MRI cloud chamber experiments on dust aerosol and cloud interactions

It is not sufficiently understood how aerosols affect cloud droplet formation and/or ice nucleation processes, and then cloud properties. A quantitative description of the relation between physico-chemical characterizations of aerosol particles and their ability of cloud formation is crucial for an improvement of numerical models.

The dynamic cloud chamber facility has been operated at Meteorological Research Institute (MRI), in order to investigate the details of the fundamental processes of cloud formation. Both temperature and pressure are automatically controlled to simulate an adiabatic expansion under a wide range of atmospheric conditions. It is equipped with laser imaging and scattering devices for sensing cloud droplet and ice crystal formation and measuring size distributions, shapes, and asphericity of aerosol and cloud particles from sub-um to a few mm in size.

This paper describes preliminary results in the experiments with respect to dust particles. In our experiments, Arizona test dust (ATD), a well-characterized standard dust sample, was investigated. The dry ATD samples were dispersed with a rotating brush disperser, and then passed through two cyclone impactor to remove particles larger than 1 um in diameter. The initial number concentrations of ATD injected into the chamber were a few hundreds particles/cc. CCNC measurements indicated that more than 80% of particles were activated at SSw of 0.4%. During expansion experiments, super-micron size particles rapidly appeared at water saturation. Concerning ATD ability as CCN, more cloud droplets formed in expansion experiments with warmer LCL temperatures even at the same evacuation rates. In mixed-phase conditions above -20degC, ATD was activated as IN at warmer temperatures than aerosols in outdoor air. At an evacuation rate corresponding to updraft velocity of 5m/s, ice nucleation continued to occur down to below -30degC and produced high concentrations of ice crystals (more than 0.3 particles/cc). Our result indicated that ATD could serve as both CCN and IN. For more detailed investigation of ice initiation processes, accurate measurements of supersaturation with respect to ice are really required.