Optical properties of non-absorbing mineral dust components and mixtures

Abstract

Mineral dust is the second largest emission by mass into the atmosphere. Aerosol particles affect the radiative forcing budget by directly scattering and absorbing light, acting as cloud condensation and ice nuclei, and by providing surfaces for heterogeneous chemistry. Factors that affect how the particles scatter and absorb light include their composition, shape, size, and concentration. In this study, we characterize the most common components of mineral dust, quartz, and aluminosilicate clay minerals. In addition, we apply our results from calcite, feldspars, quartz, and aluminosilicate clay minerals to model the optical properties of Arizona test dust (ATD). We use cavity ring-down spectroscopy to measure the extinction cross sections of size-selected particles, electron microscopy to characterize the size selection, and Mie theory as well as the discrete dipole approximation as models. For quartz, the extinction cross sections can be well modeled assuming the particles are spheroids or spheres. For clay minerals, even spheroids fail to model the extinction cross sections, potentially due to orientation effects and lift forces in our flow system. In addition, aluminosilicate clay minerals experience weak size selectivity in the differential mobility analyzer. For ATD, the extinction cross sections are best modeled by treating each component of the mixture separately in terms of shape and size distribution. Through the application to ATD, our study outlines the procedure that can be used to model the optical properties of complex airborne dust mixtures. EDITOR Thomas Kirchstetter

Hygroscopicity of internally mixed ammonium sulfate and secondary organic aerosol particles formed at low and high relative humidity

“…(𝜅CRD,Mie). 66,67 For κCRD,mie and κCRD,emp reported in this study, the presence of water vapor enhanced measured κ values by 0.04, however, this difference is within experimental error.…”

“…The aerosol to sheath flow ratio was set to 1.5:5.0 to maintain this range of particle mobility diameters without changing the sheath flow. The CRD setup in this research has previously been described in detail, and only a brief overview is presented here 6,66 . The CRD spectrometer measures extinction coefficients which are related to ring-down times according to…”

Comparing aerosol refractive indices retrieved from full distribution and size- and mass-selected measurements

“…Because models rely heavily on refractive indices to calculate aerosol optical properties, many investigations have focused on the “inverse problem” [4] of empirically retrieving the complex refractive index ( m ) m=n+ik from measured optical and morphological data by either: 1) size- (and mass-) selecting particles and measuring some combination of the extinction, scattering and absorption efficiencies ( Q ext , Q scat and Q abs , respectively) or cross-sections ( C ext , C scat and C abs , respectively) [5–24] – efficiencies are the ratio of the optical cross-section to physical cross-section – or 2) using the full distribution of aerosol particles and measuring the size distribution and at least two of the extinction ( α ext ), scattering ( α scat ), backscattering ( α bscat ) and/or absorption ( α abs ) coefficients [25–42]. Chemical species data have also been used to calculate an effective refractive index that is then compared to measured optical data [35, 43].…”

Measurements of the Imaginary Component of the Refractive Index of Weakly Absorbing Single Aerosol Particles

“…Aerosol cavity ring-down spectroscopy (A-CRDS) and cavity enhanced extinction spectroscopy (CEES) have been used extensively to measure the extinction coefficients of ensemble aerosol. [4][5][6][7][8][9] These measurements of aerosol extinction have been used to retrieve the complex refractive index of a range of aerosol species, such as secondary organic aerosol, inorganic species and mineral dust. 6,[10][11][12][13][14] Typical uncertainties for A-CRDS for the real component of the RI are n ± 0.02.…”

“…[4][5][6][7][8][9] These measurements of aerosol extinction have been used to retrieve the complex refractive index of a range of aerosol species, such as secondary organic aerosol, inorganic species and mineral dust. 6,[10][11][12][13][14] Typical uncertainties for A-CRDS for the real component of the RI are n ± 0.02. 9 The size distributions and number concentrations of the ensemble of aerosols are selected prior to extinction measurements, commonly with a differential mobility analyzer, the use of which introduces ~10% uncertainty into the extinction measurements.…”