Iron oxides and light absorption by pure desert dust: An experimental study

Abstract

Theoretical computations based on Mie theory indicate that absorption of light by desert dust aerosols should be highly sensitive to their content in iron oxides (hematite, goethite, etc.). A selective extraction method that has been developed recently now allows quantification of these minerals in aerosol samples less than 500 μg in mass. Thus it is now possible to assess experimentally the part played by iron oxide minerals on dust absorption properties. In this paper we present an adaptation of Bond et al.'s [1999] method for measuring mineral dust mass absorption efficiencies and deriving single scattering albedo values at two wavelengths (325 and 660 nm). It consists in measuring simultaneously the aerosol mass concentration with a TEOM microbalance, its scattering properties in the visible spectrum with a 3 wavelength nephelometer, and attenuation at the 2 aforementioned λ with a dual‐wavelength aethalometer. At first the method is applied to nonabsorbing (iron oxide‐free) aerosols in order to check the magnitude of the “apparent absorption” due to scattering artifacts. In good agreement with Bond et al.'s results for visible wavelengths, it is found that 2% of scattering is misinterpreted as absorption. This proportion is also found to be practically insensitive to the aerosol size distribution. After these preliminary measurements the method is applied to aerosols generated by shaking three natural soil samples collected in one of the main Chinese dust source (Gobi desert), in northern Sahara (Tunisia), and in the Sahel (Niger). For these aerosols, mass absorption efficiencies are found to range between 10−2 and 2 10−2 m2 per gram of aerosol at 660 nm and to increase linearly with the iron oxide content at the rate of 0.56 m2 per gram of iron oxide. Owing to the larger absorbing potential of iron oxides at short wavelengths, mass absorption efficiencies at 325 nm are about 6 times larger than at 660 nm. At this last wavelength the single scattering albedo (SSA) is found to decrease from 0.97 for Chinese and Tunisian aerosols to 0.95 for the Niger one that also happens to have the largest iron oxide content (6.5% in mass). At 325 nm the SSA is much lower for the three aerosols (∼0.80) than at 660 nm. These values are similar to recent results obtained close to major mineral dust sources by inversion of Sun photometer or satellite data. Finally, simple computations performed for conditions that prevail at regional scale in the vicinity of important dust sources show that, even when mineral dust is mixed with strongly absorbent particles such as black carbon (BC), the effect of iron oxides on light absorption is in the same order of magnitude as the one of BC.