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

Sde Boker, in the Negev Desert of Israel (30°51′N, 34°47′E; 470 m above sea level (asl), is a long-term station to investigate anthropogenic and natural aerosols in the eastern Mediterranean in the framework of the Aerosol, Radiation and Chemistry Experiment (ARACHNE). Ground-level measurements of physical and chemical properties of aerosols and supporting trace gases were performed during an intensive campaign in summer 1996 (ARACHNE-96). Fine non sea salt (nss)-SO₄^2- averaged 8 ± 3 μg m^-3 and fine black carbon averaged 1.4 ± 0.5 μg m^-3, comparable to values observed off the east coast of the United States. Optical parameters relevant for radiative forcing calculations were determined. The backscatter ratio for ARACHNE-96 was β = 0.13 ± 0.01. The mass absorption efficiency for fine black carbon (α_a,BCEf) was estimated as 8.9 ± 1.3 m² g^-1 at 550 nm, while the mass scattering efficiency for fine nss-SO₄^2- (α_s,nss-SO42-f) was 7.4 ± 2.0 m² g^-1. The average dry single-scattering albedo, ω₀ characterizing polluted conditions was 0.89, whereas during "clean" periods ω₀ was 0.94. The direct radiative effect of the pollution aerosols is estimated to be cooling. At low altitudes (below 800 hPa), the area was generally impacted by polluted air masses traveling over the 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 (Algerian or Egyptian deserts). The combination of measurements of SO₂, CO, CN (condensation nuclei), and accumulation mode particles allowed to characterize the air masses impacting the site in terms of a mixture of local and long-range transported pollution. In particular, the lack of correlation between SO₂ and nss-SO₄^2- indicates that the conversion of regional SO₂ into the particulate phase is not an efficient process in summer and that aged pollution dominates the accumulation mode particle concentrations.