Boundary-Layer Meteorol (2007) 122:205–216 DOI 10.1007/s10546-006-9105-1 ORIGINAL PAPER Simplified expressions for adjusting higher-order turbulent statistics obtained from open path gas analyzers M. Detto · G. G. Katul Received: 12 December 2005 / Accepted: 26 June 2006 / Published online: 21 October 2006 © Springer Science+Business Media B.V. 2006 Abstract When density fluctuations of scalars such as CO 2 are measured with open- path gas analyzers, the measured vertical turbulent flux must be adjusted to take into account fluctuations induced by ‘external effects’ such as temperature and water vapour. These adjustments are needed to separate the effects of surface fluxes respon- sible for ‘natural’ fluctuations in CO 2 concentration from these external effects. Anal- ogous to vertical fluxes, simplified expressions for separating the ‘external effects’ from higher-order scalar density turbulence statistics are derived. The level of complexity in terms of input to these expressions are analogous to that of the Webb–Pearman– Leuning (WPL), and are shown to be consistent with the conservation of dry air. It is demonstrated that both higher-order turbulent moments such as the scalar vari- ances, the mixed velocity-scalar covariances, and the two-scalar covariance require significant adjustments due to ‘external effects’. The impact of these adjustments on the turbulent CO 2 spectra, probability density function, and dimensionless similarity functions derived from flux-variance relationships are also discussed. Keywords Carbon flux · Flux correction · Higher order statistics · Open path gas analyzer M. Detto (B ) Dipartimento di Ingegneria Idraulica, Ambientale e del Rilevamento, Politecnico di Milano, P.zza L. Da Vinci 32, Italy e-mail: matteo.detto@polimi.it G. G. Katul Nicholas School of the Environment and Earth Sciences, Box 90328, Duke University, Durham, NC, USA Department of Civil and Environmental Engineering, Pratt School of Engineering, Duke University, Durham, NC, USA