Wintertime Ozone Fluxes and Profiles above a Subalpine Spruce–Fir Forest

High rural concentrations of ozone (O3) are thought to be stratospheric in origin, advected from upwind urban sources, or photochemically generated locally by 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 the authors report midwinter eddy correlation measurements of upward vertical O3 flux of 0.2 mg m22 s21 (5.6 kg km22 day21) above a subalpine canopy of Picea engelmannii and Abies lasiocarpa in the Snowy Range Mountains of Wyoming. Simultaneous below-canopy upward fluxes reached 0.1 mg m22 s21. These results corroborate similar late winter (presnowmelt) upward O3 fluxes of 0.5 mg m22 s21 (19 kg km22 day21) taken at the same site in 1992. Profile results show sustained ‘‘countergradient’’ fluxes below the canopy and sustained ‘‘with gradient’’ fluxes above the canopy. Ozone concentrations that decrease for several hours to several days correspond to simultaneously increasing positive (upward) O3 fluxes and vice versa. These phenomena, in addition to aboveand below-canopy reversed gradient patterns, suggest that O3 may be stored temporarily in either the snow base or the tree stand itself.