Articles | Volume 16, issue 8
Research article
28 Apr 2016
Research article |  | 28 Apr 2016

Comparison of eddy covariance and modified Bowen ratio methods for measuring gas fluxes and implications for measuring fluxes of persistent organic pollutants

Damien Johann Bolinius, Annika Jahnke, and Matthew MacLeod

Abstract. Semi-volatile persistent organic pollutants (POPs) cycle between the atmosphere and terrestrial surfaces; however measuring fluxes of POPs between the atmosphere and other media is challenging. Sampling times of hours to days are required to accurately measure trace concentrations of POPs in the atmosphere, which rules out the use of eddy covariance techniques that are used to measure gas fluxes of major air pollutants. An alternative, the modified Bowen ratio (MBR) method, has been used instead. In this study we used data from FLUXNET for CO2 and water vapor (H2O) to compare fluxes measured by eddy covariance to fluxes measured with the MBR method using vertical concentration gradients in air derived from averaged data that simulate the long sampling times typically required to measure POPs. When concentration gradients are strong and fluxes are unidirectional, the MBR method and the eddy covariance method agree within a factor of 3 for CO2, and within a factor of 10 for H2O. To remain within the range of applicability of the MBR method, field studies should be carried out under conditions such that the direction of net flux does not change during the sampling period. If that condition is met, then the performance of the MBR method is neither strongly affected by the length of sample duration nor the use of a fixed value for the transfer coefficient.

Short summary
Our article confronts the question of how to accurately measure fluxes of volatile chemicals between the earth's surface and the atmosphere when the possibility of using high-frequency analyzers, such as with eddy covariance techniques, is non-existent. By subsampling and averaging publically available data from FLUXNET and applying the modified Bowen ratio method (MBR), we have determined that the MBR can work when using prolonged sampling times and single average estimates of eddy diffusivity.
Final-revised paper