Recently, numerous aerial and satellite estimates of emissions -commonly called topdown (TD) estimates -have identified significant disagreements between inventory (bottom-up (BU)) and TD estimates. Since BU estimates tend to estimate lower emissions than TD methods, two structural problems with inventory methods have been identified as possible causes of this agreement. First, fugitive emissions are often highly intermittent and difficult to discover, leading under-counting the number of emitters , and error in activity data . Emissions estimates for discovered emitters most often rely on emission factors derived from prior studies. Quality of these factors varies, with some evidence indicating an inadequate representation of rare, large, emitters in the factors. Additionally, emission factors are, by definition, averages of emissions that do not capture the inherent variability for a specific source or over time. Second, the assumption of nominal process behavior for vented and combusted emissions omits process failures that have been identified as a key source of large emission events .