Quantification of Uncertainty in Emission Factors and Inventories

For both scientific and policy reasons, there is increasing demand for quantification of variability and uncertainty in emission factors and inventories. Variability refers to inherent differences in emissions among different sources or for a given source over time. Uncertainty refers to lack of knowledge regarding the true value of emissions at a given location and time period. Scientific advisory organizations such as the National Academy of Sciences and EPA’s Science Advisory Board often call for increased quantification of variability and uncertainty, either directly with respect to emissions or in integrated assessments of which emissions characterization is a component (e.g., human health risk assessment). The Office of Management and Budget has required quantification of uncertainty as part of regulatory impact assessment. EPA’s Office of the Inspector General recently released a report specifically calling for quantification of uncertainty in emission factors. Thus, it appears not to be a question of if uncertainty analysis should be done, but rather, how? This paper will provide an overview of methods for and examples of quantification of variability and uncertainty in emissions factors, and quantification of uncertainty in emission inventories, based on an accumulated body of research over a number of years and spanning a variety of emission source categories and pollutants. Furthermore, the implications of several key guidance and advisory documents, such as the Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Emission Inventories and the NARSTO Emission Inventory Assessment, will be briefly summarized. Key findings, conclusions, and recommendations will be offered regarding practical strategies for moving forward in the effort to quantify uncertainties in emissions.