Physical and Thermodynamic Properties of Secondary Organic Materials for Modeling (Final Technical Report)
- Harvard Univ., Cambridge, MA (United States)
Secondary organic material constitutes more than 50% of the submicron component of atmospheric aerosol particles. This organic material is produced by oxidation of biogenic and anthropogenic volatile organic compounds. The physicochemical properties of these aerosol particles and their variability with atmospheric conditions influence many climate-relevant endpoints, such as rates of chemical reactions, production of secondary organic material, number-diameter distributions, direct radiative forcing, and cloud condensation nuclei. The results of the project research highlight a nonlabile-to-labile transition for material representative of anthropogenic secondary organic particulate matter at a threshold humidity range. This behavior differs markedly from materials representing biogenic aerosol sources, which lack such a transition. More specifically, the mass lability of atmospheric organic particulate matter, meaning its tendency to evaporate, is an important property in the mechanisms governing the climate effects. This property is affected by both the volatility and the molecular diffusivity of the constituent organic species. Via a series of laboratory experiments, the project directly determined the mass labilities of films of secondary organic material representative of anthropogenic and biogenic organic particulate matter (PM). The evaporation rates for anthropogenic organic PM increased above a threshold relative humidity (RH) between 20% and 30%, indicating rapid partitioning above a transition RH. Below the threshold, the characteristic time for equilibration was estimated at up to 1 week for a typically sized particle. In contrast, for films representing biogenic PM, no RH threshold was observed, suggesting equilibrium partitioning is rapidly obtained for all RHs. The effective molecular diffusivity for the biogenic case was at least 1000 times greater than that of the anthropogenic case. The results allow for an improved representation of anthropogenic and biogenic secondary organic aerosol formation in climate models. The results of the project also provide new measurements on how organic aerosol particles take up water and serve as cloud condensation nuclei, thus explaining the observed complex thermodynamic behaviors using a simple thermodynamic model. The project results reconciled the discrepancy between sub-saturation hygroscopicity and super-saturation cloud condensation nuclei activity for organic aerosol particles. The key to the explanation is the phase separation between hydrophobic and hydrophilic organics and the induced surface tension lowering. The results are essential for the assessment of the aerosol indirect effect on climate. In more detail, hygroscopic growth and cloud condensation nuclei activation are key processes for accurately modeling the climate impacts of organic particulate matter. Nevertheless, the microphysical mechanisms of these processes remain unresolved. The project results focus on complex thermodynamic behaviors, including humidity-dependent hygroscopicity, diameter-dependent cloud condensation nuclei activity, and liquid-liquid phase separation in the laboratory for biogenically derived secondary organic material representative of similar atmospheric organic particulate matter. These behaviors can be explained by the non-ideal mixing of water with hydrophobic and hydrophilic organic components. The non-ideality-driven liquid-liquid phase separation further enhances water uptake and induces lowered surface tension at high relative humidity, which results in a lower barrier to cloud condensation nuclei activation. By comparison, secondary organic material representing anthropogenic sources does not exhibit complex thermodynamic behavior. The combined results highlight the importance of detailed thermodynamic representations of the hygroscopicity and cloud condensation nuclei activity in models of the Earth’s climate system.
- Research Organization:
- Harvard Univ., Cambridge, MA (United States)
- Sponsoring Organization:
- Office of Science (SC), Biological and Environmental Research (BER). Earth and Environmental Systems Science Division
- DOE Contract Number:
- SC0012792
- OSTI ID:
- 1497485
- Report Number(s):
- DOE-HARVARD-12792
- Country of Publication:
- United States
- Language:
- English
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