Open-path vs. closed-path eddy covariance measurements of the net ecosystem carbon dioxide and water vapour exchange: A long-term perspective

Abstract

The differential design, deployment and data post-processing of open-path (OP) and closed-path (CP) eddy covariance systems is a potential source of bias for ongoing global flux synthesis activities. Here we use a unique 6-year data set of concurrent CP and OP carbon dioxide (CO 2) and water vapour (H 2O) eddy covariance flux measurements above a temperate mountain grassland in Austria to explore the consequences of these differences on a long-term basis. The theoretically based transfer function approach was able to account and correct for the differences in low-pass filtering between the two systems. Corrected CO 2 and H 2O fluxes exhibited excellent 1:1 correspondence, but the CP system tended to underestimate OP H 2O fluxes during conditions of high air temperature, wind speed and global radiation, large sun angles and low relative humidity. Corrections for self-heating of the OP infra-red gas analyser had a very small effect on these relationships. Energy balance closure was slightly more favourable for the OP system. No significant differences were found for the random flux uncertainty of both systems. A larger fraction of OP data had to be excluded because of obstructions of the infra-red path by water and snow. This, however, did not translate into a correspondingly larger fraction of accepted CP flux values, because of a larger percentage of CP flux data failing on the stationarity test. Integrated over the annual cycle, the CP system yielded on average a more positive net ecosystem CO 2 exchange (25 gC m −2 year −1 vs. 0 gC m −2 year −1) and a lower evapotranspiration (465 mm year −1 vs. 549 mm year −1) as compared to the OP system.