Light scattering by dust and anthropogenic aerosol at a remote site in the Negev desert, Israel

[1] We investigated aerosol optical properties, mass concentration, and chemical composition over a 2 year period at a remote site in the Negev desert, Israel (Sde Boker, 30degrees 51'N, 34degrees 47'E, 470 m above sea level). Light-scattering measurements were made at three wavelengths (450, 550, and 700 nm), using an integrating nephelometer, and included the separate determination of the backscatter fraction. Aerosol coarse and fine fractions were collected with stacked filter units; mass concentrations were determined by weighing, and the chemical composition by proton-induced X-ray emission and instrumental neutron activation analysis. The total scattering coefficient at 550 nm showed a median of 66.7 Mm(-1) (mean value 75.2 Mm(-1), standard deviation 41.7 Mm(-1)) typical of moderately polluted continental air masses. Values of 1000 Mm(-1) and higher were encountered during severe dust storm events. During the study period, 31 such dust events were detected. In addition to high scattering levels, they were characterized by a sharp drop in the Angstrom coefficient (i.e., the spectral dispersion of the light scattering) to values near zero. Mass-scattering efficiencies were obtained by a multivariate regression of the scattering coefficients on dust, sulfate, and residual components. An analysis of the contributions of these components to the total scattering observed showed that anthropogenic aerosol accounted for about 70% of scattering. The rest was dominated by the effect of the large dust events mentioned above and of small dust episodes typically occurring during midafternoon.