Baseline maritime aerosol: methodology to derive the optical thickness andscattering properties

Satellite measurements of the global distribution of aerosol and their effe ct on climate should be viewed in respect to a baseline aerosol. In this co ncept, concentration of fine mode aerosol particles is elevated above the b aseline by man-made activities (smoke or urban pollution), while dust or se a-spray elevates the coarse mode. Using 1-3 years of measurements in 10 sta tions of the Aerosol Robotic network (AERONET) we develop a methodology and derive the optical thickness and properties of this baseline aerosol for t he Pacific and Atlantic Oceans. Defined as the median for periods of stable optical thickness (standard deviation < 0.02) during 2-6 days, the median baseline aerosol optical thickness over the Pacific Ocean is 0.052 at 500 n m with Angstrom ngstrom exponent of 0.77, and 0.071 and 1.1 respectively, o ver the Atlantic Ocean.