Aerosol Radiative Impact on Spectral Solar Flux at the Surface, Derived from Principal-Plane Sky Measurements

Abstract

Accurate measurements of the spectral solar flux reaching the surface in cloud-free conditions are required to determine the aerosol radiative impact and to test aerosol models that are used to calculate radiative forcing of climate. Spectral flux measurements are hampered in many locations by persistent broken cloud fields. Here a new technique is developed to derive the diffuse solar spectral flux reaching the surface from principal-plane measurements conducted in the last six years by the Aerosol Robotic Network (AERONET). This 50?100 instrument global network measures the principal-plane radiances in four spectral bands (0.44?1.02 ?m) approximately every hour every day. These instruments also measure the spectral optical thickness and derive the aerosol size distribution and other properties from sky measurements. The advantage of the AERONET measurements is that collimated sky radiance is measured for each 1° ? 1° field of view. Clouds and cloud shadows are rejected before the total sky brightness is reconstructed and the flux is derived. The results compare favorably with shadow band measurements and with aerosol models. Studied are smoke aerosol in Brazil; Saharan dust in Cape Verde; and urban?industrial pollution in Créteil, near Paris, France, and near Washington, D.C. The spectral attenuation of total (diffuse+direct) solar flux reaching the surface is given by f? = exp(?a? ? b?τ?), where a? is attenuation by an atmosphere with no aerosol and b? is the aerosol attenuation coefficient. Remarkably, it is found that for these sites except for the Washington, D.C., site, the spectrally averaged value of b? does not vary significantly from one aerosol type to another: {b?} = 0.35 ± 0.03 (for solar zenith angle of 50°). The measured 24-h average aerosol impact on the solar flux at the surface per unit optical thickness is ?F/?τ = ?80 W m?2 in these sites, almost independent of the aerosol type: smoke, dust, or urban?industrial pollution. In Washington, D.C., it is suspected, and demonstrated in a back of the envelope calculation, that the high amount of broken cloudiness and its correlation with the aerosol optical thickness are responsible for the apparent small aerosol forcing at the surface of ?F/?τ = ?50 W m?2.