Reply to comment on “Hydrocarbon emissions characterization in the Colorado Front Range—A pilot study” by Michael A. Levi

Abstract

The Comment by Levi (2012) on our paper, Pétron et al. (2012), presents a different interpretation of the atmospheric data and inventory estimates we used to derive our conclusions about methane emissions from oil and natural gas development in the Denver‐Julesburg Basin (DJB) in Weld County, Colorado. Levi's (2012) Comment brings up new issues that point to the need for additional information. We maintain the value of the results derived in Pétron et al. (2012), particularly that vented and fugitive methane emissions from Weld County's fossil fuel exploration and production in 2008 were likely larger and more uncertain than values reported by emission inventories. Our findings rely on the interpretation of high‐quality atmospheric observations using existing inventory data provided by the industry and regulatory agencies and on reasonable assumptions about the average vented raw gas composition. However, Levi (2012) has caused us to extend our analysis and to better characterize the uncertainties associated with his and with our approaches. In this Reply, we examine some critical limitations of the Pétron et al. (2012) and Levi (2012) interpretations of the atmospheric data using simple, two‐source emission models that incorporate inventory data sets of unknown reliability. We present new evidence that the regulatory estimates of flashing emission and regulatory modeled composition profiles for a limited number of condensate tanks, the starting point for the calculations of Pétron et al. (2012) and Levi (2012), probably do not represent the true range of these parameters for the thousands of such sources across the DJB in 2008. The results of Levi (2012) suggest that leakage in Weld County in 2008 was biased toward dry gas wells, which disagrees with current inventories of venting and fugitive emissions in U.S. oil and gas fields, including the DJB. Most importantly, the indirect flux derivations undertaken by Levi (2012) and Pétron et al. (2012) highlight two inherent shortcomings common to most emissions inventories: their reliance on the extrapolation of very limited information and the difficulty in carrying out a full uncertainty analysis of such datasets. We agree with Levi (2012) that there is an urgent need to statistically document the composition profiles and magnitudes of significant sources in oil‐ and gas‐producing fields. Observations‐based methods with established uncertainties and that are completely independent of inventory information could directly quantify emission strengths and compositions of both point and aggregated area sources, providing an objective assessment of inventory methodology and estimates.