Remote sensing of non-aerosol absorption in cloud free atmosphere

Abstract

Knowledge of the absorption of sunlight by atmospheric gases, aerosols and clouds is key to understanding climate and its variation. Several studies suggested that clouds absorb sunlight significantly more than what models predict. Other studies suggested that the anomalous absorption is present in cloud-free conditions. Here we measure absorption in cloud free atmosphere using the Aerosol Robotic Network (AERONET). Measurements of attenuation of direct sunlight are used to derive extinction optical thickness and sky measurements to derive scattering optical thickness, Residual extinction for zero scattering, cannot be caused by aerosols or known gases and would be associated with the cloud free absorption anomaly. The anomalous absorption, if exists, is assumed not to correlate with the presence of aerosol. The measurements, taken for several years in locations around the world, show that in the atmospheric windows 0.44, 0.67, 0.87 and 1.02 μm the only significant absorbers in cloud free atmosphere is aerosol and ozone. Non-aerosol absorption, defined as spectrally independent or smoothly variable, was found to have an optical thickness smaller than 0.002 ± 0.003, thereby, absorption of sunlight smaller than 1W/m2.