  Conventional and recently developed approaches for estimating turbulent
scalar fluxes under stable conditions are evaluated. The focus is on methods
that do not require fast scalar sensors such as the relaxed eddy accumulation
(REA) approach, the disjunct eddy-covariance (DEC) approach, and a novel mixing
length parametrization labelled as A22. Using high-frequency measurements
collected from two contrasting sites (Utqiagvik, Alaska and Wendell, Idaho
"during winter"), it is shown that the REA and A22 models outperform the
conventional Monin-Obukhov Similarity Theory (MOST) utilized in Earth System
Models. With slow trace gas sensors used in disjunct eddy-covariance (DEC)
approaches and the more complex signal filtering associated with REA devices
(here simulated using filtered signals from fast-response sensors), A22
outperforms REA and DEC in predicting the observed unfiltered (total)
eddy-covariance (EC) fluxes. However, REA and DEC can still capture the
observed filtered EC fluxes computed with the filtered scalar signal. This
finding motivates the development of a correction, blending the REA and DEC
methods, for the underestimated net averaged fluxes to incorporate the effect
of sensor filtering. The only needed parameter for this correction is the mean
velocity at the instrument height, a surrogate of the advective timescale.
