High rural concentrations of ozone (O3) are thought to be either stratospheric in origin, advected from upwind urban sources, or photochemically generated locally as a result of natural trace gas emissions. Ozone is known to be transported vertically downward from the above-canopy atmospheric surface layer and destroyed within stomata or on other biological and mineral surfaces. However, here we report winter-time eddy correlation measurements of vertical O3 flux above a subalpine canopy of Picea engelmannii and Abies lasiocarpa in the Snowy Range Mountains of Wyoming that indicate anomalous upward O3 fluxes. Upward fluxes of 0.5 &mgr;g m-2 s-1 (11 kg km-2 day-1) were routinely measured during the 1991-92 winter season. Decreasing O3 concentration from several hours to several days that relate to increasing positive O3 flux magnitudes and visa versa, suggest O3 may be temporarily stored in the snow base.