Prof. Miriam Freedman next to the home-built cavity ring-down spectrometer her group uses to investigate the optical properties of atmospheric aerosol particles.

Faculty Spotlight: Miriam Freedman - Understanding Aerosols’ Influence on Climate Change

Domestic dust is a nuisance when cleaning around the house. Atmospheric dust is a nuisance in its own right as one of the most difficult aerosols for scientists to study. Aerosols like mineral dust are suspended materials in the atmosphere that help in the formation of clouds and influence how solar radiation affects climate.

The challenge in studying aerosol particles is due to their dissimilar compositions and structures – these properties determine their optical properties and associated radiative effects. Most particles reflect sunlight, cooling the climate. Some particles, sent aloft from burning fossil fuels and other combustion processes, absorb sunlight and warm the climate.  Mineral dust is produced when winds lift dust from deserts and arid soils into the atmosphere.  “We don’t know whether mineral dust causes a net warming or a cooling effect,” said Miriam Freedman, assistant professor of chemistry and PSIEE co-hire.

How to effectively study aerosols is a pressing matter for scientists looking to develop accurate climate change models. More precise aerosol modeling means better assessment of the effects of climate change.

Particles of mineral dust are the second largest aerosol emission by mass into the atmosphere. Freedman, along with Daniel Veghte, a Ph.D. candidate in chemistry, used cavity ring-down spectroscopy to determine what effect the structure of a mineral dust particle has on its optical properties.

The researchers used this technique to measure the optical properties of calcium carbonate, one of the most reactive components of mineral dust. Freedman and Veghte flowed calcium particles through the cavity, filled it with laser light and measured the rate at which the light intensity decreased. The rate determines the concentration of the material inside and how much it scatters or absorbs.

Other spectroscopic techniques can provide data across multiple wavelengths, but cavity ring-down spectroscopy is a more sensitive technique for studying aerosol particles. It gives precise scattering and absorption data of size-selected particles at a single wavelength.

The results from the cavity ring-down experiments were combined with imaging of the particles, using transmission electron microscopy, and mathematical modeling of light scattering. This research demonstrates a sound method for scientists to understand how shape influences the optical properties of complex aerosol particles for the purpose of developing better climate change models that encompass net radiative effects.

Freedman is constantly looking for ways to combine surface science with spectroscopy and microscopy, pointing back to her educational roots and previous research projects.

“In college, I became very interested in basic science,” Freedman said. “I pursued fundamental chemical problems in graduate school, and then studied more applied systems as a postdoctoral researcher.”

At Swarthmore College, Freedman received a bachelor’s degree in chemistry with a minor in mathematics. She went on to earn a master’s degree in mathematics at the University of Minnesota and received her master’s and doctoral degrees in chemistry at the University of Chicago.  After attaining her Ph. D., Freedman studied atmospheric chemistry as a postdoctoral researcher at the University of Colorado at Boulder.

Searching for answers to environmental questions was one of Freedman’s long-term interests. Very much a hands-on scientist, in high school and at Swarthmore College she was active in campus organizations that collected samples from local streams, measuring nutrient levels and testing for dissolved oxygen.

As a graduate student, Freedman trained as a surface scientist looking at complex organic films. She used the technique of helium atom scattering to characterize the surface dynamics of thin polymer films. Using this technique, she gained a better understanding of the changes at the surface of a material that could affect properties of the whole film.

Today, Freedman seeks to understand the direct and indirect impacts of aerosols on climate. “I have specific, but broad research goals that I want to pursue in the field of atmospheric chemistry,” Freedman said. She actively works with students, guiding their research questions and training them as independent scholars. “Based on students’ interests, I work with them to develop a trajectory of projects that in combination would constitute a doctoral thesis while also fitting my research goals.”

Freedman and Veghte’s findings will soon be published in Analytical Chemistry. The online article “The Necessity of Microscopy to Characterize the Optical Properties of Size-Selected, Nonspherical Aerosol Particles” is available at http://pubs.acs.org/doi/pdf/10.1021/ac3017373.