Ice nucleation of ammonia gas exposed montmorillonite mineral dust particles

[摘要] The ice nucleation characteristics of montmorillonitemineral dust aerosols with and without exposure to ammonia gas were measuredat different atmospheric temperatures and relative humidities with acontinuous flow diffusion chamber. The montmorillonite particles wereexposed to pure (100%) and diluted ammonia gas (25 ppm) at roomtemperature in a stainless steel chamber. There was no significant change inthe mineral dust particle size distribution due to the ammonia gas exposure.100% pure ammonia gas exposure enhanced the ice nucleating fraction ofmontmorillonite mineral dust particles 3 to 8 times at 90% relativehumidity with respect to water (RHw) and 5 to 8 times at 100% RHw for 120 minexposure time compared to unexposed montmorillonite within ourexperimental conditions. The percentages of active ice nuclei were 2 to 8times higher at 90% RHw and 2 to 7 times higher at 100% RHw in 25 ppmammonia exposed montmorillonite compared to unexposed montmorillonite. Allmontmorillonite particles are more efficient as ice nuclei with increasingrelative humidities and decreasing temperatures. The activation temperatureof montmorillonite exposed to 100% pure ammonia was 15°C higher thanfor unexposed montmorillonite particles at 90% RHw. In the 25 ppm ammoniaexposed montmorillonite experiments, the activation temperature was10°C warmer than unexposed montmorillonite at 90% RHw. Degassingdoes not reverse the ice nucleating ability of ammonia exposedmontmorillonite mineral dust particles suggesting that the ammonia ischemically bound to the montmorillonite particle. This is the firstexperimental evidence that ammonia gas exposed montmorillonite mineral dustparticles can enhance its activation as ice nuclei and that the activationcan occur at temperatures warmer than –10°C where natural atmosphericice nuclei are very scarce.

[学科分类] 大气科学