The data for all four locations show generally the same concentration levels at any given time and all show a gradual and substantial increase during 1954. The concentrations observed at Kodiak, Alaska during and following the Spring 1952 tests are strikingly low by comparison with the 195) and 1955 concentration data. The average Sr90 activity in 106 cubic feet of surface air appears to have increased from a pre-Ivy test level of about dpm to a pre- Castle level of some LO dpm and a post-Castle level of some 200 dpm. Comparison of the Sr#0 air concentration data with the daily total fission product beta activity data obtained by the NRL group indicates "apparent" ages for the mixed fission products of from one month to several years with wide variations during any given month. No conclusions with respect to the Sr production date may be drawn from such data for a number of reasonse The size distribution of the original bomb debris is dependent on the energy yield, orientation and environment of the weapon and for each case the mean size of particulates carrying Sr90 is expected to be smaller than that of mixed fission products. Furthermore, the particulates may be further frac- tionated by the action of rains, depending on the efficiency of scavenging by rains as a function of particle size. Another complication is that imposed by the size-collection efficiency of the device itself . Additional diffi-e culties are imposed by the close spacing of tests during the last several years, Finally, there are large day to day variations in surface concentra- tion of Sr90 and other fission products, apparently due to scavenging of surface air by vegetation and to the time interval between rains, In spite of the many complicating factors indicated above which limit the meaninsfulness of any individual measurement of Sr90 concentration and its relation to total fission product activity or to other individual fission product concentration, the general features of the Sr?0 surface air concentration data shed considerable light on atmospheric circulation and storage