Evaluating the accuracy of downwind methods for quantifying point source emissions

Abstract. The accurate reporting of methane (CH4) emissions from point sources, such as fugitive leaks from oil and gas infrastructure, is important for evaluating climate change impacts, assessing CH4 fees for regulatory programs, and validating methane intensity in differentiated gas programs. Currently, there are disagreements between emissions reported by different quantification techniques for the same sources. It has been suggested that downwind CH4 quantification methods using CH4 measurements on the fence-line of production facilities could be used to generate emission estimates from oil and gas operations at the site level, but it is currently unclear how accurate the quantified emissions are. To investigate model accuracy, this study uses fence-line simulated data collected during controlled release experiments as input for eddy covariance, aerodynamic flux gradient and the Gaussian plume inverse methods in a range of atmospheric conditions. The results show that both the eddy covariance and aerodynamic flux gradient methods underestimated emissions in all experiments. Although calculated emissions had significant uncertainty, the Gaussian plume inversion method performed better. The uncertainty was found to have no significant correlation with most measurement variables (i.e. downwind measurement distance, wind speed, atmospheric stability, or emission height), which indicates that the Gaussian method can randomly either underestimate or overestimate emissions. For eddy covariance, downwind measurement distance and percent error had negative correlation indicating that far away emissions sources were likely underestimated or be undetected. The study concludes that using fence-line measurement data as input to eddy covariance, aerodynamic flux gradient or Gaussian plume inverse method to quantify CH4 emissions from an oil and gas production site is unlikely to generate representative emission estimates.