In most cases, street-level CH4 emissions detected with vehicle-based systems are remotely diagnosed and interpreted as natural gas distribution pipeline leaks from inside the vehicle or during postprocessing; there is limited evidence of direct follow-up to confirm the source in the studies. There are potentially hundreds to thousands of plumes from different and unknown sources in cities that are only detected on a single vehicle pass but remain unattributed by measurement. Persistence is problematic because detection, particularly for sources adjacent to roadways, is strongly modulated by wind conditions; if the wind direction shifts between passes or surveys, the source may be undetectable even though the emission persists. A tiered research approach may be more effective, where a vehicle is used to screen for emissions and portable methods (i.e., sniffer) are used with follow-up to identify the source (e.g., . Simultaneous measurements of CH4 and C2H6 could support disambiguation for sources like cracked pavement or sewers when the origin (natural gas or a sewage pipeline) is not immediately obvious (e.g., . While the tiered approach to measurement likely consumes more resources, it should yield a more detailed understanding of emissions sources required to guide abatement.