Environ. Sci. Technol. 1991,25,730-735
Consideration of the Chemistry of Radon Progeny A.
W. Castleman, Jr.
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802
Although the early-time chemistry and physics of radon progeny are far from being completely understood, sufficient quantitative knowledge is now available from laboratory experiments to interpret certain observations and predict their behavior and fate. Here, attention is given to the two progeny, lead and bismuth. A significant fraction of the progeny are expected to exist in the singly charged state. Any initial association with an O2molecule from the air will be rapidly replaced by polar molecules and thereby become rapidly hydrated with between five and eight molecules of water, depending on temperature and humidity. Substitution of a water molecule by an ammonia, alcohol, or perhaps other organic molecule is possible. Collisions of these cluster ion complexes with aerosol particles offer mechanisms of charge exchange, and incorporation with the particles. Failing interaction with preexisting aerosols, the clusters will remain in the singly charged state during growth and transport. The ultimate fate is then expected to be charge neutralization via recombination with cluster anions in the air, most probably NO,-(H20),.
I . Introduction Radionuclides in the atmosphere are often found associated with aerosols. This fact became evident in early studies of the widespread and persistent nature of radioactivity produced in weapons tests in the 1950s when it was also found that radioactivity from natural sources underwent similar attachment behavior (1-7). These findings raised questions about the possible impact of radioactivity-bearing aerosols on both health and ecology (8, 9). An understanding of the attachment of radionuclides to aerosols is needed not only in predicting their airborne residence time per se, but also in understanding pathways leading to the introduction of radon progeny into biological systems and the environment, including their potential of imparting physical damage or causing chemical effects through the concomitant radiological processes. Although the chemistry and physics of radon progeny are far from being completely understood, quantitative knowledge of their chemical behavior is becoming available through a number of different laboratory investigations (10-15). It is well-known that a large fraction of the progeny ions are initially charged, and a wide distribution has been reported (16) for the LY decay of 2zoRnto 21sPo, ranging from +9 to -2. In view of the large recombination energy of the highly charged progeny cation ions compared to the ionization potential of common atmospheric molecules, it is generally assumed that charge-transfer processes will rapidly lead to the singly charged state. Porstendorfer and Mercer (17) and Hopke and co-workers (10) have reported as much as 88% of the polonium atoms have a +1 charge at the end of a recoil path, and various other charged fractions have been reported for other progeny (18).
Our own research has focused on the chemistry of the ionized portion of the progeny, with emphasis on ascertaining their thermochemical and kinetic properties in terms of interactions with common atmospheric molecules and indoor pollutants. The attention has been on fundamental thermochemical information that will provide 730
Environ. Sci. Technol., Vol. 25, No. 4, 1991
a basis for modeling studies that are requisite in obtaining a complete picture of growth, attachment to aerosols, possible transport to a bioreceptor, and incorporation within. It is well recognized that reactions of the progeny ions that lead to a change in their physical or chemical states can influence their mobility, transport, and interaction with airborne molecules and aerosols. The important radon progeny ions are those comprising Po, Bi, Pb, and T1; their respective ionization potentials are 8.34, 7.29, 7.42, and 6.11 eV (19). Since there are very few atmospheric constituents that have such low ionization potentials (20), it is unlikely that the singly charged state of these progeny ions can undergo further charge-transfer processes except with ions of an opposite sign, with aerosols particles, or via a reactive association process. All of these ions have a 5d106s2electronic configuration, with lead having a further 6p', bismuth a 6p2, and polonium a 6p3 ground-state configuration. All but polonium, which has the highest ionization potential, are certain to remain in the singly charged (and probably ground electronic) state and it is expected that clustering with polar molecules will be the dominant mechanism in their early-time chemistry. Clustering affects growth, influences mobility, and hence plays a role in the overall attachment process to preexisting aerosol particles or possibly in other transformation (nucleation) mechanisms to the particulate state. Their chemical and physical form, a clustered state in particular, influences further interactions which, in turn, govern the transport and removal processes. Furthermore, their chemical form may also influence biological uptake. If the progeny are bound to molecules with extended polar groups, or alternatively with nonpolar heads extending outward, this could dramatically affect their attachment to lung tissues and ultimate incorporation within cells. These factors are presently unknown but should be considered in the future. The main thrust of the present paper is a consideration of the individual reactions that are possible in terms of their thermochemistry, how fast these reactions are likely to proceed, what ligand exchange switching reactions can take place based on their energetics and kinetics, and, finally, what is likely to be the ultimate chemical/physical/charged state of the progeny. Only Pb+ and Bi+ are considered herein.
II. Consideration of the Initial Chemistry of the Progeny Ions A. Thermochemistry of Reactions with 0 2 .It is informative to ascertain whether the progeny ions Pb+ and Bi+ can undergo chemical interaction with the oxygen molecule in air. More quantitative data are available for assessing the lead rather than the bismuth system, although conclusions are unlikely to differ appreciably. Table I shows the sequence of reaction steps that may be combined to evaluate the thermochemistry of the possible reaction Pb+ + 02
-
PbO+ + 0
(1)
Summing up the thermochemical values for the individual reaction steps shows the reaction 1is endothermic by more
0013-936X/91/0925-0730$02.50/0
0 1991 American Chemical Society
Table 11. Thermodynamic Values for the Gas-Phase Reactions of the Radon Progeny Ion Pbtd
Table I. Consideration of the Initial Chemistry" of Pbt
AHo reaction
kcal/mol
Pb(g) + ' / 2 0 2 PbO(g) '/202 0 Pb(g) + O2 PbO(g) + 0 Pb+ + e Pb(g) PbO(g) PbO+ + e Pbt + 02 PbO+ + 0
-35.27 +59.56 +24.29 -171.04 +209.40 +62.65
-
-+
---
-+
-
Pb+(M), + M = Pbt(M),tl (M = H20,NH,, CH,OH)
eV
2.72
P'+M
AH:
O J
22.4 16.9
-AHon,ntl, kcal/mol CH30Hc NH3b (27) 19.2 13.0 10.7
12.2
10.8 10.0 9.6 (9.4) (9.2)
23.3 17.2 (12.5)
"Data taken from ref 23. *Data taken from ref 25. CDatataken from ref 27. dThe values in parentheses are estimated.
-__ ReacdAssociate with a Molecule in the Air?
m
H,Oa
132 23 394 495 5,6 67 7,8
"Data taken from refs 19 and 21. Note: (9) designates gasphase atom or molecule.
_ Will_ the _Singlv Charved
n,n+l
' (PM)Q
+
Table 111. Thermodynamic Values for the Gas-Phase Reactions of the Radon Progeny Ion Bit
Bit(M),
AH: = (IT -I?) +
A$
n,n+l
(neg)
091 12
Figure 1. Thermochemical cycle used to evaluate the energetics of reaction or association of a molecule in the air, M, with a radon progeny ion, P+. For an appreciable reaction to proceed, A H o , must be a negative quantity.
than 2.7 eV. Ion reactions that are more than slightly endothermic do not proceed, and no formation of PbO+ is expected from the reaction of Pb+ with 02. The cycle shown in Figure 1 enables a consideration of the energetics of the formation of PO2+,where P+ represents any one of the progeny ions under consideration, i.e., P b or Bi and M, any molecule such as O2 in this case. Again, in terms of the exothermicity criterion, it is expected that the larger the value of IP2compared to IP1, the less likely a reaction. The terms IP1,and IP2denote the ionization potentials of P and PM, respectively. On the basis of the limited thermochemical data available in the literature (19,21,22), h H o I is determined to be about -17.5 kcal/mol (-0.76 eV), showing that an interaction is possible. Nevertheless, as discussed in a later section, we have experimentally established that this is a very slow threebody association reaction. Furthermore, in view of the enhanced bonding of Pb+ [and any other simple ion, for that manner (22)] to molecules that not only have large polarizabilities, but also appreciable dipole moments, it is expected that even if Pb+.02 is formed, rapid ligandexchange reactions will take place that lead to the loss of 0, from the cluster via substitution of another molecule. A further consideration of the cycles shown in Figure 1 reveals the situation for the case of M = H 2 0 or other atmospherically important molecules. Indeed, by use of the thermochemical data shown in Table 11, along with the cycle outlined, it is readily calculated that water, ammonia, and alcohol would displace O2 from the cluster. In view of its high concentration, clustering with water would probably dominate the sequence. This can be deduced from the following reactions Pb+(or B P ) + O2 s Pb+.02 (2) Pb+.02 + H20 s Pb+*H20+ 0
2
(3)
2,3 394 4,5 56
+ M = Bi+(M),+I (M = H,O, NH3) -AHon,ntl,kcaljmol "3" H,Ob 35.5 23.2 13.4
22.8
17.7 14.0 12.0 10.5 9.7
"Data taken from ref 26. bData taken from ref 14.
Where the thermodynamic reference state is chosen as 1 atm, and employment of the appropriate approximation of partial pressures, pi,related to the standard state and used in place of activities, the concentration ratio of any of the above species can be determined from standard thermodynamic considerations. For example, in the case of reaction 3, the ratio of the concentrations, [i], can give
where AGO, is the Gibbs free energy for reaction 3 at temperature T , the p's designate partial pressures, and the brackets denote concentrations. Since the entropy terms will not be greatly different, the exponential term will be dominated by the enthalpy change for the reaction and the result will be a value in excess of 3 X lo3. Hence, this will more than offset the ratio of the partial pressures of water and 02,which are expected to be less than 0.01. When the data in Table I11 for Bi+ associated with water are considered, similar behavior is expected for this progeny ion, too, and clustering with water will dominate over that with 02.However, in view of the somewhat stronger enthalpy of clustering of NH3 with Bi+ compared to Pb+, in an atmosphere containing appreciable concentrations, some attachment or incorporation of ammonia may occur in the case of Bi+. The equilibrium constant for replacement of water by ammonia clustered to the ion would be lo9. This only requires ammonia concentrations in the order of 0.2 ppb to yield a concentration of ion clusters containing ammonia to be comparable to those containing only water. B. Consideration of the Cluster Distributions of Radon Progeny Ions. At the present time there are insufficient data to make a complete analysis of the compositional distributions likely for the radon progeny ions. The primary information lacking is that for cluster systems
-
Environ. Sci. Technol., Vol. 25, No. 4, 1991
731
0 1
-
-
1
PARTIAL PRESSLRE OF I\ ATER TORR
1 PARTIAL PRESCURE OF \YATER, TORR
3 3
10
10-4
10-3
10-2
IO-'
10-2
10-1
PARTIAL PRESSURE Of'.AhihlOVIA, T O K R
1
10
- c
-
E I
z
-
I I
-s a-
c - v -I = -1 v
-=
C I
1
PAR'IAL PPES--RE CF \'- i T E R TORR
10
Flgure 2. Predicted distributions for clustering about the radon progeny ions as a function of the partial pressure of the clustering water molecule (a) Pb' at 300 K (Data taken from ref 23); (b) distribution calculated at 250 K ( w - 9 O F ) from thermodynamic measurements made in our laboratory; (c) Bi' at 300 K, calculated from thermodynamic measurements made in our laboratory
of mixed composition. Nevertheless, with present knowledge it is possible to gain some insight into the degree to which clustering can affect size (collision cross section) and mobility for the case of individual atmospheric molecules interacting with the ions. It is instructive to first consider the influence of water vapor concentration and temperature on the behavior of the lead and bismuth systems (23,241. Parts a-c of Figure 2 show the probable extent to which Pb+ and Bi+ will become hydrated in air. It is obvious that under normal (ambident home) conditions of temperature and humidity, the average lead ion will be hydrated with more than five water molecules, six dominating the situation for high humidities. Referring to Figure 2b, it can be readily seen that at colder temperatures (example corresponds to --9 O F ) the hydration number can easily be shifted to higher values. The situation is not expected to be greatly different from Bi+, as can be seen from Figure 2c. 732
Environ. Sci. Technol., Vol. 25, No. 4, 1991
10-3
10'
P4RTIALPKESSLRE OF i I E T H l h O L 1 0 R l i
Flgure 3. Predicted distributions for clustering about the radon progeny ions as a function of partial pressure of the clustering molecule. (a) NH about Pb+ at 300 K; (b) NH, about Bi' at 300 K; (c) CH,OH about Pb' at 300 K.
Ammonia and methyl alcohol are two other molecules that we have studied (25-27) and found to bind strongly to the radon progeny ions. Appropriate plots of the distributions are shown in Figure 3a-c. Despite the strong bonding, at the low partial pressures expected for ammonia and alcohols in air (perhaps a few ppm, corresponding to Torr), only a few ligands of these would be expected to be bound to the ions in the absence of water. Although no quantitative data are yet available for the mixed systems, it is likely that hydration will dominate; but, perhaps one molecule of alcohol and/or ammonia will replace a molecule of hydration.
-
III. Consideration of the Ion Reaction Kinetics There are a number of different kinetic pathways that need to be considered in order to fully account for the chemistry of the radon progeny ions. As discussed in the prior section dealing with clustering complexes, association is most likely to be the dominant process in the case of Pb+
and Bi+. We have found that atom substituent abstraction is possible upon the interaction of certain metal ions of high ionization potential with organic molecules, e.g., Cu+ with amines (28). Our considerations show that this is unlikely to be an important mechanism for the case of lead ions, but in the case of bismuth, this has been found to be a possible pathway that requires further consideration. Charge transfer with trace contaminants contained in air is another possible mechanism but, for the reasons discussed earlier, is unlikely to be important for the two ions under consideration. A final mechanism that needs to be considered is that of ion-ion recombination with attention to the cluster systems, and we discuss this possibility in a later section. In order to elucidate the kinetics, one needs to consider the three-body reactions that can lead to association processes.
P+ + M Z(P+*M)* kl
k-1
(7)
Here, k l and k2 represent the rate constants for the (forward) collision, k, represents the reverse rate constant, and B designates a third-body (air) molecule, which can assist in the formation of a stable cluster through the removal of energy upon collision. The overall reaction can be expressed by
k(3)
P+ + M + B P+-M + B (9) where the conventional Lindemann mechanism (29) leads to
The overall conversion rate of a bare ion to the clustered state can be determined by solving eq 9, and employing eq 10 for the overall rate coefficient. Rate data can be most readily determined for k@)at low and moderate pressures, as we have done for reactions of Pb+ and Bi+ (12), and other ions as well (30). The fractional amount of conversion, f i , for a given ion becomes = e-kJ3'[Bl[M1t
(11) where t is time. The product ko(3)[B]increases and then levels off with pressure, ultimately approaching a finite maximum value. A t low pressures the overall rate coefficient approaches fi
k0(3)= klkz/k-, (12) a value that can be measured experimentally. In the atmosphere and for the ions and molecules under consideration here, it is likely that the reactions are occurring in the transition pressure regime. But, a consideration of the low-pressure rate constant relationships gives insight into the processes that are likely to be impeded through slow kinetics. Most importantly, the use of the lowpressure limiting values given by eq 12 is especially valuable since employing them in eq 11 leads to a deduction of the maximum conversion of an ion to a cluster complex. We have investigated a number of association reactions and found that the three-body association rates differ very little for the two ions under consideration, but vary dramatically with the nature of the clustering ligand. This is seen by referring to the data listed in Table IV, which show that the rate constants span over nearly 3 orders of magnitude with ammonia displaying low rates and monomethylamine having a high rate of association. In terms
Table IV. Measured Association Rate Coefficientsn for Pb+ and Bi'
rate coeff k q , cm6 s-l Pb+ Bi+
ligand 0 2 "3
CHsC1 CH30H CHzSH CH3NH2
53.2 x 10-31 1.6 x 10-29
8.1 x 6.1 X 1.6 x 21.1 x
2.0 x
10-29
10-29
5.8 X 10-27 10-27
2.1 x 10-27 21.2 x 10-26
"Data taken from ref 12.
of generalities, which may show exceptions, it is found that the larger or the more complex the clustering molecule, the higher the rate constant for association. This generalization is in accord with the finding of a very slow rate of attachment of O2 to the ions. It is also found that the greater the bond energy, the faster the association; this is due to the dependence of the bond energy on the lifetime of the complex (29) where a smaller value of k-l is indicative of a long lifetime against unimolecular dissociation. In general, termolecular clustering will be more rapid for molecules having large dipole moments and large polarizabilities. Switching reactions are also found to proceed facilely, and extensive investigations of these have been made in our laboratory, with particular attention to Na+ associated with several ligands, including water, ammonia, and methanol switching with CH,CN, CH3COCH3,CH,CHO, CH,COOH, CH,COOCH,, NH,, CH,OH, and CH30C2H4OCH, (31). We have ascertained that the rates of these ligand-exchange reactions can be accurately calculated from the method of Su and Chesnavich (32) for those cases where the reaction is exothermic. Thermochemical information on mixed-cluster systems is generally required in deducing the energetics. But, lacking these data, it is generally reasonable to assume that molecules that are more polar will replace (switch out) less polar ones for systems where the polarizabilities are not much different. For molecules of similar dipole moments, polarizability will dominate. Of course, interactions between the ligands themselves can also facilitate the exchange process, e.g., hydrogen bonding between the ligands.
IV. Consideration of Ion Charge Exchange with Aerosol Particles It is generally found that aerosol undergo charging and ultimately display a Boltzmann-type distribution of charge (33):
N* - = 2e-p2q2/2rkT
NO Here, N designates the number density of the charged (k) and uncharged (0)aerosols, p the number of charges of unit charge q, and r the radius of the particle, and k and T designate the Boltzmann constant and temperature, respectively. Due to the charged nature of the aerosols, ion clusters readily interact with them. It is possible to write the time dependence of an ion population as follows: dn, = F dt where lo-' Icy Ilo4. Here, cy represents an appropriate coefficient for ion-ion recombination and p an appropriate coefficient (approximately related to a geometrical cross section) for the ions interaction and ultimate charge exchange with particles of number density, N. The symbol Environ. Sci. Technol., Vol. 25, No. 4, 1991
733
Table V. Considering the Ultimate Fate of Radon Progeny Ions/Ion Cluster-Ion Cluster Recombination Energetics”
n(H20)
NOB-(H@), (electron affinity), eV
Pb+(H,O), (ionization potential), eV
0 1
3.90 4.53
2
5.15
crossing
7.42 6.54 5.72
3
5.75
c,
5.19
4
point
5
6
4.72 4.29 3.87
Data taken from ref 22.
F designates the rate of ion (or cluster) formation and n the number concentration of the ions. On the basis of considerations of the ranges of numerical values for a and P, it is expected that the ratio of the extent of ion-ion recombination compared to aerosol loss will be about unity in clean air, and much less than 1 under most tropospheric conditions where there is a high concentration of preexisting aerosol particles. As discussed earlier, for these ions of low ionization potential and reactivity, one may expect them to remain in a singly charged state until encountering an aerosol particle. It is striking to compare recent findings of radon progeny size distributions measured with newly developed sophisticated instrumentation (34). Typically, distributions peaking around 1-1.2 nm are detected. We have carefully investigated the reactions of Pb+ and Bi+ with O2and HzO and, as discussed above, have found that even if a transient radon progeny oxide does exist, the oxygen will be rapdily replaced by the polar H 2 0 molecule. Subsequent rapid hydration will occur, the extent of which depends on temperature and humidity as indicated in section 11. On the basis of thermodynamic measurements reported therein, it would be expected that the radon progeny ions would hydrate in very short times. Between five and eight water molecules can be taken up, with a more likely peaking of the cluster distribution occurring around six water molecules. The sizes estimated for these clusters are in accord with the experimental observations of the progeny species having a diameter of -1 nm in room air.
V. Mechanisms of Nucleation and Ultimate Fate of t h e Charged Species It is interesting to speculate on the ultimate fate of the radon progeny clusters that do not encounter an aerosol particle. Homogeneous nucleation is not of atmospheric importance and the heterogeneous (interaction with aerosol particles) processes will dominate (35). The interactions of the charged small clusters with other clusters such as sulfuric acid-water clusters, which may be present under SOz oxidation conditions at high humidities, can also lead to a heteromolecular growth process and ultimately incorporation of the progeny with a growing aerosol particle. Ion-induced nucleation is another possibility, but is unlikely to be a major mechanism under the concentrations expected under indoor radon levels (35). Hence, we are left with a consideration of the ion cluster-ion recombination process (36, 37) referred to in an earlier section. Ferguson (38) has considered the evolving cluster distributions for atmospheric ions. It is well-known that cosmic ray processes dominate the ionization at high altitudes, while ionization due to a mixture of cosmic rays and the secondary effects of radon progeny influence ionization near ground level ( 3 ) . Nevertheless, in all regions of the atmosphere there are between a few hundred 734
Environ. Sci. Technol., Vol. 25, No. 4, 1991
and a thousand ion pairs per cubic centimeter (7) and the considerations of Ferguson and co-workers show that protonated water, ammonia, and alcohol clusters will dominate the positive ion spectrum while NO3-, possibly bound to HN03, or perhaps HS04- bound with water and nitric acid, will be the dominate negative ion species. Since Coulombic repulsion will prevent an interaction between hydrated radon progeny positive ions and other positive ion clusters, the dominant mechanism, if ion cluster-ion recombination does take place, is expected to be with the NO3- hydrates. It is interesting to speculate on the possible neutralization products and the possibility of electrolytic condensation nuclei being formed. The relevant thermochemical data are shown in Table V. The first column in the table indicates the degree of total hydration, while the second column lists the electron affinity of the neutral species leading to the complex NO,-(H,O),. The third column lists the ionization potential of the neutral complex Pb+(H20), or alternatively the recombination energy for the charged species. The point under consideration is that the electron affinity of the nitrate increases with hydration, while the recombination energy of the radon progeny ion decreases. It is readily apparent that at a value of n = 3, that is, three waters bound to NO, and three to Pb+, there is a crossing (based on the energetics of the infinitely separated charges). At this point the two species could become bound into one “droplet” by Coulombic attraction, but an electron would not be transferred from the negative to the positive species. Of course, proper account must be taken of the energetics associated with bringing the charges together from infinite separation, and this would add a few waters of hydration to the considerations shown. Nevertheless, it is possible, and it is interesting to speculate that a small electrolytic “droplet” could be formed by this ion-ion recombination process that could further enhance the growth processes. Such processes would also lead to the “apparent” neutralization of the progeny ions. This is a very plausible explanation for the rapid loss of charge of radon progeny ions where no viable charge-transfer mechanism has been proposed to account for neutralization by other direct collisional charge-transfer processes.
VI. Conclusions The availability of some thermochemical and reaction rate data now enable an evaluation of the likely mechanisms leading to the chemical and physical transformations of the radon progeny ions following their production. Bi+ and Pb+ are likely to undergo clustering with water vapor and other atmospheric constituents such as ammonia and alcohols (and possibly other organics). Hydration with about five to eight molecules is likely, with the incorporation of a molecule of ammonia and perhaps an organic one being very probable. Collisions of these cluster ion complexes with aerosol particles offer a mechanism of charge exchange, and incorporation with the particles. Failing interaction with preexisting aerosols, the clusters will remain in the singly charged state during growth and transport. The ultimate fate is then expected to be charge neutralization via recombination with cluster anions in the air. Acknowledgments I thank S. W. Sigsworth and B. C. Guo for helpful discussions during the course of this work. Much of the data on which this review is based comes from their data as referenced in bibliography. Registry No. Rn, 10043-92-2;NH,, 7664-41-7;CH,OH, 67-56-1.
Literature Cited Reiter, E. A. Atmospheric Transport Processes. Part 2: Chemical Tracers; TID-25314; U. S. Atomic Energy Commission, USAEC Division of Technical Information Extension: Washington, DC, 1971. Israel, H.; Dolezalek, H. Atmospheric Electricity, Fields, Charges, Currents; Israel Program for Scientific Translations, Ltd.: Jerusalem, Israel, 1973; Vol. 11. Bricard, J.; Pradel, J. In Aerosol Science;Davies, C. N., Ed.; Academic Press: New York, pp 1986; 87-110. Mohnen, V. Investigation of the Attachment of Neutral and Electrically Charged Emanation Decay Products to Aerosols. AERE-Trans 1106, H. M. Stationery Office, 1967. Subba Ramu, M. C.; Vohra, K. G. Tellus 1969, 3, 395. Roffman, A. J . Geophys. Res. 1972, 77, 5883. Junge, C. E. Air Chemistry and Radioactivity; Academic Press: New York, 1963. Raabe, 0. R. Health Phys. 1969, 17, 177. Chamberlain, A. C.; Dyson, E. D. Br. J . Radiol. 1956,29, 317. Hopke, P. K. Enuiron. Int. 1989, 15, 299. Su, Y. F.; Cheng, Y. S.; Newton, G. J.; Yeh, H. C. Attachments of Lead-212 Clusters to Monodisperse Aerosols in the Ultra Pure Air. Proceedings 1989 AAAR Meeting, October 9-13, 1989, Reno, NV, 1989. Sigsworth, S. W.; Castleman, A. W. Jr. Chem. Phys. Lett. 1990, 168, 314. Guo, B. C.; Conklin, B. J.; Castleman, A. W. Jr. J . Am. Chem. SOC.1989, 111 , 6506. Guo, B. C.; Purnell, J. W.; Castleman, A. W. Jr. Chem. Phys. Lett. 1990, 168, 155. Leuchtner, R. E.; Farley, R. W.; Harms, A. C.; Funasaka, H.; Castleman, A. W. Jr. Int. J. Mass Spectrom. Ion Processes, in press. Szucs, S.; Delfosse, J. M. Phys. Rev. Lett. 1965, 15, 163. Porstendorfer, J.; Mercer, T. T. Health Phys. 1979,15,191. Maiello, M. L.; Harley, N. H. Health Phys. 1989, 57, 51. Handbook o f Chemistry and Physics 67th ed.; Weast, R. C., Astle, M. J., Beyer, W. H., Eds.; CRC Press, Inc.: Boca Raton, FL, 1986-1987. (a) J . Phys. Chem. Ref. Data 1977, 6 (Suppl. 1). (b) Ionization Potential and Appearance Potential Measurements 1971-1981; NSRDS-NBS 71; U S . Department of Com-
merce, Government Printing Office: Washington, DC, 1982. (21) JANAF Thermochemical Tables, 3rd ed.; Chse, M. W., Jr., Davies, C. A,, Downey, J. R., Jr., Frurip, D. J., Mcdonald, R. A., Syverud, A. N., eds.; National Burea of Standards: Washington, DC, 1987. (22) Keesee, R. G.; Castleman, A. W. Jr. J . Phys. Chem. Ref. Data 1986, 15, 1011. (23) Tang, L. N.; Castleman, A. W. Jr. J. Chem. Phys. 1972,57, 3638. (24) Tang, L. N., Castleman, A. W. Jr. J . Chem. Phys. 1974,60, 3981. (25) Gleim, K. L.; Guo, B. C.; Keesee, R. G.; Castleman, A. W. Jr. J . Phys. Chem. 1989, 93, 6805. (26) Castleman, A. W., Jr. Chem. Phys. Lett. 1978, 53, 560. (27) Guo, B. C.; Castleman, A. W. Jr. Znt. J. Mass Spectrom. Ion Processes 1990, 100, 665. (28) Sigsworth, S. W.; Castleman, A. W. Jr. J . Am. Chem. SOC. 1989, 11 1, 3566. (29) Meot-Ner, M. In Gas Phase Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 1, p 197. (30) Castleman, A. W., Jr.; Sigsworth, S.; Leuchtner, R. E.; Weil, K. G.; Keesee, R. G. J . Chem. Phys. 1987,86,3829. (31) Yang, X. L.; Castleman, A. W. Jr. J . Chem. Phys. 1990,93, 2405. (32) Su, T.; Chesnavich, W. J. J. Chem. Phys. 1982, 76, 5183. (33) Castleman, A. W., Jr.; Keesee, R. G. In The Stratospheric Aerosol Layer; Whitten, R. C., Ed.; Springer-Verlag: New York, 1982; Vol. 28, pp 69-92. (34) Discussions a t DOE/OHER Contractors Meeting, Fort Collins, CO; session on Fundamental Processes in Radon Transport Dynamics and Modeling, September 2&22,1989. (35) Castleman, A. W., Jr. Enuiron. Sci. Technol. 1988,22,1265. (36) Mohnen, V. A. In Mesopheric Models and Related Experiments; Fiocco, G., Ed.; Reidel: Dordrecht, The Netherlands, 1971; pp 210-219. (37) Arnold, F. Nature 1980, 284, 610. (38) Ferguson, E. E.; Fehsenfeld, F. C.; Albritton, D. L. In Gas Phase Ion Chemistry;Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 1, pp 45-82.
Received f o r review December 5,1989. Accepted November 27, 1990. Financial support by the U. S. Department of Energy, Grant DE-FG02-88-ER60668, is gratefully acknowledged.
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