Observation of playa salts as nuclei in orographic wave clouds

During the Ice in Clouds Experiment – Layer Clouds (ICE-L), dry lakebed, or playa, salts from the Great Basin region of the United States were observed as cloud nuclei in orographic wave clouds over Wyoming. Using a counterflow virtual impactor in series with a single-particle mass spectrometer, sodium-potassium-magnesium-calcium-chloride salts were identified as residues of cloud droplets. Importantly, these salts had similar mass spectral signatures to playa salts with elevated cloud condensation nuclei (CCN) efficiencies similar to sea salt. Using a suite of chemical characterization instrumentation, the playa salts were observed to be internally mixed with oxidized organics, presumably produced by cloud processing, and carbonate. These salt particles were preferentially observed as residues of large droplets (>19 µm) compared to smaller droplets (>7 µm). In addition, a small fraction of silicate-containing playa salts were hypothesized to be important in the observed heterogeneous ice nucleation. While the high CCN activity of sea salt has been demonstrated to play an important role in cloud formation in marine environments, the presence of playa salts as cloud nuclei in continental North America has not been shown previously. This study suggests that the impact of playa dust on cloud formation may be significant due to the cloud-forming ability of playa dust, global abundance of playas, and increasing potential for playa dust storms as a result of land use changes.