[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, 30°51 0 N, 34°47 0 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 À1 (mean value 75.2 Mm À1 , standard deviation 41.7 Mm À1 ) typical of moderately polluted continental air masses. Values of 1000 Mm À1 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 Å ngström 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.

Abstract. Spectral measurements of the aerosol particle scattering coefficient a•. and the aerosol optical depth ro were conducted at Ouranoupolis (Greece, 40ø23'N, 23ø57'E) and at Sde Boker (Israel, 30ø5 I'N, 34ø47'E) between June and September 1998. Measurements were related to 5-day three-dimensional back trajectories at 950, 850, and 550 hPa to assess the influence of long-range transport from particular source regions to the aerosol load at the two sites. Our measurements show that the eastern Mediterranean basin is moderately to highly polluted during summer. Daily average ors values at 550 nm were typically in the range of 30-200 Mm '• at both sites. The range obtained for the summer regional aerosol optical depth ro was 0.03-0.52 at 500 nm. Enhanced aerosol extinction was related to transport of polluted air masses from western and eastern Europe. High-altitude transport of mineral dust from northern Africa was observed at both sites, particularly in Israel.

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