Physical and chemical characteristics of aerosols over the Negev Desert (Israel) during summer 1996

Sde Boker, in the Negev Desert of Israel (30 degrees 51'N, 34 degrees 47'E; 470 m above sea level (asl), is a long-term station to investigate anthrop ogenic and natural aerosols in the eastern Mediterranean in the framework o f the Aerosol, Radiation and Chemistry Experiment (ARACHNE), Ground-level m easurements of physical and chemical properties of aerosols and supporting trace gases were performed during an intensive campaign in summer 1996 (ARA CHNE-96). Fine non sea salt (nss)-SO42- averaged 8 +/- 3 mug m(-3) and fine black carbon averaged 1.4 +/- 0.5 mug m(-3), comparable to values observed off the east coast of the United States. Optical parameters relevant for r adiative forcing calculations were determined, The backscatter ratio for AR ACHNE-96 was beta = 0.13 +/- 0.01. The mass absorption efficiency for tine black carbon (alpha (a),(BCEf)) was estimated as 8.9 +/- 1.3 m(2) g(-1) at 550 nm, while the mass scattering efficiency for fine nss-SO4-2 (alpha (s,n ss-SO42-f)) was 7.4 +/- 2.0 m(2) g(-1). The average dry single-scattering a lbedo, omega (0) characterizing polluted conditions was 0.89, whereas durin g "clean" periods omega (0) was 0.94. The direct radiative effect of the po llution aerosols is estimated to be cooling. At low altitudes (below 800 hP a), the area was generally impacted by polluted air masses traveling over t he Balkan region, Greece, and Turkey. Additional pollution was often added to these air masses along the Israeli Mediterranean coast, where population and industrial centers are concentrated. At higher altitudes (700 and 500 hPa), air masses came either from eastern Europe or from North Africa (Alge rian or Egyptian deserts). The combination of measurements of SO2, CO, CN ( condensation nuclei), and accumulation mode particles allowed to characteri ze the air masses impacting the site in terms of a mixture of local and lon g range transported pollution. In particular, the lack of correlation betwe en SO2 and nss-SO42- indicates that the conversion of regional SO2 into the particulate phase is not an efficient process in summer and that aged poll ution dominates the accumulation mode particle concentrations.