128 Health Physics The gummed film data were occasionally not consistent with other types of measurements Someof the difficulties m interpreting the gummed film data were the lack of mformation on the exact date of counting and any counting efficiency corrections that might have been madeto the origmal data Because much of the fallout at the sites of the gummed film stations was associated with very heavy precipitation, the abihty of gummed film to retam fallout particles was likely to have been very low at times These various factors resulted m high uncertamty im our interpretations of the gummed film data, particularly for the purpose of making quantitative mterpretations Despite various limitations, the gummed film data were good indicators of the specific calendar days on whichfallout occurred at the samplingsites and unequivocally demonstrated the contmuation of fallout for several days after a mayor test The gummed film data and the HASL automatic gamma-ray momtors were both imdicatve that the HASL air surveys were often conducted prior to the end offallout deposition and, occasionally, even conducted prior to the arrival of fallout Thus, some earher reported deposition estimates based only on the air survey data were too low,particularly in the southem Marshall Islands where muchofthe fallout occurred several days after the detonations Use of an atmospheric transport model Becausethere are no knownfallout momtormg data for the Marshall Islands prior to the 1952 Ivy series (except at the test site atolls), it was necessary to use a meteorological model and archival meteorological data to estimate fallout deposition at the atolls for tests August 2010, Volume 99, Number 2 Modeling the transport and deposition of particles released from a nuclear weapons test 1s a complex and highly uncertam exercise, even if perfect mformation on the spatial variations im wind speeds and directions over the entire region 1s available The actual activity-particle size distribution im a nuclear debris cloud varies with the particular type and yield of the tests, the height of the burst and the local topography Nodataof these types are available for the Pacific tests Thus, the amountof °7Cs attached to particles of a particular size released from a particular altitude could only crudely be estrmated for the tests of mterest based on limited data from Nevadatests In order to relate the geographic pattern of deposition and number of deposited particles estimated by HYSPLIT to the ’Cs deposition as a function of yield, debris cloud size, and altitude, we developed a crude model to describe the relative numbers of particles released from the cap of the stabihzed mushroom cloud at various altitudes as well as from the stem (Morozet al 2010) An actvity-particle size distribution was also estimated based on data of beta activity as a function of particle size from measurements followmg NTS tests, modified slightly to reflect the fact that '*’Cs tends to be depleted on the larger particles due to a predisposition for the larger particles to be deposited prior to the formation of ‘Cs from its gaseous precursor, °’Xe The total amount of ‘Cs released was estimated from the esti- mated fission yield of each test (UNSCEAR 2000, Hicks 1981, 1982) Note, however, that even the fission yields for US TN tests are only an estimate (UNSCEAR 2000), since those data are still classified Trajectory Comparisons were made of the geographic deposition patterns predicted by HYSPLIT with historical groundoraircraft radiological momtorimg data Based on comparisons for tests where momitoring data were available, 1t appears that when HYSPLITpredicted fallout in 2007), was developed and 1s maintamed by the National mated deposition generally agrees with measured '*’Cs to under certam conditions, can be apphed to estrmating dispersion of fallout from nuclear testmg Our application of the HYSPLIT model for estrmatmg Marshall Islands fallout 1s discussed m a compamion paper m this meteorological model could be used to only provide crude estates of fallout where no measurements were made A more rehable use for HYSPLIT was to support our mterpolations of deposition at atolls from real measurements of deposition at nearby atolls Although the HYSPLIT model estimates are very uncertam, m somecases they were the only source of any imformation on fallout deposition and, m general, supported numerous anecdotal reports of substantial fallout prior to 1952 at someatolls, m particular at Ujelang from two 1951 tests, Dog and Item The HYSPLITpredictions werealso used to complete the fallout deposition pattern for 1956 and 1958 tests for which only a limited number ofatolls were monitored for radioactive debris The 1956 conducted m 1946, 1948, and 1951 here, Hybrid Smgle-Particle The model used Integrated (HYSPLIT) (Draxler and Hess 1997, Draxler et al Oceanic and Atmosphenc Admmustration (NOAA) and issue (Moroz et al 2010) The HYSPLIT code models the fallout deposition downwind from available meteorological data at the timeofthe test, usmg reanalysis data sets of historical meteorological measurements extrapolated to a sparse grid The model simulates the transport and deposition ofparticles of different sizes onigmating at different altitudes at the location of the test The HYSPLIT model does not simulate the weapons debris cloud itself or the radioactivity associated with particles of a given size the areas where fallout was actually observed, the estwithm a factor of 10 (Moroz et al 2010) Thus, the