The quantiﬁcation of greenhouse gas emissions requireshigh precision measurements made with high spatial resolution.Here we describe measurements of carbon dioxide (CO2)and methane(CH4) conducted using Purdue University’s AirborneLaboratory for Atmospheric Research (ALAR), aimed at thequantiﬁcation of the “footprints” for these greenhouse gasesfor Indianapolis, IN. A cavity ring-down spectrometer measuredatmospheric concentrations, and ﬂask samples were obtainedat various points for comparison. Coupled with pressure,temperature, and model-derived horizontal winds, thesemeasurements allow for ﬂux estimation. Long horizontaltransects were ﬂown perpendicular to the wind downwind ofthe city. Emissions were calculated using the wind speedand the difference between the concentration in the plumeand the background concentration. A kriging method is appliedto interpolate the measured values to a vertical plane tracedout by the ﬂight pattern within the mixed layer. Results show theurban plume is clearly distinguishable in the downwindconcentrations for most ﬂights. Additionally, there is largevariability in the measured day-to-day emissions ﬂuxes as wellas in the relative CH4 and CO2 ﬂuxes. Uncertainties in themethod are discussed, andits potential utilityin determiningsectorbased emission factors is shown.