Ice nucleation of ammonia gas exposed montmorillonite mineral dust particles
The ice nucleation characteristics of montmoril- lonite mineral dust aerosols with and without exposure to ammonia gas were measured at different atmospheric tem- peratures and relative humidities with a continuous flow dif- fusion chamber. The montmorillonite particles were exposed to pure (100%) and diluted ammonia gas (25 ppm) at room temperature in a stainless steel chamber. Therewas no signif- icant change in the mineral dust particle size distribution due to the ammonia gas exposure. 100% pure ammonia gas expo- sure enhanced the ice nucleating fraction of montmorillonite mineral dust particles 3 to 8 times at 90% relative humidity with respect to water (RHw) and 5 to 8 times at 100% RHw for 120 min exposure time compared to unexposed montmo- rillonite within our experimental conditions. The percent- ages of active ice nuclei were 2 to 8 times higher at 90% RHwand 2 to 7 times higher at 100%RHwin 25ppm ammo- nia exposed montmorillonite compared to unexposed mont- morillonite. All montmorillonite particles are more efficient as ice nuclei with increasing relative humidities and decreas- ing temperatures. The activation temperature of montmoril- lonite exposed to 100% pure ammonia was 15◦C higher than for unexposed montmorillonite particles at 90% RHw. In the 25ppm ammonia exposed montmorillonite experiments, the activation temperature was 10◦C warmer than unexposed montmorillonite at 90% RHw. Degassing does not reverse the ice nucleating ability of ammonia exposed montmoril- lonite mineral dust particles suggesting that the ammonia is chemically bound to themontmorillonite particle. This is the first experimental evidence that ammonia gas exposed mont- morillonite mineral dust particles can enhance its activation as ice nuclei and that the activation can occur at temperatures warmer than –10◦C where natural atmospheric ice nuclei are very scarce.