In contrast, plume detections on midstream facilities indicate that major, known, emission sources are infrequently and unpredictably identied. Many midstream facilities with known large emissions were never detected by aircraft methods, and no midstream facility, even those detected multiple times, was detected in more than half of overights. These data indicate that midstream emissions -even known, reported, large emission sources -are not detected in a representative fashion by the aircraft method used in this study. The impact of this issue is dicult to assess, but data suggest that midstream emissions estimated by plumes may be signicantly low; detection frequencies below 50% alone suggest midstream emissions estimated this way could be half actual emissions, assuming individual plume estimates are correct and unbiased.