PhD Defense: Adriana Rocha Lima
Location
Physics : 401
Date & Time
July 21, 2015, 2:00 pm – 4:00 pm
Description
TITLE: Optical, microphysical, and compositional properties of volcanic ash, dust, and other atmospheric aerosols.
ABSTRACT: There are a large variety of aerosol particles in Earth's atmosphere. These tiny particles can scatter and absorb solar radiation, affect cloud properties, and impact air quality and climate. The quantification of aerosol effects in the atmospheric processes requires the knowledge of their optical, microphysical, and compositional properties.
The imaginary part of the complex refractive index, Im(m), of the aerosol particles is the essential parameter needed to quantify the aerosol absorption. This study presents a method to derive the Im(m) combining a large number of measurements of the aerosol optical and microphysical properties. The aerosol spectral mass absorption efficiency, αabs, was derived from optical reflectance measurements from the ultraviolet (UV) to the near infrared (NIR) wavelengths. Gravimetric mass and a new densimetry method were used to determine, respectively, the mass and the grain density of the aerosol particles. Particles' size and aspect ratio distributions were obtained using scanning electron microscopy (SEM). Finally, X-ray fluorescence and X-ray diffraction were used to obtain information about the composition of selected samples.
This work shows results from in situ and laboratory measurements for the characterization of several types of aerosol particles. The first part has a detailed study of the volcanic ash particles from the April-May (2010) Eyjafjallajökull eruption. The values for the imaginary refractive index were found to vary between 0.005 to 0.001i for the fine mode and from 0.003 to 0.001i for the mixed mode for the wavelengths ranging from 300 to 2500 nm. This is the first consistent spectral measurement of refractive index for the Eyjafjallajökull volcanic ash.
The second part of this work contains results of in situ and laboratory measurements of Saharan dust collected in Algeria and Mauritania during the Fennec-The Saharan Climate System experiment in 2011. This experiment showed differences in the spectral dependence of the Im(m) for the fine and the mixed modes of the dust particles. Im(m) was found to range from 0.008 to 0.001i for Algeria and 0.005 to 0.002i for Mauritania. Additionally, a significant variability in composition between dust collected in Algeria and Mauritania was observed.
The third part presents results for the imaginary refractive index of hematite and a comparison of mass absorption efficiencies for volcanic ashes from different eruptions, for dust from different locations, and for natural pigments. The results show a significant variation in the spectral dependence of aerosol absorption over the whole spectrum but, in particular, over the UV-Visible regions.
The analyses of natural pigments revealed that goethite can have stronger absorption than hematite in the UV, depending on their relative concentration in the particles. The determination of goethite and hematite content in global dust aerosols still requires significant effort.
The results of this study can help to constrain assumptions of aerosol properties in models and satellite remote sensing, and reduce uncertainties in predicting particulate transport, aerosol forcing, and climate.
ABSTRACT: There are a large variety of aerosol particles in Earth's atmosphere. These tiny particles can scatter and absorb solar radiation, affect cloud properties, and impact air quality and climate. The quantification of aerosol effects in the atmospheric processes requires the knowledge of their optical, microphysical, and compositional properties.
The imaginary part of the complex refractive index, Im(m), of the aerosol particles is the essential parameter needed to quantify the aerosol absorption. This study presents a method to derive the Im(m) combining a large number of measurements of the aerosol optical and microphysical properties. The aerosol spectral mass absorption efficiency, αabs, was derived from optical reflectance measurements from the ultraviolet (UV) to the near infrared (NIR) wavelengths. Gravimetric mass and a new densimetry method were used to determine, respectively, the mass and the grain density of the aerosol particles. Particles' size and aspect ratio distributions were obtained using scanning electron microscopy (SEM). Finally, X-ray fluorescence and X-ray diffraction were used to obtain information about the composition of selected samples.
This work shows results from in situ and laboratory measurements for the characterization of several types of aerosol particles. The first part has a detailed study of the volcanic ash particles from the April-May (2010) Eyjafjallajökull eruption. The values for the imaginary refractive index were found to vary between 0.005 to 0.001i for the fine mode and from 0.003 to 0.001i for the mixed mode for the wavelengths ranging from 300 to 2500 nm. This is the first consistent spectral measurement of refractive index for the Eyjafjallajökull volcanic ash.
The second part of this work contains results of in situ and laboratory measurements of Saharan dust collected in Algeria and Mauritania during the Fennec-The Saharan Climate System experiment in 2011. This experiment showed differences in the spectral dependence of the Im(m) for the fine and the mixed modes of the dust particles. Im(m) was found to range from 0.008 to 0.001i for Algeria and 0.005 to 0.002i for Mauritania. Additionally, a significant variability in composition between dust collected in Algeria and Mauritania was observed.
The third part presents results for the imaginary refractive index of hematite and a comparison of mass absorption efficiencies for volcanic ashes from different eruptions, for dust from different locations, and for natural pigments. The results show a significant variation in the spectral dependence of aerosol absorption over the whole spectrum but, in particular, over the UV-Visible regions.
The analyses of natural pigments revealed that goethite can have stronger absorption than hematite in the UV, depending on their relative concentration in the particles. The determination of goethite and hematite content in global dust aerosols still requires significant effort.
The results of this study can help to constrain assumptions of aerosol properties in models and satellite remote sensing, and reduce uncertainties in predicting particulate transport, aerosol forcing, and climate.
