Observations from four periods (three late springs and one early summer) at temperate forest sites in western and eastern Canada offer the first estimation of how the concentrations of submicron forest organic aerosol mass (SFOM) from the oxidation of biogenic volatile organic compounds (BVOC) vary over the ambient temperature range of 7 degrees C to 34 degrees C. For the measurement conditions of clear skies, low oxides of nitrogen and within approximately one day of emissions, 50 estimates of SFOM concentrations show the concentrations increase exponentially with temperature. The model that is commonly used to define terpene emissions as a function of temperature is able to constrain the range of the SFOM values across the temperature range. The agreement of the observations and model is improved through the application of an increased yield of SFOM as the organic mass concentration increases with temperature that is based on results from chamber studies. The large range of SFOM concentrations at higher temperatures leaves open a number of questions, including the relative contributions of changing yield and of isoprene, that may be addressed by more ambient observations at higher temperatures. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.
Xylenes are important constituents of many liquid fuels, as well as precursors of secondary organic aerosols (SOAs). To examine the mechanisms for formation of SOAs in the atmosphere, the abstraction reaction of p-xylene with OH and the secondary degradation channels of its intermediates were first and extensively investigated with density functional theory at the B3LYP/6-31+G (d, p) level. The result indicates that H-abstraction from methyl groups is a barrier-less path while that from phenyl groups require a free energy barrier of approximately 2.8 kcal mol(-1). Upon formation of p-xylyl, further addition by O-2 readily occurs to form peroxy radical. Subsequently, possible degradation channels for the formation of main products (p-tolualdehyde and p-quinone methide) have been determined in presence of NO. The free energy profile constructed shows that the entire reaction process is exothermic. In addition, the dipole moment of p-tolualdehyde is higher than that of p-xylene, consistent with their relative hygroscopic values. This indicates that the degradation products of p-xylene can readily immerse into the SOA phase, while p-xylene may be subject to further atmospheric degradation to form non-volatile compounds. (C) 2011 Elsevier B.V. All rights reserved.
A thermal-elastoplastic constitutive model is proposed for particle-filled composites in this paper. Particles are assumed to be linear thermoelastic while the matrix follows the thermal-elastoplastic responses with the generalized Ramberg-Osgood relation. Based on the micromechanics methodology and homogenization procedures, the effective thermal-mechanical constitutive functions are derived including the macroscopic Helmholtz free energy and the macroscopic yield function.First, it is assumed that in the case of plastic unloading or stress-strain state being in the macroscopic yield surface, the constitutive relation of the composites is linear thermoelastic expressed by the macroscopic Helmholtz free energy. The micromechanics-based thermoelastic properties of the composite are obtained including the effective elastic moduli, thermal expansion coefficients, and specific heats.Furthermore, with the concept of linear comparison composites, the variational principle is extended to consider the thermal effect, from which the lower bound of the macroscopic stress potential for the nonlinear composites can be computed. The associated macroscopic plastic strain is defined, and the macroscopic yield function in the temperature-strain space is therefore determined.Finally, the above two constitutive functions are combined with the thermal-elastoplastic constitutive theory proposed by Huang (1994) to develop the loading-unloading criterion in the temperature-strain space and the incremental thermal-elastoplastic constitutive relations for particulate composites. The results can be useful in the study of the thermomechanical behavior of particle-filled composites at elevated temperatures.
A thermal-elastoplastic constitutive model is proposed for particle-filled composites in this paper. Particles are assumed to be linear thermoelastic while the matrix follows the thermal-elastoplastic responses with the generalized Ramberg-Osgood relation. Based on the micromechanics methodology and homogenization procedures, the effective thermal-mechanical constitutive functions are derived including the macroscopic Helmholtz free energy and the macroscopic yield function.First, it is assumed that in the case of plastic unloading or stress-strain state being in the macroscopic yield surface, the constitutive relation of the composites is linear thermoelastic expressed by the macroscopic Helmholtz free energy. The micromechanics-based thermoelastic properties of the composite are obtained including the effective elastic moduli, thermal expansion coefficients, and specific heats.Furthermore, with the concept of linear comparison composites, the variational principle is extended to consider the thermal effect, from which the lower bound of the macroscopic stress potential for the nonlinear composites can be computed. The associated macroscopic plastic strain is defined, and the macroscopic yield function in the temperature-strain space is therefore determined.Finally, the above two constitutive functions are combined with the thermal-elastoplastic constitutive theory proposed by Huang (1994) to develop the loading-unloading criterion in the temperature-strain space and the incremental thermal-elastoplastic constitutive relations for particulate composites. The results can be useful in the study of the thermomechanical behavior of particle-filled composites at elevated temperatures.