Detection of a plume from the active face of a landfill may depend partly on the detection limits of the measurement methods used and the time at which observations are made. First, the aircraft and satellite platforms used in recent studies have 5-10 kg CH4/h , see also and ~100 kg CH4/h detection limits, respectively. Thus, the active face may dominate total landfill emissions generally (e.g., , but plumes from smaller active faces or those accepting less waste may be below the detection limits of these methods, or atmospheric conditions may result in non-detects. Topography of the active face and operator traffic may hamstring efforts to corroborate this hypothesis with on-or near-surface methods . Second, remote sensing methods that rely on measuring CH4 abundance in a column of air using solar backscatter are limited to daytime observations. Emissions from a landfill's active face may only be detectable or elevated during daytime working hours when waste is being added to the active face or managed. Landfill operators may add a thin layer of cover soil or another barrier over the active face at the end of each working day to control odors, birds, and prevent the wind from mobilizing dust/aerosols off the surface before removing it at the start of the next working day. It is unknown whether this is standard industry practice, but covering the active face temporarily at night may temporarily modify its emissions rate. Thus, higher daytime emissions rates and observational density may contribute to TD estimates of point-sources > GHGRP self-reported rates for the Charlotte Motor Speedway Landfill (1003099) and Newby Island Landfill (1006179). Elevated daytime emissions rates from urban sources of CH4 are not unique to landfills; incomplete combustion from manufacturing or intentional releases from other sources during daytime operations may also be higher.