Long-term measurements of aerosol optical and physical properties over the Eastern Mediterranean: Hygroscopic nature and source regions

Abstract

Long-term aerosol optical and physical properties such as aerosol optical thickness and angstrom exponent were obtained from AERONET ground based sun sky radiometer located at the rural coastal Erdemli (36.6°N, 34.3°E) site in the Eastern Mediterranean from January 2000 to December 2014. Aerosol Optical Thickness (AOT) and Angstrom Exponent (α) exhibited a great short-term variability, varying up to couple of fold from day to day. The lowest AOT values was found in winter due to the removal of particles by wet deposition. Elevated AOT values with low α440-870 was observed in spring owing to sporadic mineral dust transport from North Africa and the Middle East. High AOT at 440 nm and α440-870 were characterized in summer because of high gas-to-particle conversion, sluggish air masses and absence of rain. Enhanced AOT and low α (0.7) and fine mode fraction (47%) were associated with air flow from North Africa and the Middle East with a slight decrease from 440 nm to 1020 nm, showing importance of coarse particles. Air flow from Mediterranean Sea, Eastern Europe, Western Europe and Turkey, on the other hand, showed comparatively higher α (>1.1) and fine mode fraction (>62%). Three classes of aerosol were identified namely non-hygroscopic dominated, moderately hygroscopic and hygroscopic dominated. Non-hygroscopic particles were dust dominated aerosol population originated from North Africa (Southern Tunisia, Western Libya, Libya/Chad border, southern Egypt northeastern Mauritania, north Mali, southern Algeria, Northwestern Niger) and the Middle East (extending from Iraq to eastern Saudi Arabia). Moderately hygroscopic particles were a mixture of mineral dust and anthropic aerosols when the dust reached receptor site after passing through industrialized and populated sites (such as Spain, France and Italy). Hygroscopic particles, however, were ascribed to nearly mesoscale formation of secondary aerosols under the prevailing summer conditions.