Mineral dust aerosol plays a critical role in the atmosphere. Dust affects the Earth's radiation balance by direct absorption and scattering of light across the spectrum from infrared (IR) to ultraviolet (UV). Atmospheric dust particles also serve as sites for cloud nucleation indirectly affecting albedo, and as reactive surfaces for tropospheric reactions altering the chemical balance for important gas phase species such as SO2. Correctly modeling the effects of dust in weather, climate, and air quality requires accurate information about dust loading and composition, size and shape (CSS) distributions, as well as proper treatment of dust optical (scattering and absorption) properties. Dust loading and CSS distributions can be obtained by optical remote sensing. However, remote sensing dust retrievals also depend critically on an accurate treatment of aerosol optical properties. Thus, uncertainties in dust optical properties can lead to errors in estimated dust loading, and CSS distributions with deleterious consequences for weather and air quality forecasts and climate modeling.
Intellectual merit. We will investigate dust optical properties across the IR-UV spectrum through laboratory measurements and modeling analyses. The study will focus on authentic dust samples. Aerosol extinction and light scattering properties will be analyzed by measurements of particle CSS distributions through real-time in situ single particle time-of-flight mass spectrometry and particle sizing, and various ex situ methodologies. Since the particle CSS distributions will be measured simultaneously with the optical properties, detailed comparisons with theoretical simulations will be possible, with few (or no) adjustable parameters.
The main goals of this work are to establish methods for using spectroscopic and polarimetric measurements to infer mineral dust aerosol CSS distributions, and to explore how these distributions may be altered by atmospheric aging. The qualitative insight and quantitative data provided by this work can be incorporated into remote-sensing retrieval algorithms, improving the reliability of aerosol radiative transfer models for dust retrievals and climate forcing calculations, and thus transforming our understanding of the impact of dust on atmospheric chemistry, dynamics, and climate.
Broader impacts. The proposed activities offer tremendous opportunities for post-doctoral fellows and students to develop as independent scientists. This research includes aspects of experimental aerosol science, light scattering, spectroscopy, and reaction kinetics studies, combined with an extensive theoretical modeling program. Students participate in all facets of the work, presenting their results at seminars and conferences. The PIs, through a long standing collaboration, have successfully mentored students and post-doctors at all levels in preparation for professional careers in academia and industry. More than half of the students involved in the research program have been women or from underrepresented minorities. The program also maintains active collaborations with faculty from small colleges in state Iowa, enhancing their research and teaching.
Mineral dust aerosol, consisting largely of wind-blown soil, plays an important role in determining the chemical and physical equilibrium of the Earth's atmosphere. Dust affects the Earth's radiation balance by direct absorption and scattering of light across the spectrum from the infrared to the ultraviolet. Atmospheric dust particles also serve as sites for cloud nucleation, indirectly affecting the Earth's albedo, and provide reactive surface for tropospheric reactions that can alter the chemical balance for important trace gas species such as SO2 and NO2. Correctly modeling the effect of dust in climate calculations requires accurate information about dust loading and dust particle composition, size, and shape (CSS) distributions, as well as proper treatment of dust optical (scattering and absorption) properties. Dust loading and CSS distributions can be obtained by optical remote sensing. However, dust retrievals from remote sensing data also depend on accurate treatment of aerosol optical properties. Thus, uncertainties in dust optical properties lead to errors in estimated dust loading and CSS distributions, with deleterious consequences for climate modeling. We have applied a range of laboratory experiments to study dust optical properties and how those properties may vary with dust mineralogy or be altered by atmospheric aging processes. Experimental work has been supported by theoretical efforts using advanced light scattering models. Our goal has been to investigate the spectral and light scattering properties of aerosol samples that reflect the complexity of atmospheric mineral dust. We have found that theoretical models based on a uniform spheroid shape approximation can accurately reproduce dust absorption and scattering properties in many (though not all) cases provided appropriate shape distribution models are applied. However, shape distributions based on simple imaging methods such as electron microscopy often underestimate the importance of particles with extreme shape factors (i.e. spheroid aspect ratios >3). Rather we found that in many cases it is possible to infer dust particle shape distributions from analysis of the aerosol infrared extinction spectrum. Our work has uncovered important correlations between dust particle size, mineralogical composition, and shape factors. Thus a "one shape fits all" approach for modeling the optical properties of mineral dust is inadequate. Accounting for correlations between particle mineralogy and shape factors significantly improves the accuracy of theoretical models for dust absorption in the infrared and for light scattering properties in the visible. We have tested these improved models on authentic dust samples of Saharan sand, Iowa loess, and diatomaceous earth, and on a commonly used proxy for complex atmospheric mineral dust, Arizona road dust. We have also determined more accurate optical constants for a range of important atmospheric aerosol mineral and organic components including iron oxides, carboxylic acids, and humic materials. Our studies of the effects of dust processing by organic acids show that both physical coating of the particles and chemical reactions are possible depending on the system, and that these processes can alter dust CSS distributions. While the effects of these interactions on infrared optical properties are fairly muted, the effect on the visible light scattering properties can be quite significant. In some cases it appears that the effect of atmospheric processing will be to moderate the effective particle shape distributions making the particles appear somewhat closer to spherical. We have used the results from these studies to improve remote spectral characterization and modeling approaches for dust optical properties that account for differences in dust mineralogy and the effects of atmospheric aging. Results from these research studies have been published in a series of papers in the open scientific literature and have been discussed at several scientific meetings. Our raw data has also been shared with colleagues internationally who continue to use our results in modeling atmospheric field data. It is expected that improvements in the accuracy of climate modeling will help to better inform public policy discussions of climate change. This NSF funded project has supported the research efforts and training of several undergraduate and graduate students, including several women. NSF supported activities under this grant provide unique opportunities for students to develop as independent scientists. The project is interdisciplinary and requires students to learn aspects of physics, chemistry, geology, environmental, and atmospheric science. Our research helps students develop experience and specific skills in experimental aerosol science, analytical characterization methods, light scattering, spectroscopy, and reaction kinetics studies, and theoretical modeling of aerosol optical properties. Our students come to the project from different departments and with different backgrounds. Working together the students learn to be part of an interdisciplinary research team. As part of their training, students participate in local meetings and seminars, and present their findings at national conferences and professional society meetings. Our more senior graduate students also get significant experience in mentoring new graduate students and undergraduate research students.
