Simulation of mineral dust effects on UV radiation levels (original) (raw)

2000, Journal of Geophysical Research: Atmospheres

The role played by aerosols on UV radiative transfer in the atmosphere is very uncertain. This is especially true regarding mineral dust. To determine the sensitivity of the UV levels to the presence of this atmospheric specie, we have simulated the UV irradiance with different vertical distributions of mineral dust. We have used a discrete ordinates radiative transfer model to obtain the UV levels both at sea level and at 3000 m. We have computed the aerosol single-scattering albedo, the phase function, and the asymmetry factor by Mie scattering theory. The background aerosol profiles were taken from WCRP [ 1986] models, whereas the radiative properties of mineral dust have been calculated from the aerosol size distribution obtained during Saharan dust invasions at Tenerife island (28.5øN, 16.3øW). The values for aerosol optical depth assumed as input for the model calculations 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 Saharan dust)/F (no Saharan dust) by 2-4%. In addition, we observe that to the same total optical depth the diffuse UV levels depend not only on the vertical dust distribution but also on the backg.round aerosol vertical distribution. We have computed differences for the diffuse radiation fluxes of about 5% bet•veen a maritime and a continental model to the same mineral dust vertical distribution. 1. Introduction Atmospheric aerosols modi.fy the energy balance in the Earth-atmosphere system and also the UV radiation levels at the surface. This component can affect climate both directly, by radiation scattering and absorption, and indirectly, producing changes in the cloud droplet concentration and size distribution. The role played by aerosols on the UV radiative transfer in the atmosphere is very uncertain. It is necessary to decrease this uncertainty in order to improve the modeling of the energy transfer due to the important role played by UV radiation in the biosphere. Early studies about the influence of aerosols in the Earthatmosphere system energy budget has been mainly centered on particles from anthropogenic sources, such us non-sea-salt sulfhte (nss SO•), aerosols f•om biomass burning, etc. They have shown that the effect of these compounds is comparable to greenhouse effect gases but opposite in sign. For example, Kiehl and Briegleb [1993] and Taylor and Penner [1994] suggest a global average direct radiative tbrcing (AF) in the range of AF =-0.3 and-0.9 W/m 2. For biomass burning aerosols, Penner et al. [1992] estimate a direct radiative forcing around-1 W/m 2. Nevertheless, recent studies have pointed out the importance of the mineral dust in the determination of the Copyright 2000 by the American Geophysical Union. Paper number 1999JD901058. 0148-0227/00/1999JD901058509.00 atmospheric radiative properties, mainly in the oceanic regions where this is the dominant aerosol component. Modeling studies indicate that the directly radiative forcing by mineral dust could be significant on a global scale and dominant on regional scales. Authors such us Tegen et al. [1996] propose a value of-1 W/m 2, whereas Sokolik and Toon [1996] have estimated a mineral dust forcing of about-0.25 W/m 2 over land and-0.6 W/m 2 over oceans. On the other hand, Tegen and Lacis [ 1996] have shown that the mineral dust radiative forcing depends on the vertical distribution of this atmospheric constituent. These authors have pointed out that the changes in the tropospheric radiation levels are a function of the effective radius of the aerosol distribution for different vertical distributions. Herman et al. [1997] have studied the distribution of the UV-absorbing aerosols by radiance differences between the 340 and the 380 nm channels from the Nimbus 7 Total Ozone