Sensitivity of cloud droplet formation to the numerical treatment of the particle mixing state

The importance of the particle mixing state and hygroscopicity to cloud droplet activation was investigated using an adiabatic cloud parcel model. The particle mixing state was described in terms of the hygroscopic growth properties. Various aerosol types were considered, and the input parameters describing the particle size distribution and water uptake properties were taken from the field measurement data reported in the literature. A series of sensitivity studies were conducted where the treatment of the particle mixing state was simplified as compared to the reference model configuration that resolved particle populations both as a function of the size and hygroscopicity. The results show that describing the hygroscopicity and mixing state of an aerosol population by a single parameter induced differences of up to 12% compared to the reference case in the cloud droplet concentrations (CDNCs) in marine and continental background areas. In urban and rural environments where particles display a high degree of external mixing, the relative differences in CDNC were up to 35% if the particles were assumed to be internally mixed. It was also found that a computationally efficient and relatively accurate approach is to treat less and more hygroscopic particles separately using mean properties of each population. In this case, the relative differences in CDNC were generally below 20%. The model calculations also suggest that the mixing state of a particle distribution is reflected in the size‐resolved activation efficiency (fraction of particles activated into cloud droplets versus particle dry diameter).