Global scale variability of the mineral dust long-wave refractive index: a new dataset of in situ measurements for climate modeling and remote sensing

Abstract

Modeling the interaction of dust with long-wave (LW) radiation is still a challenge because of the scarcity of information on the complex refractive index of dust from different source regions. In particular, little is known about the variability of the refractive index as a function of the dust mineralogical composition, which depends on the specific emission source, and its size distribution, which is modified during transport. As a consequence, to date, climate models and remote sensing retrievals generally use a spatially invariant and time-constant value for the dust LW refractive index. In this paper, the variability of the mineral dust LW refractive index as a function of its mineralogical composition and size distribution is explored by in situ measurements in a large smog chamber. Mineral dust aerosols were generated from 19 natural soils from 8 regions: northern Africa, the Sahel, eastern Africa and the Middle East, eastern Asia, North and South America, southern Africa, and Australia. Soil samples were selected from a total of 137 available samples in order to represent the diversity of sources from arid and semi-arid areas worldwide and to account for the heterogeneity of the soil composition at the global scale. Aerosol samples generated from soils were re-suspended in the chamber, where their LW extinction spectra (3–15 µm), size distribution, and mineralogical composition were measured. The generated aerosol exhibits a realistic size distribution and mineralogy, including both the sub- and super-micron fractions, and represents in typical atmospheric proportions the main LW-active minerals, such as clays, quartz, and calcite. The complex refractive index of the aerosol is obtained by an optical inversion based upon the measured extinction spectrum and size distribution. Results from the present study show that the imaginary LW refractive index (k) of dust varies greatly both in magnitude and spectral shape from sample to sample, reflecting the differences in particle composition. In the 3–15 µm spectral range, k is between ∼ 0.001 and 0.92. The strength of the dust absorption at ∼ 7 and 11.4 µm depends on the amount of calcite within the samples, while the absorption between 8 and 14 µm is determined by the relative abundance of quartz and clays. The imaginary part (k) is observed to vary both from region to region and for varying sources within the same region. Conversely, for the real part (n), which is in the range 0.84–1.94, values are observed to agree for all dust samples across most of the spectrum within the error bars. This implies that while a constant n can be probably assumed for dust from different sources, a varying k should be used both at the global and the regional scale. A linear relationship between the magnitude of the imaginary refractive index at 7.0, 9.2, and 11.4 µm and the mass concentration of calcite and quartz absorbing at these wavelengths was found. We suggest that this may lead to predictive rules to estimate the LW refractive index of dust in specific bands based on ...

Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave spectrum: a simulation chamber study

“…Caponi et al: Spectral-and The experiments in this work have been performed in the 4.2 m 3 stainless-steel CESAM (Wang et al, 2011). The CE-SAM has been extensively used in recent years to simulate, at sub-and super-saturated conditions, the formation and properties of aerosols at concentration levels comparable to those encountered in the atmosphere (Denjean et al, 2015a, b;Brégonzio-Rozier et al, 2015, 2016Di Biagio et al, 2014, 2017. CESAM is a multi-instrumented platform, equipped with 12 circular flanges to support its analytical environment.…”

“…The particle size distribution is routinely measured by a combination of (i) a scanning mobility particle sizer (SMPS, mobility diameter range 0.02-0.88 µm), composed of a differential mobility analyzer (DMA, TSI Inc. model 3080) and a condensation particle counter (CPC, TSI Inc. model 3772); (ii) a SkyGrimm optical particle counter (Grimm Inc., model 1.129, optical equivalent diameter range 0.25-32 µm); and (iii) a WELAS optical particle counter (PALAS, model 2000, optical equivalent diameter range 0.5-47 µm). Full details of operations and data treatment of the particle counters are provided in Di Biagio et al (2017).…”

“…The first quartz membrane sample ("total") was collected without a dedicated size cutoff using an in-housebuilt stainless-steel sampler operated at 5 L min −1 . However, as detailed in Di Biagio et al (2017), the length of the sampling line from the intake point in the chamber to the filter entrance was 50 cm, resulting in a 50 % cutoff of the transmission efficiency at 10.6 µm particle aerodynamic diameter. This fraction is therefore indicated as PM 10.6 in the following discussion.…”

“…The experiments were conducted at ambient temperature and at a relative humidity < 2 %. As in Di Biagio et al (2014Biagio et al ( , 2017, dust aerosols were generated by mechanical shaking of the parent soils, previously sieved to < 1000 µm and dried at 100 • C for about 1 h to remove any residual humidity. About 15 g of soil was placed in a Buchner flask and shaken for about 30 min at 100 Hz by means of a sieve shaker (Retsch AS200).…”

Aethalometer multiple scattering correction &lt;i&gt;C&lt;/i&gt;&lt;sub&gt;ref&lt;/sub&gt; for mineral dust aerosols

“…This consisted of recalculating the OPC calibration curve for different complex refractive index values. For dust aerosols the refractive index was varied in the range 1.47-1.53 (n) and 0.001-0.005i (k) following the literature (see Di Biagio et al, 2017) and D g was set at the mean ± one standard deviation of the values obtained for the different n and k. For kaolinite the OPC diameter conversion was performed by setting the refractive index at 1.56-0.001i. For ambient air the refractive index was set at 1.60-0.01i, a value that represents a medium absorbing urban polluted aerosol (see Di Biagio et al, 2016).…”