Consequences of uncertainties in CO2 density for estimating net ecosystem CO2 exchange by open-path eddy covariance

Errors in the estimation of CO2 surface exchange by open-path eddy covariance, introduced during the removal of density terms [Webb et al. Quart J Roy Meteorol Soc 106:85-100, (1980) - WPL], can happen both because of errors in energy fluxes [Liu et al. Boundary-Layer Meteorol 120:65-85, (2006)] but also because of inaccuracies in other terms included in the density corrections, most notably due to measurements of absolute CO2 density (rho(c) ). Equations are derived to examine the propagation of all errors through the WPL algorithm. For an open-path eddy covariance system operating in the Sierra de Gador in south-east Spain, examples are presented of the inability of an unattended, open-path infrared gas analyzer (IRGA) to reliably report rho(c) and the need for additional instrumentation to determine calibration corrections. A sensitivity analysis shows that relatively large and systematic errors in net ecosystem exchange (NEE) can result from uncertainties in rho(c) in a semi-arid climate with large sensible heat fluxes (H-s) and (wet) mineral deposition. When rho(c) is underestimated by 5% due to lens contamination, this implies a 13% overestimation of monthly CO2 uptake.