Development of a new parameterization for the effect of nitric acid coating on the ice nucleation efficiency of mineral dust particles

Atmospheric mineral dust particles are often observed to be coated with the secondary soluble materials (sulfate, nitrate, organics, etc.) while being transported far downwind of emission sources. Ice nucleation efficiencies of such coated particles are poorly understood, as a result, governing ice nucleation processes are very crudely represented in the atmospheric models. In this study, we investigated the effect of nitric acid coating on the ice nucleating properties of dust particles for conditions commonly found in both mixed-phase and cirrus clouds. At these conditions, laboratory experiments indicated that coated dust particles are poor ice nuclei compared to bare particles in the deposition mode, while their efficiencies were largely unaffected in the condensation/immersion freezing mode of ice nucleation. This suggests that ice nucleation parameterization used by climate models should account for chemistry effects in some way. In addition, we derived ice nucleation parameterization from these laboratory measurements and implemented into the single column version of Community Atmospheric Model Version 5 (CAM5). In order to calculate the coated fraction of the dust population, the reactive uptake of nitric acid gas is calculated based on the size distribution of dust particles and the respective reactive uptake co-efficients. We find that dust aging due to coating has significant implications on the simulated ice crystal number and ice water content, especially for pure ice phase clouds. These results along with their impact on cloud optical properties and precipitation will be discussed.