HR: 08:00h
AN: A31B-01 INVITED     [PDF]
TI: Chemistry of Cosmogenic Radioisotopes in the Stratosphere
AU: * Rowland, F S
EM: rowland@uci.edu
AF: Donald Bren Research Professor, Department of Chemistry University of California, Irvine, Irvine, CA 92697-2025 United States
AB: Bombardment of Earth's atmosphere by Gev cosmic ray protons produces two neutrons/cm$^{2}$/second, which react with $^{14}$N nuclei to form $^{14}$C. The initial stratospheric reaction of the bare $^{14}$C atom with O$_{2}$ forms $^{14}$CO, which is subsequently oxidized to $^{14}$CO$_{2}$ by HO. The $^{14}$CO$_{2}$ is then the basis for age-dating studies of inorganic carbon, and of organic carbon after incorporation by photosynthesis. Proton spallation of light nuclei (chiefly $^{14}$N, $^{16}$O) produces various isotopic fragments, many of them radioactive, containing 8 or fewer protons and 10 or fewer neutrons. For geophysical studies, the most important of the radioisotopes so produced are $^{3}$H, $^{7}$Be, and $^{10}$Be. Proton spallation of $^{40}$Ar produces many isotopes with atomic weight less than 40, including $^{34}$Cl, $^{36}$Cl, $^{38}$Cl, $^{39}$Cl, $^{35}$S, $^{38}$S, $^{22}$Na, $^{24}$Na, $^{32}$Si, $^{32}$P, and $^{33}$P, all of which have been detected in the atmosphere. The "million year" isotopes ($^{10}$Be, $^{36}$Cl) are useful for geological dating. The short-lived isotopes were collected both from rainwater captured at ground level, and on filter paper at 18 kilometers altitude carried by an RB-57 aircraft ({\bf1970}, J. A. Young, C. W. Thomas, N. A. Wogman and R. W. Perkins, {\it JGR}, 75, 2385). The radioisotopes were measured with multidimensional gamma ray spectroscopy, which was able to detect $^{24}$Na and $^{38}$Cl without prior chemical separation. Count rates as low as 0.1 count/minute were monitored, even in the presence of 10$^{7}$ counts/minute of fallout fission products from nuclear testing in the atmosphere. This fallout background is now greatly reduced because of the four decades old ban on nuclear testing in the atmosphere. The stratospheric collection of $^{24}$Na (15 hour half-life) was interpreted as scavenging of the radiosodium by particulate matter, and retention on filter paper with an efficiency of 100 percent within the statistical accuracy in comparison with its production in argon tanks carried on the aircraft. The efficiency of collection of radiochlorine atoms was only about 1/3, which was suggested as incomplete transfer to particles in the much shorter time available before radioactive decay. The chemistry of chlorine atoms such as $^{38}$Cl (37 minute half-life) and $^{39}$Cl (55 minutes) is probably still homogeneous gas phase in nature. Only small fractions might have reached H$^{38}$Cl or H$^{39}$Cl in the time available since nuclear creation of these atoms. Because cosmic ray intensity does not vary diurnally, the atmospheric chemistry of atomic radiochlorine atoms could be investigated under the very different free radical conditions found during night-time.
DE: 0317 Chemical kinetic and photochemical properties
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting