Laboratory investigations of the impact of mineral dust aerosol on cold cloud formation
Dust particles represent a dominant source of particulate matter (by mass) to the atmosphere, and their emission from some source regions has been shown to be transported on regional and hemispherical scales. Dust particles' potential to interact with water vapor in the atmosphere can lead to important radiative impacts on the climate system, both direct and indirect. We have investigated this interaction for several types of dust aerosol, collected from the Southwestern United States and the Saharan region. A continuous flow diffusion chamber was operated to measure the ice nucleation ability of the dust particles in the temperature range of relevance to cirrus and mixed-phase clouds (-65<T<-20 degrees C). In most experiments, particles were size selected using a differential mobility analyzer prior to sampling to give information on heterogeneity of the sample with size, generally in the range of diameters 100-400 nm. All dust nucleated ice heterogeneously in the deposition mode colder than about -40 degrees C, but required droplet activation in the exclusively heterogeneous ice nucleation regime warmer than -36 degrees C. Ice nucleated on 1% of dry generated dust particles of a given type at a similar relative humidity with respect to ice irrespective of temperature between -60 and -40 degrees C, however differences in relative humidity for ice nucleation was observed between the different dust types. The Saharan dust types exhibited a dependency on particle size below 500 nm. Additional data were collected during the International Workshop on Comparing Ice Nucleation Measurement Systems (ICIS, 2007) which indicated that ice nucleation on larger, polydisperse dust particles occurs at warmer temperatures than found for the smaller particles. When particles were coated with secondary organic aerosol (SOA) species, higher relative humidity was required for ice nucleation below -40 degrees C, similar to that required for homogeneous nucleation of sulfates. However, ice nucleation was still observed on SOA-coated dust at warmer temperatures than are required for homogeneous nucleation of sulfates, indicating that condensation freezing occurs without any apparent deactivation for temperatures between -25 and -35 degrees C.