Abstract
Abstract. Realistic size equivalence and shape of Saharan mineral dust particles are derived from in-situ particle, lidar and sun photometer measurements during SAMUM-1 in Morocco (19 May 2006), dealing with measured size- and altitude-resolved axis ratio distributions of assumed spheroidal model particles. The data were applied in optical property, radiative effect, forcing and heating effect simulations to quantify the realistic impact of particle non-sphericity. It turned out that volume-to-surface equivalent spheroids with prolate shape are most realistic: particle non-sphericity only slightly affects single scattering albedo and asymmetry parameter but may enhance extinction coefficient by up to 10 %. At the bottom of the atmosphere (BOA) the Saharan mineral dust always leads to a loss of solar radiation, while the sign of the forcing at the top of the atmosphere (TOA) depends on surface albedo: solar cooling/warming over a mean ocean/land surface. In the thermal spectral range the dust inhibits the emission of radiation to space and warms the BOA. The most realistic case of particle non-sphericity causes changes of total (solar plus thermal) forcing by 55/5 % at the TOA over ocean/land and 15 % at the BOA over both land and ocean and enhances total radiative heating within the dust plume by up to 20 %. Large dust particles significantly contribute to all the radiative effects reported. They strongly enhance the absorbing properties and forward scattering in the solar and increase predominantly, e.g., the total TOA forcing of the dust over land.
Highlights
Saharan mineral dust is one of the most abundant aerosols in the Earth’s atmosphere and is transported from Africa to America over the Atlantic Ocean (Parkin et al, 1972; Doherty et al, 2008; Prospero et al, 2010)
Varying imaginary part (IM) in the reasonable range between 0.0008 and 0.008 and De,max we found that a decrease of IM counteracts an increase of De,max (Fig. 6)
Large mineral dust particles have a big influence on the radiative impact of the dust
Summary
Saharan mineral dust is one of the most abundant aerosols in the Earth’s atmosphere and is transported from Africa to America over the Atlantic Ocean (Parkin et al, 1972; Doherty et al, 2008; Prospero et al, 2010). Its main aims were to investigate typical size distributions, the mineralogical as well as chemical composition and the realistic particle shape of the dust particles close to the source of their insertion into the atmosphere This information can be applied to simulate macro-physical ensemble optical properties such as the extinction as well as backscatter coefficient and the lidar ratio to be compared. We wish to study the effect of maximum particle size in a dust size distribution on optical properties and forcing in order to simulate the cut-off problem In this context we would like to note that independent remote sensing techniques, e.g., to derive micro-physical particle properties from radiometric radiation measurements at the ground surface (Dubovik et al, 2002), may lead to SSA in the visible range, which is significantly larger than dust SSA calculated with in-situ data (Cattrall et al, 2003).
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