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Accounting for greenhouse gas emissions from the degradation of chemicals in the environment

Muñoz, Ivan, Rigarlsford, Giles, i Canals, Llorenç Milà, King, Henry
The international journal of life cycle assessment 2013 v.18 no.1 pp. 252-262
Monte Carlo method, accounting, air, carbon dioxide, carbon footprint, environmental fate, global warming, greenhouse gas emissions, greenhouse gases, life cycle assessment, materials life cycle, methane, models, nitrous oxide, sediments, uncertainty
PURPOSE: The degradation of chemicals in the environment is often excluded from life cycle assessment (LCA) studies. This paper describes a method to account for greenhouse gas (GHG) emissions from degradation, namely carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). MATERIALS AND METHODS: A multi-media fate model is proposed to estimate the distribution and degradation of chemicals released to the environment. The environmental distribution, along with emission factors and assumptions for each compartment (air, water, soil and sediments), is used to calculate emissions of CO₂, CH₄and N₂O associated with the initial release of a chemical to either the air, water or soil compartments. These emissions, along with global warming potentials (GWP), are converted to CO₂equivalents from degradation (GWPdₑg). The method is described and evaluated with nine organic chemicals. RESULTS AND DISCUSSION: Applying the proposed method to the example chemicals shows that all the end products (CO₂, CH₄and N₂O) have the potential to significantly contribute to GWPdₑg, depending on the chemicals’ composition and environmental fate. These emissions, taken to represent the end-of-life stage of these nine chemicals, are shown to be significant when compared with the corresponding cradle-to-gate emissions. For three of the nine chemicals, the degradation emissions in the environment exceeded the cradle to gate phase. CONCLUSIONS: The method enables the accounting of GHG emissions that are currently neglected in many LCA and carbon footprint studies. The model has been parameterized for a generic environmental scenario, but it can be adapted for more site-specific conditions. Uncertainty in this method is associated to the potential lack of chemical-specific data to use in fate modelling, the generic evaluative environment used in fate models and the uncertainty associated with emission factors. A Monte Carlo analysis on the latter shows that the highest uncertainty is expected for chemicals where N₂O emissions are the main contributor to GWPdₑg.