ABSTRACT
Morphology-dependent resonances (MDRs) can serve as a sensitive probe of the size and composition of microspheres. While the utilization of MDRs to characterize homogeneous spheres is now routine, analysis of spherical particles with more complicated refractive index profiles can be extremely difficult and time consuming. In ultraviscous and glassy aerosol particles, the concentration profile of water during sorption often contains a sharp front that propagates from the particle surface to the particle center over time. Here we show that the MDR positions associated with this type of concentration profile closely match those of a spherical core-shell profile. Due to the similarities, a core-shell model can be used to simplify the analysis of MDR positions that are observed during water uptake by high-viscosity aerosol particles. We examined the applicability and limitations of this core-shell model in the tracking of water sorption by single particles. Overall, the core-shell model allows for the radial position of a sharp diffusion front to be readily found using MDR positions observed during water sorption, making the analysis of light-scattering measurements much faster and less error prone than previously used fitting schemes. Additionally, methods for calculating MDRs in spherical core-shell particles are also discussed.
ABSTRACT
Isotopic exchange experiments that utilize D2O and H2O have received attention as a method for studying water diffusion in high viscosity aerosol particles. However, the mathematical models used to retrieve diffusion coefficients from these measurements have yet to be critically examined. Here, two models for the isotopic exchange of D2O and H2O in spherical particles are analyzed and compared. The primary difference between the two models is the choice of boundary condition at the surface of the spherical particle. In one model, it is assumed that the concentration of D2O at the surface is fixed, while in the other model, it is assumed that, at the particle surface, the concentration of D2O in the condensed phase is in equilibrium with D2O vapor. Closed-form expressions for the two boundary value problems that describe these physical models are found and discussed. Then, specific examples of aqueous droplets containing either sucrose, citric acid, and shikimic acid are examined with both models. It is found that at low relative humidities the choice of boundary condition has a negligible effect on the predicted lifetime of isotopic exchange, while at high relative humidities predicted lifetimes can differ by orders of magnitude. The implication of this result is that the choice of model can greatly affect diffusion coefficients retrieved from experimental measurements under certain conditions. Finally, discrepancies between diffusion coefficients measured using isotopic exchange and water sorption and desorption experiments are discussed.
ABSTRACT
We use the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on both Terra and Aqua satellites to present new high-resolution mapping of major atmospheric dust source points in the Middle East region on the basis of the improved version of the recently developed Middle East Dust Index (MEDI) applied to 70 dust storms, which occurred during the period between 2001 and 2012. Results indicate that 247 different source points have participated in dust storm generation in the Middle East region in which Iraq and Syria are the highest efficient sites for dust storm generation in this region, respectively. Using extracted indices for Deep Blue algorithm, identified dust sources were classified into three levels of intensity. The frequency of occurrence approach, the relationship between high atmospheric dust content and its number of occurrences, is also used to identify sensitive source points. High-intensity dust storms are mainly located west of Iraq and the border of Iraq and Syria. We will discuss the implications of our results in understanding the global dust cycle.
