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The volatility of organic aerosols (OA) has emerged as a property of primary importance in understanding their atmospheric life cycle, and thus abundance and transport. However, quantitative estimates of the thermodynamic (volatility, water solubility) and kinetic parameters dictating ambient-OA gas-particle partitioning, such as saturation concentrations (C*), enthalpy of evaporation (Delta H-vap), and evaporation coefficient (gamma(e)), are highly uncertain. Here, we present measurements of ambient-OA volatility at two sites in the southeastern US, one at a rural setting in Alabama dominated by biogenic volatile organic compounds (BVOCs) as part of the Southern Oxidant and Aerosol Study (SOAS) in June-July 2013, and another at a more anthropogenically influenced urban location in North Carolina during October-November 2013. These measurements applied a dual-thermodenuder (TD) system, in which temperature and residence times are varied in parallel to constrain equilibrium and kinetic aerosol volatility properties. Gas-particle partitioning parameters were determined via evaporation kinetic model fits to the dual-TD observations. OA volatility parameter values derived from both datasets were similar despite the fact that measurements were collected in distinct settings and seasons. The OA volatility distributions also did not vary dramatically over the campaign period or strongly correlate with OA components identified via positive matrix factorization of aerosol mass spectrometer data. A large portion (4070 %) of measured ambient OA at both sites was composed of very-low-volatility organics (C* <= 0.1 mu g m(-3)). An effective Delta H-vap of bulk OA of similar to 80-100 kJ mol(-1) and a gamma(e) value of similar to 0.5 best describe the evaporation observed in the TDs. This range of Delta H-vap values is substantially higher than that typically assumed for simulating OA in atmospheric models (30-40 kJ mol(-1)). TD data indicate that gamma(e) is on the order of 0.1 to 0.5, indicating that repartitioning timescales for atmospheric OA are on the order of several minutes to an hour under atmospheric conditions. The OA volatility distributions resulting from fits were compared to those simulated in the Weather, Research and Forecasting model with Chemistry (WRF/Chem) with a current treatment of secondary organic aerosol (SOA) formation. The substantial fraction of low-volatility material observed in our measurements is largely missing from simulations, and OA mass concentrations are underestimated. The large discrepancies between simulations and observations indicate a need to treat low-volatility OA in atmospheric models. Volatility parameters extracted from ambient measurements enable evaluation of emerging treatments for OA (e.g., secondary OA using the volatility basis set or formed via aqueous chemistry) in atmospheric models.