Aerosols contribute to the Earth's radiation balance directly by absorbing and scattering light and indirectly by nucleating cloud droplets which increase planetary albedo. Both effects depend on hygroscopic growth, but the contribution of organics to aerosol hygroscopicity is not well understood--limiting efforts to model those effects. This study will address several important questions regarding aerosol hygroscopicity through field measurements, laboratory experiments, and modeling: (1) What is the contribution of water-soluble organic carbon (WSOC) to hygroscopic growth under sub- and super-saturated conditions?; (2) What are the chemical and physical properties of WSOC?; and (3) How well can Köhler theory and thermodynamic equilibrium modeling account for observed hydroscopic growth and cloud condensation nucleus (CCN) activity of mixed inorganic and organic aerosols? To answer these questions, aerosol samples will be collected at the Desert Research Institute's high-elevation Storm Peak Laboratory (SPL) during summer. Detailed inorganic and organic composition of the samples will be measured with state-of-the-art analytical techniques to identify heretofore unspeciated high molecular weight WSOC. Water extracts of the samples and isolated WSOC will be re-aerosolized to directly measure their contribution to hygroscopic growth and CCN activity. Measured growth factors and CCN activity will be reconciled using Köhler theory and structure-based thermodynamic activity models. The results of this work will improve ability to simulate hygroscopic growth for aerosols of mixed inorganic and organic composition.
The explicit treatment of high molecular weight WSOC compounds produced in this project will be incorporated into the on-line version of the E-AIM thermodynamic equilibrium model for use by researchers and students worldwide. Ultimately, the project's results will provide information that can be incorporated into large-scale climate models to better predict direct and indirect aerosol radiative effects, such as hygroscopic-growth curves for water soluble organics and parameterizations of Köhler theory for aerosols of different inorganic and organic composition. The research will be incorporated into student projects as part of the Geoscience Research at Storm Peak (GRASP) activity, which engages students from underrepresented groups. During the winter of 2010, SPL will also host atmospheric science field courses for several universities. The project will allow the students taking these courses to work together with project researchers during the field deployment. Graduate undergraduate students from the involved research groups participate in laboratory experiments, data analysis, and publication of the results.
This project was part of a collaborative effort by researchers from multiple universities to characterize the physical and chemical properties of the organic fraction of suspended particles (a.k.a. aerosols) collected over several weeks from a mountaintop location in Colorado. Large volumes of air were passed through filters and the constituents of the particles that were trapped on them were dissolved into water (aqueous) and organic solvent solutions. The aqueous solutions were then split and one portion was further processed to isolate just the water soluble organic compounds. Our group received the total (organic + inorganic) and organic-only aqueous extracts and generated from them billions of solution droplets that were then dried to produce suspended aerosol particles having residual composition identical to that of the extract solutes. The interaction of these uniform composition particles with water vapor under subsaturated (relative humidity, RH, < 100%) and supersaturated (RH just above 100%) was then measured in our lab using a tandem differential mobility analyzer (TDMA) and cloud condensation nuclei counter (CCNc), respectively. Analysis of the samples required a little over a month of continuous operation of the pair of instruments. This two-step approach in which large samples of particles are collected and then used to generate new particles having uniform composition is far more difficult than use of identical instruments to measure the properties of the original particle population, but offers the only way to isolate the properties of soluble organic compounds from those of the soluble inorganic and insoluble compounds that are mixed together in the same ambient particles. A graduate student and two undergraduates were primarily responsible for the preparation, calibration, and ultimate use of the instruments. Though there has long been recognition that particles composed of water soluble organic compounds typically do not take up as much water as do their counterparts composed of water soluble inorganic species, there are few direct measurements of the behavior of the complex mixture of hundreds or thousands of organic species present in ambient particle populations. Our analyses showed that particles composed of just the water soluble organic compounds grew by about 33% in volume when brought from a low humidity environment to an RH of 80%, whereas particles composed of the total soluble fraction more than doubled in volume. Similarly, particles composed only of the soluble organic compounds had to be larger to form cloud droplets at relative humidity exceeding 100% than those composed of the inorganic + organic mixture. Interestingly, the organic-only particles interacted with water more strongly under supersaturated conditions than can be reconciled with their behavior under subsaturated conditions. In a sense, our findings from this project represent more of a mid-point of the larger collaborative effort than an ending. Our data and those provided by other participants that detail the composition of the extracts are now being integrated in thermodynamics models developed by yet other team members. That synthesizing effort will be completed over the next year and will be reported separately.
