Assessing the Potential of Laser-induced Condensation in Dusty Arid Environments

Abstract

Atmospheric turbidity plays a crucial and controversial role in the hydro-climatology of arid regions, with atmospheric aerosols both acting as rainfall inhibitors and enhancers. Aerosols and dust thus represent an active component of the climatology of arid lands, inducing strong feedbacks on the already fragile hydrological regime of these regions. Aircraft observations and model simulations show that cloud development is strongly modulated by dust-cloud interactions at the micro scales, during the drop formation process. However, the influence of aerosols and dust on precipitation remains poorly understood, mainly due to our limited knowledge of the dynamical processes that - acting over a wider range of spatial and temporal scales - drive cloud formation and trigger precipitation. In this thesis, we first focused on establishing a link between atmospheric turbidity - an integral measure of dust/aerosol load in the lower atmosphere - and precipitation in hyper arid regions, focusing in particular on the case study of the Arabian Peninsula. Over the study region, this link turned out to be particularly strong, positive, and localized in space, resulting in intense convective rainfall events such as the ones that take place over the Peninsula during the Spring, in connection with elevated regional dust transport and local advection of water vapor from the Red Sea, the Gulf and the Arabian Sea. These results encouraged us to better understand the characteristics of dust-water interactions at the Nano-scales, thus focusing on nanoscale surface wettability, defined as the complex interplay of surface chemistry and morphology on the tendency of a liquid to spread on a solid substrate. Aerosol wettability is known to possess a crucial role in triggering (or suppressing) condensation, based on a broad range of boundary conditions. Despite its fundamental importance, the Nanoscopic view of wettability still escapes to our full understanding. This gap of knowledge is usually attributed to the lack of high enough resolution or sensitivity of current methods under bioactive conditions or moisturized environments. We hence developed an 'ad-hoc' methodology to investigate the wettability of atmospheric aerosols and dust, based on the joint use of sessile drop contact angle, environmental electron scanning microscopy condensation, and atomic force microscopy measurements. Such methodology was first tested in the characterization of calcite dust wettability, a very common mineral dust in the United Arab Emirates (UAE). We found and described a direct relationship between wettability at macro- and Nano-scales. We also found that calcite wettability transitions mostly depend on the aging of the considered surface. Since our methodology further allows the decoupling of morphology and chemistry, we were further able to establish that wettability evolution due to aging at the Nanoscale is a consequence of chemistry alterations only. Such ability of separating chemical/photochemical effects from the ones due to surface morphology is crucial for the future investigation of photochemical surface modifications induced by optical techniques, and for an efficient quantitative assessment of the role of these modifications in inducing condensation on natural and artificial nuclei of aggregation.

Date of Award	May 2017
Original language	American English