- Spectral measurements of the aerosol particle scattering coefficient sigmas and the aerosol optical depth taua were conducted at Ouranouspolis (Greece, 40°23'N, 23°57'E) and at Sde Boker (Israel, 30°51'N, 34°47'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 sigmas values at 550 nm were typically in the range of 30-200 Mm-1 at both sites. The range obtained for the summer regional aerosol optical depth taua 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.