Potential impact of Owens (dry) Lake dust on warm and cold cloud formation

摘要

Owens lake bed is one of the largest sources of particulate matter in the western hemisphere. Because of the fine-grained nature of this dust, it is easily lofted above the atmospheric boundary layer, where it may affect cloud formation. Further, unlike many other soil types, it has a large fraction of soluble material (～3–37% of the mass on an organic-matter-free basis), so it can impact both warm and cold clouds. In this study, we measure the hygroscopicity, cloud condensation nucleus (CCN) activity, and ice nucleating ability of Owens (dry) Lake dust. Our studies confirm that a fraction of the Owens (dry) Lake dust (35% by number for particle sizes ～200 nm) is quite hygroscopic and has CCN activity comparable to that of sodium chloride (50 nm particles activate at 0.33% supersaturation). These hygroscopic dust particles can readily initiate droplet formation at modest supersaturations in the atmosphere and may significantly impact liquid clouds in the southwestern United States. The less hygroscopic fraction had only modest CCN activity. However, such particles may still activate as cloud droplets at atmospheric supersaturations and could potentially modify cloud properties if particles with diameters larger than ～300 nm are present. The less hygroscopic fraction also was found to nucleate ice heterogeneously at relative humidities significantly lower than required for homogeneous freezing of aqueous particles at temperatures colder than -40°C. In convective systems, this dust may be lofted to the upper troposphere where it may impact convection strength and onset conditions for ice formation in cold clouds.