Vertical profiles and transport of HTO in the troposphere

Water-vapor samples were collected from the surface to 9.2-km altitude over Scottsbluff, Nebraska from November 1965 to January 1967 and over the eastern Pacific and Death Valley from July 1966 to August 1967. During spring, the resulting profiles of the T/H ratio generally show an increase with altitude. However the mixing ratio of tritiated water vapor decreases with altitude, reaches a minimum at 7 km and increases again above that level. The shape of the mixing ratio profile is shown to be a product of the vertical transport of HTO by precipitation and eddy diffusion and two source regions of HTO vapor: the tropopause and the continental surface. The profile is further modified by two processes. Below the freezing level at around 3 km, ice crystals falling from above inject their high T content into the environmental water vapor which acts as a source of HTO vapor. Below 2 km, horizontal advection of oceanic air with lower T content plays a role. The profiles show also that in 1966 the influence of HTO re-evaporated from the continental surface is limited to altitudes below 2 km. The re-evaporated HTO nevertheless considerably modifies the seasonal variation of the T content in rain even over the oceans. The theoretical shape of the steadystate profile above the freezing level is calculated and fitted to the experimental data for the mixing ratio. The resulting vertical profile for the average mixing ratio in spring 1966 is 〈M〉 = 4.4 · 106 · exp [(z - 3.37)/1.91] + 1.47 · 108 · exp [(z - 3.37)/2.42] [HTO molecules/kg air] where z is the altitude above ground in kilometers.