Uncertainties in modeling secondary organic aerosols: three-dimensional modeling studies in Nashville/western Tennessee

The formation of secondary organic aerosols (SOA) is simulated for the Nashville/western Tennessee domain using three recent SOA modules incorporated into the three-dimensional air quality model, CMAQ. The Odum/Griffin et al. and CMU/STI modules represent SOA absorptive partitioning into a mixture of primary and secondary particulate organic compounds (OC), with some differences in the formulation of the absorption process and the selection of SOA species and their precursors. Empirical representations based on measured laboratory SOA yields are used for condensable organic products in both these modules. The AEC module simulates SOA absorption into organic and aqueous particulate phases, and a representation based on an explicit gas-phase mechanism is used in the AEC module. Predicted SOA concentrations can vary by a factor of 10 or more. In general, the gas-phase mechanistic approach predicts a higher yield of SOA than those based on laboratory yields. There exist some differences in the two empirical modules despite their similar basis on experimental data. All three modules predict a dominance of SOA of biogenic origin as compared to SOA of anthropogenic origin. The causes for differences among the three SOA modules include the representation of terpenes, the mechanistic versus empirical representation of SOA-forming reactions, the identities of SOA, and the parameters used in the gas/particle partitioning calculations. Two sensitivity studies show that formation of water-soluble SOA and temperature dependence may be areas of key uncertainties affecting current models.     

Possibility of charge contrast imaging of polymeric materials

Preliminary results illustrate the possibility of charge contrast imaging (CCI) of polymeric materials. Possible CCI images of low-density polyethylene and polyvinyl chloride reveal details that may aid in the characterization of the microstructure of polymeric materials. These pictures were obtained with a Hitachi S-3000N variable pressure scanning electron microscope with the environmental secondary electron detector (ESED).     

Biological Removal of Gaseous VOCs from Automotive Painting Operations

Among the pollutants that automotive plants produce, volatile organic compound (VOC) emissions due to paint solvents from painting operations are the largest in quantity. The current control process, based on vapor-phase adsorption followed by thermal oxidation, is costly to install and operate. At Ford, a cost-effective method of removing VOCs has been investigated that involves converting an existing spraybooth scrubber system to a biological reactor. This paper reports the results of a pilot-scale investigation in which two activated-sludge bioreactors, one with and the other without powdered activated carbon (PAC), were operated in parallel for 16 months. The primary findings include (1) The biological VOC removal process was technically feasible, and a scrubber system at a typical assembly plant is sufficiently large to handle the solvent loading used under normal vehicle-production conditions; (2) as compared to the adsorption∕thermal oxidation process, the biological process was found to be comparable in VOC removal efficiency, an order of magnitude more cost-effective in capital cost, and a factor of two more cost-effective in operating and maintenance cost; and (3) the bioreactors effectively captured and degraded hydrophilic paint solvents (methyl ethyl ketone, n-butanol, and butyl cellosolve) that were fed via the vapor phase. Toluene, a hydrophobic solvent, was also well-captured and degraded (74 to 91% without PAC and 86 to 93% with PAC).     

On stress intensities induced by direct resistance heating

The temperature field and mode I stress intensity caused by resistance heating are calculated for a model problem. The results indicate that, for a constant far-field electric current, the stress intensity is proportional to the crack length to the power 5/2. Consequently, the effect of localized heating becomes especially important as the crack length increases and may, if ignored, cause significant errors during the more severe thermal fatigue tests that utilize direct resistance heating.

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Résumé On calcule pour un problème de modélisation le champ de température et l'intensité de contrainte de Mode I provoqué par un chauffage par résistance. Les résultats indiquent que, pour un courant électrique constant appliqué, l'intensité de contraintes est proportionnelle à la longueur d'une fissure à la puissance 5/2.En conséquence, l'effet d'un chauffage local devient spécialement important lorsqu'on augmente la longueur de fissure, et peut, s'il est ignoré, induire des erreurs graves au cours d'essais de fatigue thermique de sévérité accrue qui recourent au chauffage direct par effect Joule.