate cloud droplets from non-activated seed particles during data analysis allowing the characterization of the cloud droplet formation properties without CCN counter data. Two types of experiments, clouds without irradiation (N-IC) and under UV-visible light irradiation conditions (IC), have been conducted in the CESAM multiphase atmospheric chamber. Measured cloud droplet lifetimes were in good agreement with atmospheric droplet lifetimes. The achieved supersaturation in the cloud was mostly sensitive to the initial relative humidity in the chamber. The comparison between the cloud formation pattern of N-IC and LC was also investigated. Under illumination conditions, the generated clouds clearly showed a gradual activation of seed particles into droplets and thus of the microphysical properties like LWC and droplet concentration, while under dark conditions, clouds faced a flash activation of seed particles. Because this phenomenon may also impact the air/water partitioning of semi-volatile compounds, and it should be considered for further studies, especially in further multiphase photochemical studies implying water-soluble volatile organic compounds in the CESAM chamber.