ALBERT

Description: We develop the thermodynamic underpinnings of a two-dimensional volatility basis set (2-D-VBS) employing saturation concentration (Co) and the oxygen content (O:C) to describe volatility, mixing thermodynamics, and chemical evolution of organic aerosol. This is an extension of our earlier one-dimensional approach employing C* only (C*=γ Co, where γ is an activity coefficient). We apply a mean-field approximation for organic aerosol, describing interactions of carbon and oxygen groups in individual molecules (solutes) with carbon and oxygen groups in the organic-aerosol solvent. In so doing, we show that a linear structure activity relation (SAR) describing the single-component Co of a molecule is directly tied to ideal solution (Raoult's Law) behavior. Conversely, non-ideal solution behavior (activity coefficients) and a slightly non-linear SAR emerge from off-diagonal (carbon-oxygen) interaction elements. From this foundation we can build a self-consistent description of OA mixing thermodynamics, including predicted saturation concentrations and activity coefficients (and phase separation) for various solutions from just four free parameters: the carbon number of a hydrocarbon with a 1 μg m−3 Co, and the carbon-carbon, oxygen-oxygen, and non-ideal carbon-oxygen terms. This treatment establishes the mean molecular formula for organics within this 2-D space as well as activity coefficients for molecules within this space interacting with any bulk OA phase described by an average O:C.

Description: Permafrost soils in arctic and boreal ecosystems store twice the amount of current atmospheric carbon that may be mobilized and released to the atmosphere as greenhouse gases when soils thaw under a warming climate. This permafrost carbon climate feedback is among the most globally important terrestrial biosphere feedbacks to climate warming, yet its magnitude remains highly uncertain. This uncertainty lies in predicting the rates and spatial extent of permafrost thaw and subsequent carbon cycle processes. Terrestrial ecosystem influences on surface energy partitioning exert strong control on permafrost soil thermal dynamics and are critical for understanding permafrost soil responses to climate change and disturbance. Here we review how arctic and boreal ecosystem processes influence permafrost soils and characterize key ecosystem changes that regulate permafrost responses to climate. While many of the ecosystem characteristics and processes affecting soil thermal dynamics have been examined in isolation, interactions between processes are less well understood. In particular connections between vegetation, soil moisture, and soil thermal properties affecting permafrost conditions could benefit from additional research. In particular, connections between vegetation, soil moisture, and soil thermal properties affecting permafrost could benefit from additional research. Changes in ecosystem distribution and vegetation characteristics will alter spatial patterns of interactions between climate and permafrost. In addition to shrub expansion, other vegetation responses to changes in climate and disturbance regimes will all affect ecosystem surface energy partitioning in ways that are important for permafrost. Lastly, changes in vegetation and ecosystem distribution will lead to regional and global biophysical and biogeochemical climate feedbacks that may compound or offset local impacts on permafrost soils. Consequently, accurate prediction of the permafrost carbon climate feedback will require detailed understanding of changes in terrestrial ecosystem distribution and function and the net effects of multiple feedback processes operating across scales in space and time.