Laserfiche WebLink
<br />"Dry ice is solid carbon dioxide, condensed from the atmosphere. <br />When it is released into clouds, it sublimates; that is, it <br />changes from the solid state back to a vapor in the atmosphere. <br />(Prior to sublimation, the dry ice will cool a portion of the <br />cloud sufficiently to freeze water driplets which, in turn, <br />can begin a growth process that may result in precipitation <br />reaching the ground.") 'Carbon dioxide is a natural constituent <br />of the atmosphere, present normally in a concentration of about <br />400 ppm by weight. Carbon dioxide is added to the atmosphere as <br />a result of the decomposition of vegetable matter and the <br />combustion of carbon-containing fuels. It is withdrawn from the <br />atmosphere by growing vegetation as part of the process of <br />photosynthesis of carbohydrates. Carbon dioxide is highly <br />water-soluble and it is exchanged constantly between the atmos- <br />phere and the oceans. Within the atmosphere, it moves freely in <br />response to large-scale processes of diffusion.' [1, p. B-13J <br />Thus, because the sublimation process simply returns carbon <br />dioxide vapor to the atmosphere from whence it came in compara- <br />tively insignificant amounts, it is environmentally benign as a <br />cloud seeding agent. <br /> <br />liThe programmatic Statement states the foll owi ng concerni ng <br />silver iodide release from ground-based generators: <br /> <br />'The American Conference of Governmental Industrial <br />Hygienists has set a threshold limit value for silver in <br />workroom air, under conditions of continuous exposure, at <br />0.01 mg/m3. According to a study made for the National <br />Institute of Environmental Health Sciences, atmospheric <br />silver emissions have yet to produce any reported ambient <br />air concentrations approaching this threshold limit value. <br />This is so despite the presence in many parts of the <br />country of industrial areas that emit more than a hundred <br />times as much silver into the atmosphere as a cloud-seeding <br />generator and do so almost continuously. For this reason <br />it is considered that the ground-based silver iodide smoke <br />generator does not constitute a health hazard provided <br />continuous exposure at a very short distance from the <br />generator is avoided.' [1, p. B-2J <br /> <br />liThe programmatic Environmental Statement continues: <br /> <br />'The pathways followed by silver iodide after it reaches <br />the ground are uncertain. Accumulations of silver iodide <br />from cloud seeding in soil, vegetation, and surface waters <br />have not been large enough to measure. It has not been <br />found in surface waters such as streams and lakes in the <br />same concentration found in newly fallen precipitation. It <br />is likely that some of the silver iodide is trapped in the <br />uppermost layers of the ground and that some of it is <br />carried by percolating water or other transport processes <br />to deeper layers of the soil or underwater muds where it is <br />sequestered from the biosphere. The part of it that is <br /> <br />22 <br />