Models play a vital role in predicting environmental fates of pollutants, which is critical for effective remediation. However, many fate and transport models for complex mixtures, e.g. petroleum products, do not incorporate the individual compounds, which are responsible for toxicity and environmental persistence. In this research, a diesel/water microcosm mimicked an environmental fuel spill with simulated weathering by evaporation and irradiation. Temporal changes in composition were assessed by gas chromatography-mass spectrometry (GC-MS) and time of flight mass spectrometry (ToF-MS) with atmospheric pressure chemical ionization (APCI). During evaporation, first-order kinetic rate constants were calculated for selected compounds and employed to develop predictive models, based on GC retention indices. Models were initially developed for compounds from individual classes (normal alkane, branched alkane, alkyl benzene, and polycyclic hydrocarbon) and later expanded to include compounds from all classes (comprehensive model). Using the comprehensive model, the rate constants were predicted with an average error of 10%, whereas the class specific models resulted in less error (4 - 8%). A model was also developed that incorporated varying temperature (5 to 35 °C), allowing for the prediction of the rate constants over environmentally relevant temperatures (16 % error). Using the rate constant, the fraction remaining of individual compounds was determined. The fraction remaining of individual compounds was used to calculate the fraction remaining of the total fuel (± 6%), and was in good agreement with currently available evaporation models. The variable-temperature model successfully applied to predict the fraction remaining of other petroleum products, demonstrating applicability beyond diesel fuel. The variable-temperature model was also used to predict chromatographic profiles of a fuel after evaporation, estimated the length of time a fuel has been evaporated using the predicted chromatogram, and estimate the time to reach a specific percent evaporated for an individual compound or for the entire fuel. First-order kinetic rate constants were also determined for diesel fuel irradiated with simulated sunlight for 10 hours by GC-MS and APCI-ToF-MS. The decay of hydrocarbons and formation of oxygenated compounds began within the first hour of irradiation. Using GC-MS, a two-fold increase in the rate constant was observed during irradiation (0.004 - 1.211 h-1) than predicted from the variable-temperature evaporation model (0.000 - 0.379 h-1). Compounds unlikely to evaporate also decayed, indicating they were precursors to photooxidation. In the APCI-ToF-MS, rate constants were determined for decay of hydrocarbons (0.003 - 0.210 h-1) and formation of oxygenated compounds (0.002 - 1.173 h-1). The kinetic rate constants developed in this work provided valuable information about changes in individual compounds during the weathering of petroleum products. Predicting changes in individual compounds provides additional information not available in most current models impact assessment and guide remediation of petroleum releases.
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