A. Quirantes

Aerosol transport over the western Mediterranean basin: Evidence of the contribution of fine particles to desert dust plumes over Alborán Island

Eight months (June 2011 to January 2012) of aerosol property data were obtained at the remote site of Alboran Island (35.95°N, 3.03°W) in the western Mediterranean basin. The aim of this work is to assess the aerosol properties according to air mass origin and transport over this remote station with a special focus on air mass transport from North Africa. For air masses coming from North Africa, different aerosol properties showed strong contributions from mineral dust lifted from desert areas. Nevertheless, during these desert dust intrusions, some atmospheric aerosol properties are clearly different from pure mineral dust particles. Thus, Angstrom exponent α(440–870) presents larger values than those reported for pure desert dust measured close to dust source regions. These results combine with α(440, 670) − α(670, 870) ≥ 0.1 and low single scattering albedo (ω(λ)) values, especially at the largest wavelengths. Most of the desert dust intrusions over Alboran can be described as a mixture of dust and anthropogenic particles. The analyses support that our results apply to North Africa desert dust air masses transported from different source areas. Therefore, our results indicate a significant contribution of fine absorbing particles during desert dust intrusions over Alboran arriving from different source regions. The aerosol optical depth data retrieved from Sun photometer measurements have been used to check Moderate Resolution Imaging Spectroradiometer retrievals, and they show reasonable agreement, especially for North African air masses.

Scattering cross sections of randomly oriented coated spheroids

Abstract The extended boundary condition method (EBCM), or T-matrix method, has been applied to the determination of scattering cross sections of coated spheroidal particles in random orientation. We report the effects of varying size and core/shell ratio for a number of axial ratios. Different scattering properties of equal-volume prolate and oblate spheroids, as well as the effect of nonsphericity on thin-shell-coated systems are considered. We discuss some convergence strategies for coated-particle EBCM calculations.

The scattering of light by a suspension of coated spherical particles: effects of polydispersity on cross sections

The effect of polydispersity on extinction cross sections is studied for a particle system composed of coated, spherical particles. Aden - Kerker equations, governing the light-scattering behaviour of the system, are formulated in a way that makes them more suitable for computation purposes. Differences found between light scattered by non-absorbing and highly absorbing particles show the importance of using accurate refractive indices, especially when dealing with their imaginary part, which accounts for absorption.

Assessment of African desert dust episodes over the southwest Spain at sea level using in situ aerosol optical and microphysical properties

Desert dust (DD) aerosols reach the El Arenosillo observatory (southwest Spain) following two characteristic pathways at sea level, each showing significant differences in its aerosol microphysical and optical properties. These differences, in turn, determine the influence on the radiative forcing over the region. For these events, the meteorological scenarios show a depression located over North Africa at ground level. A Mediterranean pathway occurs when: (1) the depression is located over North Africa and the Mediterranean Basin or (2) it is coupled to a high pressure in higher latitudes. A North Africa pathway is observed when the depression is located only over North Africa. In our inventory, there are clear DD episodes under the Mediterranean flow, whereas other specific DD events take place under a mixture of Mediterranean and North African flows. The pure Mediterranean flow is associated with a higher increase of particle volume and scattering coefficient within the sub-micron than the super-micron size ranges. This result indicates that the contribution to the radiative forcing through the scattering processes over the region for particles with D<1 µm is larger than for particles with D>1 µm. In contrast, the episodes with a mixture of Mediterranean and North African flows show a similar effect of sub- and super-micron size ranges on radiative forcing. The size range with the largest impact on the scattering processes is 0.3 µm<D<0.6 µm. Similar temporal variability of in situ and columnar-integrated aerosol properties on episodes with mixed flow suggests that the amounts of aerosols within the super-micron size range at the surface and aloft are correlated. During the episodes with pure Mediterranean flow, the amounts of aerosol vary independently within all size ranges, but the particle size distributions at surface and aloft are similar.

Determination of size/shape parameters of colloidal ellipsoids by photon correlation spectroscopy☆

The usefulness of photon correlation spectroscopy (PCS) for the determination of particle size in colloidal suspensions is well established. Most PCS experiments measure the time correlation function of polarized scattered light. However, if the scattered light is depolarized, dynamic and structural information on the dispersed particles can be made available, which is difficult to obtain by other techniques. One of the reasons for the less frequent use of this technique is that the depolarized signal is orders of magnitude weaker than full signals, and hence more sensitive to experimental errors. Depolarized light scattering by monodisperse suspensions of ellipsoidal hematite (a-FezO3) particles was employed for the determination of the size (equivalent volume spherical diameter) and shape (axial ratio) of the particles. From the dependence of the decay constant of the photocount correlation function with the scattering angle, information on both quantities was obtained for ellipsoids with sizes between 100 and 200 nm and axial ratios ranging from 0.19 to 0.59. Reasonable agreement was obtained with data deduced from transmission electron microscope pictures.

Cross section calculations of randomly oriented bispheres in the small particle regime

Abstract The T-matrix is used to calculate the extinction cross section of bispherical particle systems in random orientation for a monospherical size parameter x =0.01. Differences between bispherical and monospherical (Mie) results are shown for a range of values of the refractive index. It is found that the size of the T-matrix that needs to be calculated can be large, thus preventing simple dipole approximations from being used. Once the T-matrix is computed, however, only a small number of terms is needed to obtain cross section values.

Dielectric relaxation and distribution function of relaxation times in dilute colloidal suspensions

Abstract This paper describes an empirical approach to the theoretical distribution function of relaxation times (DFRT) that best characterizes the dielectric behaviour of colloidal suspensions, according to the model proposed by DeLacey and White (E.H.B. DeLacey and L.R. White, J. Chem. Soc., Faraday Trans. 2, 77 (1981) 2007; DW hereafter). It is well known that this model predicts a dielectric response that cannot be given in terms of a single relaxation time for the system, even if the colloidal particles are homogeneous spheres. Hence, the suspension must be characterized by a distribution of relaxation times; the dielectric response of a dilute colloidal suspension is described in this work using a reasonable DFRT, and the results are compared to the predictions of the DW model. It is demonstrated that such a DFRT is also a suitable approximation to the results obtained using more recent theories, incorporating into the explanation of the dielectric properties of the suspension the possibility of surface conductivity in the inner part of the double layer.