Attenuation of concentration fluctuations of water vapor and other trace gases in turbulent tube flow W. J. Massman1 and A. Ibrom2 1US Forest Service, Rocky Mountain Research Station, 240 West Prospect, Fort Collins, CO 80526, USA 2Bio Systems Department, Risø National Laboratory, DTU, Frederiksborgvej, 4000 Roskilde, Denmark
Abstract. Recent studies with closed-path eddy covariance (EC) systems have indicated
that the attenuation of fluctuations of water vapor concentration is
dependent upon ambient relative humidity, presumably due to
sorption/desorption of water molecules at the interior surface of the tube.
Previous studies of EC-related tube attenuation effects have either not
considered this issue at all or have only examined it superficially.
Nonetheless, the attenuation of water vapor fluctuations is clearly much
greater than might be expected from a passive tracer in turbulent tube flow.
This study reexamines the turbulent tube flow issue for both passive and
sorbing tracers with the intent of developing a physically-based
semi-empirical model that describes the attenuation associated with water
vapor fluctuations. Toward this end, we develop a new model of tube flow
dynamics (radial profiles of the turbulent diffusivity and tube airstream
velocity). We compare our new passive-tracer formulation with previous
formulations in a systematic and unified way in order to assess how sensitive
the passive-tracer results depend on fundamental modeling assumptions. We
extend the passive tracer model to the vapor sorption/desorption case by
formulating the model's wall boundary condition in terms of a
physically-based semi-empirical model of the sorption/desorption vapor
fluxes. Finally we synthesize all modeling and observational results into a
single analytical expression that captures the effects of the mean ambient
humidity and tube flow (Reynolds number) on tube attenuation.
Citation: Massman, W. J. and Ibrom, A.: Attenuation of concentration fluctuations of water vapor and other trace gases in turbulent tube flow, Atmos. Chem. Phys., 8, 6245-6259, doi:10.5194/acp-8-6245-2008, 2008.