Laboratory study of heterogeneous ice nucleation in deposition mode of montmorillonite mineral dust particles aged with ammonia, sulfur dioxide, and ozone at polluted atmospheric concentrations

Heterogeneous ice nucleation in deposition mode of montmorillonite mineral dust aerosol particles exposed to atmospheric trace gases (ammonia, sulfur dioxide, and ozone) was studied at temperatures warmer than -40 degrees C with a continuous flow diffusion chamber. Pure and typical polluted atmospheric concentrations of ammonia, sulfur dioxide, and ozone gases were used to age montmorillonite mineral dust aerosol particles at room temperature and atmospheric pressure in a stainless steel chamber. Ammonia-, sulfur dioxide-, and ozone-exposed montmorillonite mineral dust aerosols act as ice nuclei in heterogeneous deposition freezing at warmer temperatures than required for homogeneous freezing. The ice nucleation efficiency of montmorillonite mineral dust aerosols increased about two times due to exposure to ammonia at a typical atmospheric concentration of about 100 ppt. This is the first experimental evidence for the enhancement of the ice nucleation efficiency of montmorillonite mineral dust aerosols by ammonia gas at typical atmospheric concentrations. Montmorillonite exposure to either pure (100%) or 45 ppm sulfur dioxide or to ozone at 200 ppb shows no clear evidence for changing the ice nucleation efficiency of montmorillonite mineral dust particles. Thus, we conclude that enhancements atmospheric trace gases (e. g., sulfur dioxide and ozone) due to anthropogenic activities have no significant impact on the heterogeneous ice nucleation of montmorillonite mineral dust particles.