Abstract
Abstract. Iron is a major component of atmospheric aerosols, influencing the light absorption ability of mineral dust, and an important micronutrient that affects oceanic biogeochemistry. The regional distribution of the iron concentration in dust is important for climate studies; however, this is difficult to obtain since it requires in-situ aerosol sampling or simulation of complex natural processes. Simultaneous studies of aerosol chemical composition and radiometric measurements of aerosol optical properties, which were performed in the Negev desert of Israel continuously for about eight years, suggest a potential for deriving a relationship between chemical composition and light absorption properties, in particular the spectral single-scattering albedo. The two main data sets of the present study were obtained by a sun/sky radiometer and a stacked filter unit sampler that collects particles in coarse and fine size fractions. Analysis of chemical and optical data showed the presence of mixed dust and pollution aerosol in the study area, although their sources appear to be different. Spectral SSA showed an evident response to increased concentrations of iron, black carbon equivalent matter, and their mixing state. A relationship that relates the spectral SSA, the percentage of iron in total particulate mass, and the pollution components was derived. Results calculated, using this relationship, were compared with measurements from dust episodes in several locations around the globe. The comparison showed reasonable agreement between the calculated and the observed iron concentrations, and supported the validity of the suggested approach for the estimation of iron concentrations in mineral dust.
Highlights
The presence of iron (Fe) in aeolian dust is of wide interest in climate studies due to its biogeochemical and radiative impacts (Jickells et al, 2005; Mahowald et al, 2005)
We present an analysis of aerosol absorption obtained from radiometric measurements (Sect. 4.1.), aerosol chemistry obtained from in-situ surface sampling (Sect. 4.2. and 4.3.), their integration (Sect. 4.4.), and development of a physical model for establishing the relationship between aerosol spectral absorption and chemistry (Sect. 4.5)
It should be mentioned here that the black carbon (BC) concentration is measured by a light reflectance technique so that other species (e.g., Fe) or even just an increase of total mass concentration on the filter during a dust event may contribute to the BC signal by additional light absorption
Summary
The presence of iron (Fe) in aeolian dust is of wide interest in climate studies due to its biogeochemical and radiative impacts (Jickells et al, 2005; Mahowald et al, 2005). Koven and Fung (2006) employed a similar concept of absorption spectral dependence for the characterization of dust composition; they studied sun/sky radiometer measurements through the whole atmospheric column (AERONET) and incorporated the mineralogical effects by using different modeled relative fractions of hematite, silicate and BC. We use this data for deriving coefficients which relate spectral SSA and concentrations of Fe and BC in a developed equation This approach avoids usage of bulk or modeled optical properties of a material, or absorption properties measured on aerosol samples after they have been removed from atmosphere. Specific objectives were: (1) to correlate data sets for events where ground level aerosol chemistry was found to be representative for the total column optical measurement; (2) to estimate the contamination of atmospheric dust by BC and its role in aerosol spectral absorption; and (3) to derive a relationship between iron concentrations and differential aerosol spectral absorption
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