Climatology of Sr-90 in Surface Air: A Simple Model using Diffusion and Scavenging

Abstract

The evolution of Sr-90 distribution following an instantaneous stratospheric or tropospheric source in an annulus defined by latitude walls is investigated by means of a very simple model that uses 12 stratospheric and 12 tropospheric annular boxes and which assumes diffusional transport between boxes and wet and dry scavenging from tropospheric boxes. Coefficients for interbox transport are expressed in terms of half-residence times, and these are adjusted to be consistent with reasonable eddy diffusivities and/or variable local stratospheric residence times suggested by previous studies. Precipitation scavenging is proportional to precipitation rate, for which a sinusoidal variation was assumed, with annual mean, amplitude of oscillation and phase all determined by climatology. The spring peak and fall minimum in tropospheric air are obtained at all latitudes. At middle and high latitudes, these extremes result primarily from annual oscillation of mixing downward from the stratosphere, while at low latitudes they result from the phase of the large annual oscillation of precipitation scavenging. The only reasonable way to obtain, in this model, the progressive delay of the spring maximum with latitude in middle latitudes is by a corresponding delay of the maximum rate of transport across the tropopause, a delay suggested by variations of stratospheric mass. For reasonable values of vertical and horizontal exchange coefficients and a megaton midlatitude source, half-residence times are obtained for the Northern Hemisphere stratosphere, total stratosphere, and Northern minus Southern Hemisphere burdens that agree with observation. Half-residence times for the northern and total stratosphere increase by about two months over a period of several years, as concentration is depleted in middle and high latitudes where transfer to the troposphere is rapid. Maximum concentration develops over the equator. Tropospheric debris from a tropospheric source is initially rapidly depleted by precipitation scavenging, and after a few months the tropospheric burden is small compared to the stratospheric debris acquired by initial upward diffusion. Thereafter, the stratosphere becomes the source, and both burdens slowly decrease at the rates given by stratospheric residence times.