The Effects of Desert Particles Coated with Sulfate on Rain Formation in the Eastern Mediterranean

Measurements of aerosol composition in the eastern Mediterranean reveal that sulfate is found in most aerosol particles. Some of the large particles contain mixtures of chemicals such as sulfate and sea salt. The most striking observation is the discovery that mineral dust particles often get coated with sulfate and other soluble materials. The amount of soluble material on these particles is found to be related to their surface area, suggesting that the deposition process could be surface dependent. The mechanism by which sulfate is found on some of the mineral dust particles is believed to originate from evaporating cloud drops, which were originally nucleated on sulfate cloud condensation nuclei (CCN) and subsequently collected dry interstitial mineral dust particles. The presence of soluble material on mineral dust particles, converts the latter into effective giant CCN. This is further corroborated by the fact that the few large drops near the bases of convective clouds near the coast of Israel sometimes contain both dust and sulfate. Calculations show that the presence of such large CCN could be instrumental in producing large drops (20–40 µ m), which would accelerate precipitation development through drop growth by collection. Ice crystal concentration in these types of clouds was found to be much higher than expected based on previously reported measurements of ice nuclei. These high concentrations are believed to be produced by one or more of the ice multiplication processes such as the Hallett–Messop mechanism and/or the enhanced nucleation of ice under supersaturation conditions. Both ice multiplication mechanisms are possible when large drops are formed. These findings paint out that many of the rain clouds in this region have mixed characteristics (between maritime and continental). Since cloud seeding with ice nuclei attempts to increase ice crystal content in the clouds in order to enhance rain, the use of such seeding techniques in these clouds seems to be fruitless since any additional ice would not enhance rain or may even reduce it.