Atmospheric Aerosol Optical Properties in the Persian Gulf Region

Aerosol optical depth measurements over Bahrain acquired through the ground-based Aerosol Robotic Network are analyzed. Optical depths obtained from ground-based sun/sky radiometers showed a pronounced temporal trend, with a maximum dust aerosol loading observed during the March to July period. The aerosol optical depth probability distribution is rather narrow with a modal value of about 0.25. The Angstrom parameter frequency distribution has two peaks. One peak around 0.7 characterizes a situation when dust aerosol is more dominant, the second peak around 1.2 corresponds to relatively dust-free cases. The correlation between aerosol optical depth and water vapor content in the total atmospheric column is strong (correlation coefficient of 0.82) when dust aerosol is almost absent (Angstrom parameter is greater than 0.7), suggesting possible hygroscopic growth of fine mode particles or source region correlation, and much weaker (correlation coefficient of 0.45) in the presence of dust (Angstrom parameter is less than 0.7). Diurnal variations of the aerosol optical depth and precipitable water were insignificant. Angstrom parameter diurnal variability (~20-25%) is evident during the April-May period, when dust dominated the atmospheric optical conditions. Variations in the aerosol volume size distributions retrieved from spectral sun and sky radiance data are mainly associated with the changes in the concentration of the coarse aerosol fraction (variation coefficient of 61%). Geometric mean radii for the fine and coarse aerosol fractions are 0.14 μm (s.d.=0.02) and 2.57 3 μm (s.d.=0.27), respectively. The geometric standard deviation of each fraction is 0.41 and 0.73 respectively. In dust-free conditions the single-scattering albedo (SSA) decreases with wavelength, while in the presence of dust the SSA either stays neutral or increases slightly with wavelength. The changes in the Angstrom parameter derived from ground-based nephelometer and co-located sunphotometer during the initial checkout period were quite similar. 4