Optical properties and dominant types of aerosols in the marine environments surrounding the East Mediterranean - Middle East (EMME) region during the AQABA cruise

This study analyzes the optical properties (scattering, absorption coefficients, single scattering albedo) of aerosols in the marine boundary layer of oceanic areas surrounding the East Mediterranean – Middle East (EMME) region. It aims  to explore the spatio-temporal variability of aerosols, their atmospheric mixing state, sources and dominant types in a way to assess their role on solar radiation and climate. The current analysis uses measurements obtained in the framework of the AQABA (Air Quality and climate change in the Arabian Basin) cruise, during a two month (1^st July - 1^st September 2017) period. The cruise consisted of a round trip onboard of a research vessel from south of France to Kuwait, crossing the central-east Mediterranean Sea, Red Sea, Arabian Sea and Persian Gulf.

Aerosol scattering and absorption coefficients of both submicron (PM₁) and supermicron (PM₁₀) particles were measured, using a polar nephelometer (Aurora 4000 Ecotech Inc) and a dual spot aethalometer (Model AE-33, Magee Scientific), respectively. The meterorological and atmospheric conditions during the whole cruise campaign in July-August 2017 were consisted with local and regional climatology, without intense dust outflows from the arid/desert lands in the Middle East. FLEXPART air mass back-trajectories indicated the potential impact of the continental emissions to examined oceanic regions.

Both scattering and absorption coefficients for PM₁ and PM₁₀ particles exhibited higher values along the ship cruise in the southern Red Sea, due to continental outflow from east Africa, and in the Persian Gulf due to mixing of natural dust with anthropogenic emissions from the industrial sector and oil refineries. The east Mediterranean exhibited moderate aerosol loading, with intermediate values of scattering Ångström Exponent (SAE) (around 1-1.5), which increased over the Persian Gulf, suggesting enhanced anthropogenic impact against desert dust, while over the Gulf of Aden and the west Arabian Sea, SAE values were very low revealing dust dominance. The absorption Ångström Exponent (AAE) values remained close to 1, indicative of Black Carbon from fossil-fuel combustion, while they increased at regions dominated by dust aerosols, even without high aerosol loading i.e. in the Gulf of Aden and the Arabian Sea.

Using the SAE vs. AAE classification scheme, key aerosol types were identified along the ship cruise. The results showed contrasting aerosol characteristics and types for the various sub-regions. The “BC-dominated” type clearly prevailed over the East Mediterranean and Suez Canal, while coarse particles mixed with BC dominated in the Gulf of Aden and the Arabian Sea, where the “dust type” also appeared. In the Persian Gulf, the mixing of anthropogenic pollution with marine aerosols, resulted in a dominant “small/low absorption” aerosol type, characterized by fine aerosols with low spectral dependence of the absorption coefficient.