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The Hygroscopicity and CCN Potential of Organic Aerosol Moieties

Active Dates 8/1/2022-7/31/2025
Program Area Atmospheric System Research
Project Description
The concentration, chemical composition, and size of aerosol particles govern their role as cloud condensation nuclei (CCN) and play a major role in altering cloud properties. Until recently, the hygroscopic (water-seeking) properties of CCN were dominated by inorganic compounds, such as nitrates, sulfates and chlorides, but as emissions controls have reduced emissions of nitrogen and sulfur oxides, the organic fraction of aerosol has played an increasing role. Yet, the organic fraction is much more complex than the inorganic fraction, being composed of thousands of compounds with complex functionality that is frequently difficult to relate to aerosol particles' tendency to adsorb water, or hygroscopicity.

With recent advances in the science, predicting the hygroscopicity of the organic fraction of atmospheric aerosols may be possible:

1.    The principal Investigator of this project has developed a new theory of aqueous solutions that represents solution thermodynamic properties from infinite dilution (e.g., nearly pure water) to near pure solute (e.g., very little water). The theory has a few key properties that are important for understanding the role of the organic fraction in aerosol-cloud interactions: (a) It predicts the amount of water bound to solute and the amount that is free. The unbound fraction of solvent per solute molecule is the ideal solution theory used to derive kappa-Kohler theory, which is RH/(1-RH), where RH is the relative humidity. (b) The bound fraction depends on the functional groups on the organic molecule--groups of atoms in the molecule that dictate the molecule's behavior in chemical reactions. In recent work, Wexler compared the theory's predictions to estimates of free and bound water on sugars, and found that predictions agree with the literature, and that the amount of water bound to each sugar is proportional to the number of oxygen atoms in the molecule – that is, to the OH functional groups and the ring- and between ring- oxygens.

2.    Dillner, the co-Investigator of this project, and colleagues have developed a method for assessing a range of molecular properties of the organic fraction using Fourier Transform-Infrared Spectroscopy (FT-IR) including identifying and quantifying the number of functional groups in an aerosol sample. This method also assesses the number of carbon atoms, aerosol mass, elemental carbon content, organic carbon content and organic mass content, in addition to properties related to the inorganic fraction such as the amount of ammonium, nitrate, and sulfate.

3.    Recently, the National Park Service reached an agreement with the Department of Energy to establish an Interagency Monitoring of Protected Visual Environments Program (IMPROVE) site at the Southern Great Plains (SGP) Atmospheric Research Measurement (ARM) facility. Dr. Dillner’s group routinely measures FTIR spectra from all collected IMPROVE PM2.5 samples, so these spectra are now available for samples from the ARM SGP site.

Thus, the time is right to further develop the thermodynamic theory of aerosol solutions and the FTIR method for quantifying functional groups to more accurately estimate the hygroscopic properties of atmospheric aerosol, especially the organic fraction. In collaboration with Rahul Zaveri of Pacific Northwest National Laboratory, this new thermodynamic understanding will also be incorporated into the appropriate parts of Department of Energy-supported atmospheric models.
Award Recipient(s)
  • University of California, Davis (PI: Wexler, Anthony)