Baseline maritime aerosol: Methodology to Derive the optical thickness and scattering properties

Satellite measurements of the global distribution of aerosol and their effect on climate should be viewed in respect to a baseline aerosol. In this concept, concentration of fine mode aerosol particles is elevated above the baseline by man‐made activities (smoke or urban pollution), while dust or sea‐spray elevates the coarse mode. Using 1–3 years of measurements in 10 stations of the Aerosol Robotic network (AERONET) we develop a methodology and derive the optical thickness and properties of this baseline aerosol for the 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 nm with Ångström exponent of 0.77, and 0.071 and 1.1 respectively, over the Atlantic Ocean.