fe — + @ ie) Fy J Sr® DEPOSITION, MC/SQ MILE 5 _ L. a tie ar hare PRECIP, iN. JEMAMSJASONDJFMAMSJASONOJFMAMIJASONOVFMAMSJJA SOND SE MAMI JASOND 4954 4955 1956 4957 Pe joao | i Fo | NEW YORK Fig. 5 — Sr” deposition, New York, 1954-1957. troposphere. It is believed, without much question, that the Be’ content of the lower stratosphere is much higher than the troposphere. This is due both to the greater production rate and the absence of precipitation scavenging in the stratosphere. The meteorological model of exchange between the stratosphere and the troposphere propounded by the authors, Brewer, Dobson, and others, is shown in Fig. 7 in the preceding paper. It indicates that air leaves the stratosphere only in the temperate or polar latitudes and not in the tropical or equatorial latitudes. It is this meteorological picture, with details too technical to treat here, which we feel explains both the peak of stratospheric fallout in the temperate latitudes and a seasonal variation in the fallout rate. 2 RESIDENCE TIME The hold-up time of Sr*® in the stratosphere is also a meteorological problem. Dr. Libby has estimated that the removal is at an exponential rate of about 10 per cent of the stratospheric content coming out each year. The meteorologist objects to the concept of a fixed percentage of the stratospheric content being removed each year; in his view the percentage depends upon the distribution in the stratosphere as well as on the total amount present. The meteorologist is only now ready to begin to treat the stratospheric-tropospheric exchange problem properly. By default, then, the exponential removal rate must be accepted as a first approximation. However, it will be argued that the amount of Sr®® removed up to the end of 1957 has been at an average annual rate of 20 per cent or higher rather than 10 per cent. In Fig. 7 (this paper) the ordinate shows the amountof Sr” in the stratosphere at the time given by the abscissa. The upward jogs are the injections by the United States and U.S.S.R. high-yield tests. The gently sloping straight lines show the decrease in stratospheric content computed at 20 per cent per year. Hence, the ordinate is on a logarithmic scale. In total, some 36 Mt have been added to the stratosphere up to the end of 1957. By the end of 1957, since 23 Mt are left, 13 Mt of the 36 Mt must have been deposited out, if the rate of removal is 20 per cent per year. It was seen in Fig. 5 that our estimate of the amount of stratospheric deposition to the same date was also roughly 13 Mt. Thus this evidence, the 333 “aS.