Modification of the relaxed eddy accumulation technique to maximize measured scalar mixing ratio differences in updrafts and downdrafts

A modification to the relaxed eddy accumulation (REA) flux measurement tech nique is proposed which maximizes the scalar mixing ratio difference in upd rafts and downdrafts. This technique was developed with the goal of measuri ng the stable isotope (C-13/C-12 and O-18/O-16) ratios of updraft and downd raft air and thus the net fluxes of (OO2)-O-13-O-16 and (COO)-C-12-O-18-O-1 6. Current mass spectrometer precision is small relative to measured isotop ic gradients in CO, in the Earth's boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampl ing concept of hyperbolic hole analysis to control sampling of air during o nly those turbulent events which contribute most strongly to the flux. Inst ead of basing updraft/downdraft sampling decisions strictly on vertical win d velocity, CO2 mixing ratio ([CO2]) fluctuations or those of another scala r are also used. Simulations using 10-Hz data show that a wind-based/scalar -based sampling threshold can achieve a factor of 2.7 increase in scalar up draft/downdraft [CO2] differences over simple REA. During midday periods wi th strong photosynthetic fluxes, up/down [CO2] differences with HREA of 8-1 0 ppm are possible, compared with 3-5 ppm for the best conventional REA cas e. Corresponding isotopic differences can likely be resolved with current m ass spectrometers using this approach.