2. Un-inventoried, large, emission sources exist on many facilities. One emitter of this type was discovered on one facility during TD measurements (a leak in a fuel gas system, included in data presented here and corrected the day after identication). No similar large sources were found on the other 14 facilities. Therefore, if large emitters were to explain the gap between the TD estimates, multiple such sources must exist on most facilities and these sources must have unknown characteristics that made them undetectable by Solution 1 overights. While there is no evidence for this type of large, systematic, emission source on midstream facilities, it cannot be ruled out. Per-source screening and measurements would likely be required to either identify these sources or increase condence that they do not exist. Given the size of the TD/OEI disagreement, all three of the above causes deserve evaluation. The above analysis indicates that, for midstream facilities, an `informed' inventory will likely require a more comprehensive program than simply performing periodic TD measurements at a facility; full-facility estimates need to be augmented with additional, on-site, diagnostic work and measurements. For example, many vents or other sources on midstream facilities could be metered and monitored over extended periods, and compared to TD estimates.