Simulation of mineral dust effects on UV radiation levels

The role played by aerosols on UV radiative transfer in the atmosphere is v ery uncertain. This is especially true regarding mineral dust. To determine the sensitivity of the UV levels to the presence of this atmospheric speci e, we have simulated the UV irradiance with different vertical distribution s of mineral dust. We have used a discrete ordinates radiative transfer mod el to obtain the UV levels both at sea level and at 3000 m. We have compute d the aerosol single-scattering albedo, the phase function, and the asymmet ry factor by Mie scattering theory. The background aerosol profiles were ta ken from WCRP [1986] models, whereas the radiative properties of mineral du st have been calculated from the aerosol size distribution obtained during Saharan dust invasions at Tenerife island (28.5 degrees N, 16.3 degrees W). The values for aerosol optical depth assumed as input for the model calcul ations are 0.2 (at 550 nm) for clean background aerosols and 0.3 (at 550 nm ) for the mineral dust component. From the results we can conclude that the dust vertical size distribution can affect the irradiance ratio F (with Sa haran dust)/F (no Saharan dust) by 2-4%. In addition, we observe that to th e same total optical depth the diffuse UV levels depend not only on the ver tical dust distribution but also on the background aerosol vertical distrib ution. We have computed diffuse radiation fluxes of about 5% between a mari time and a continental model to the same mineral dust vertical distribution .