The role of eddy covariance in atmospheric monitoring of CO2 leaks

Eddy Covariance (EC) is considered a key atmospheric technique for quantifying CO2 leakage. However the complex and localised heterogeneity of a CO2 leak above the background environmental signal violates several of the critical assumptions made when implementing the EC technique, including:

That horizontal gradients in CO2 concentration are zero. That horizontal and vertical gradients in the covariance of CO2 and orthogonal wind directions are zero.

The ability of EC measurements of CO2 flux at the surface to provide information on the location and strength of CO2 leakage from below ground stores was tested during a 144 kg/day release event (27 March - 13 June 2012) at the Ginninderra controlled release facility. We show that the direction of the leak can be ascertained with some confidence although this depends on leak strength and distance from leak. Elevated CO2 levels are seen in the direction of the leakage area, however quantifying the emissions is confounded by the potential bias within each measurement through breaching of the assumptions underpinning the EC technique. The CO2 flux due to advection of the horizontal CO2 concentration gradients, thought to be the largest component of the error with the violation of the EC technique's assumptions, has been estimated using the modelling software Windtrax. The magnitude of the CO2 flux due to advection is then compared with the measured CO2 flux measured using the EC technique, to provide an initial assessment of the suitability of the EC technique to quantifying leakage source rates. Presented at the 2013 CO2CRC Research Symposium