The multispecies analysis of daily air samples collected at the NOAA BoulderAtmospheric Observatory (BAO) in Weld County in northeastern Colorado since 2007shows highly correlated alkane enhancements caused by a regionally distributed mixof sources in the Denver-Julesburg Basin. To further characterize the emissions of methaneand non-methane hydrocarbons (propane, n-butane, i-pentane, n-pentane and benzene)around BAO, a pilot study involving automobile-based surveys was carried out duringthe summer of 2008. A mix of venting emissions (leaks) of raw natural gas and flashingemissions from condensate storage tanks can explain the alkane ratios we observe in airmasses impacted by oil and gas operations in northeastern Colorado. Using the WRAPPhase III inventory of total volatile organic compound (VOC) emissions from oil andgas exploration, production and processing, together with flashing and venting emissionspeciation profiles provided by State agencies or the oil and gas industry, we derive a rangeof bottom-up speciated emissions for Weld County in 2008. We use the observed ambientmolar ratios and flashing and venting emissions data to calculate top-down scenariosfor the amount of natural gas leaked to the atmosphere and the associated methane andnon-methane emissions. Our analysis suggests that the emissions of the species wemeasured are most likely underestimated in current inventories and that theuncertainties attached to these estimates can be as high as a factor of two.