Clusters Elmihfing gas-to-particle conversion processes
By A. W Cnstkman,J,:
Why is there currently so much interest in the subject of line particles? First, fine particles are ubiquitous throughout the universe: They are present in inters t e h media (e.g., in dense clouds as well as comets); they are ultimately responsible for the rings around Saturn, and, in some theories of planetary evolution, they are invoked to explain the origin of planets themselves (2). In high altitudes of our own planet there are layers of meteoritic ablation where dust layers are known and noctilucent clouds form, possibly enhanced by ioninduced nucleation processes (2). For more down-to-earth reasons, fine partcles are of interest to environmental Scientists, who must contend with them in ways more germane to our everyday life in terms of visibility and health. For many years the community of atmospheric Scientists was divided into two groups: those dealing with prob lems of visibility effects, clouds, and condensation processes and those interested in chemical transformations. The latter considered heterogeneous p m esses to be of secondary importance and of third-order interesf; they usually attempted to account for everything by considering only gas-phase reactions. But one need only r e a l i i that gasphase acid molecules in the troposphere end up in particles and precipitation, or consider the discrepancies in homogeneous model predictions having to do with the ozone bole (3) to appreciate that a l l phases of the atmosphere are intimately coupled. In the broad context, particles are central to the atmospheric processes that we as scientists attempt to describe. My group and I began in the early 1970s to work on clusters and ultraline particles to provide new information on some of the processes that at that time were identi6ed as potentially important. In this article, I will provide an over-
A. Welford Costlemon, Jr:
I
At the Third Chemical Congress of North America in June in Toronto, A. Wetford Castleman, Jr., received the ACS Award for Creative Advances in Environmental Science and Technology In his award address on clusters-weakly bound aggregates among ahnospherically important species-he elucidated their properties. reactions, and processes for the formation d a e w l s from gaseous precursors, a field in which he and his students have been pioneers. Castleman and hs colleagues were the first to demonstrate that sulfur dioxide in the atmosphere is oxidired by reaction with hydroxyl radicals. His research shaved that ths readion is the only important gaephase oxidation proc658 and that the kinetics of the r e a e tion are not fast enough to account for all the sulfur dioxide oxidation in the environment. C&eman. Evan Pugh hofessor of C h e m i i at the Pennsylvania State Unkrsity, received an honorary doctorate from the University of tnnsbruck, Austria, last year and h a s publiihed more than 250 papers. He received hls B.Ch.E from Rensse laer porylechnic Institute in 1957 and his Ph.D. at the Polytechnic lnstnute d h w Min 1969. At Pennsvhra-
view of recent research on fine particles, tie in some points not previously brought together, show some approaches we have tried, and examine problems currently of interest to researchers in this field. As part of our early work, we performed scoping experiments along with free-radical-scavenging experiments and established that OH rather than HO, as thought at the time was responsible for homogeneous oxidation of SO,. In addition, we showed that the reaction in humid air produced sulfatecontaining particles (@. ' h g and I reported one of the first definitive measurements (5, 6) of the rate of this reaction of value in modeling the atmosphere, albeit these measurements were based on a competitive reaction scheme-a technique that has now been supplanted by more sophisticated meth-
ods.
Several possible scenarios regarding the fate of H S q have been suggested, including reaction with another OH or O,, perhaps forming an addition product. More recent work (7)confirms our early suggestion that SO3 is produced, probably by reaction with 4 (5). Subsequent steps in the mechanisms leading to sulfate aerosol formation are still an active area of investigation. Work in our laboratory (5, 8) indicates the rapid association reaction of with H20, recent work by Lee (9) establishes that the direct fourenter reaction is expectedly slow. More recently we have undertaken a study to show that Hz0.S03 rapidly rearranges to form H2S04 (20). Using an electrostatic quadrupole-focusing tecbnique, we showed that the product of the reactions displays refocusing behavior identical to H2S04, indicating that the ultimate product is the sulfate tively invealgaticg the dynamics of molecule rather than S03.H20, which formation, h s e r photoionizatbn, di5 would have a larger dipole and signifisociatm and SDBclroscOw. and r e cantly different focusing characterisactions and bdnding of 'g'mase tics. clusters. Following Junge's discovery (22) in I
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the late 1950s of a sulfate particle layer in the stratosphere, considerable interest in the origin of the particle layer arose as researchers sought to determine if the layer was derived from natural in situ reactions or from man-made activities and particles carried into the stratosphere. Through an analysis of temporal distributions of sulfate, we determined that volcanic emissions were dominant contributors to the layer (12). Sulfur-containing gases from the natural biosphere, probably carried upward in equatorial regions, were likely responsible for the persistent “background” stratospheric sulfate aerosols. Findings of an enhanced isotopic enrichment in 34S with altitude revealed the in situ reaction as the important sulfate contributor (13). Recent studies of isotope enrichments in water clusters (14) confirm that clustering mechanisms involve unimolecular dissociation between growth steps and lead to a cumulative enrichment in the heavy isotope. This effect is related to the differences in the zero-point energies of the molecules incorporated in the clustering species. Our work has focused on cluster distributions, growth, properties, and reactions to shed light on the factors governing the formation and stability of prenucleation embryos, and of nucleation processes leading to new particle formation (1.5-17). Nucleation has been a controversial subject, with classical energy barrier approaches working in some cases and not others. A study of cluster ions has been particularly useful (18-21), because deducing their thermochemical properties has enabled an assessment of the validity of classical expressions. Entropy factors have been found to be of major importance in the structural aspects of small clusters, which is why some systems with little structural ordering in the prenucleation embryos are validly described by classical approaches, whereas other, more ordered systems are not (22). An empirical scaling method of accounting for these factors, which has been suggested by our work, awaits more exact theoretical formulations of this difficult problem. We have established that cluster properties for ligands attached to ions can now be related to those of the condensed phase by proper scaling of the summed successive enthalpies (19). In the context of atmospheric processes, studies of hetermolecular clusters are particularly revealing. Even under conditions where homogeneous nucleation might be be operative, heteromolecular processes would dominate (20, 21, 23) (unless the rate of condensation on preexisting aerosols exceeds these processes). This is due to 1266 Environ. Sci. Technol., Voi. 22, No. 11, 1988
the fact that bonding to ions, or among with concomitant C10N02 and HC1 redissimilar molecules, is frequently moval and C12 generation. N205,which greater than between those comprising undergoes only an extremely slow gasthe homogeneous system, and this phase reaction with H20, may undergo lowers the energy barrier to an extent fast conversion with polar stratospheric that the species of mixed composition clouds (PSCs) or even hydrated clusters will serve as the (prenucleation) em- of sufficient size (28), although reacbryos responsible for nucleation (23). tions with bromine compounds also Consider, for example, the enhanced may be important and provide a viable stability of mixed ion clusters com- alternative mechanism that needs to be prised of H20 and SO2 bound to C1- considered (33). (24), over the pure systems comprised Negative ions serve as bases and are of the same number of ligands, an ef- strongly bound with acid molecules fect that has been observed in several ( 3 4 3 3 , the bonding of HN03 to NO3systems studied in our laboratory. exceeding 1 eY NO3- (HN03),(H20), Of particular current interest are is expected to have a large electron afstudies of the stability, properties, and finity, the electron affinity of NO3- begrowth of clusters comprised of acids ing already greater than 4 e y further and bases. In studies of nitric acid wa- solvation leads to a proportionate inter clusters, where deuterated species crease (36). Reaction with H2S04 (37) were investigated to allow for mass as- leads to HS04- as the core ion, and hysignments without the ambiguities drates of HS04- bound with HN03 caused by mass degeneracies, we stud- molecules have been detected in the upied distributions arising from mixed per atmosphere (36). clusters comprised of protonated nitric C1- and its hydrates are likely to be acid and water formed after electron transient species in the upper atmosimpact ionization. A local minimum in phere (39; C1- rapidly reacts with the size distribution at D+ (DN03) HN03 to form HC1 and NO3-. An inter(D2O)4 provided some circumstantial esting reaction involves C10N02, evidence that the distributions corres- which leads to C12 formation and NO3-, pond to the hydrate size where the acid but whether this reaction is of any conmolecules form ion pairs. Calculations sequence is unknown, and doubtful due based on thermochemical studies of ion to expected low concentrations. But it hydrates confirm this possibility. Inter- is interesting to note that solid hydrates estingly, this minimum occurs with ex- of HC1, which have been suggested as actly the same composition as with the contenders in the mechanisms proposed S03-H20 system (Le., H + .(H20)4 to involve PSCs, have crystal structures SO3). Ammonia reacts in a facile dis- that indicate the presence of Cl-(H20), placement reaction with the loss of H 2 0 groups (29). Hence there is another tie(25, 26). Similar studies with other in with cluster chemistry, where studies acid-HzO cluster systems (e.g., HC1- of the cluster reactions may help eluciH20 and NH3-H2S)have revealed min- date heterogeneous reactions such as ima in the cluster distribution, as have C10N02 HC1+ C12 HN03 studies of systems incorporating NH3. Cluster reactions between the atmospherically important molecules NH3 and C10N02 H20 --+ HOC1 HN03 and with SO2 have been performed in our laboratory (27). A reaction known In passing, it is worthy of mention to produce solids of various color, depending on stoichiometry, and reactions that studies of accommodation coeffibetween NH3 and HCl (and HI for cients (29, 30) are needed for assessing comparison) have been performed the reactions of various atmospheric (28). NH3 is found to be more easily species with ice-like systems, including incorporated with SO2 clusters than the influence caused by acid molecules. vice versa, possibly because of more Cluster research provides an ideal alterfacile insertion of NH3 into a less de- native for obtaining mechanistic data. fined (SO& structure than the more The presence of ions is expected in the highly ordered one expected for NH3 heterogeneous substrates, where even pure water self-dissociates into hyclusters. Studies of ionic clusters not only pro- drates of H+ and OH-. Working with vide basic information for unraveling cluster ions in a thermalized flow reacgas-to-particle formation processes and tor maintained at a specific (stratosrelated nucleation mechanisms but also pheric) temperature enables investigaare taking on added importance as a tion at the molecular level of reaction, possible mechanism to explain the product evolution, and an evaluation of ozone hole problem (28). Several re- possible incorporation of such species searchers (3, 28-32) have suggested into the “hydrate lattice.” At large clusthat heterogeneous or ion chemistry ter sizes, the data will approach the bemay hold a partial key by providing a havior of the bulk system, and the reacmechanism for N205 and HN03 loss, tions with ions also may have relevance
+
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if these mechanisms prove feasible. Finally, there is the question of the mechanism of PSC formation and the incorporation of HN03. As revealed by our work, the clusters of HN03 and H 2 0 are expected to be stable. Because of the strong bonding of HN03 with the terminal negative ion NO3-, which displaces even HzO, and the known presence of proton hydrates in the upper atmosphere, an attractive explanation for PSC formation and HN03 incorporation is that H 3 0 + ( H 2 0 ) , - N 0 3 (HN03), recombination initiates the process. The “charged ends” of the resulting zwitterion would certainly enhance further growth. Whether nucleation actually proceeds by ion-induced, ion cluster-ion cluster recombination or neutral hetermolecular processes awaits further study. What is certain is that cluster research will have an important role in answering this as well as many other as yet to be identified important atmospheric processes where homogeneous and heterogeneous chemistry come together.
Acknowledgments I owe a large debt of gratitude to my group-past and present-members and colleagues, postdocs, and students who worked, discussed, and unraveled with me various aspects of these problems. They share equally in the recognition bestowed upon me. Unfortunately, I can’t do them all justice in the short space available. The continuing interest and financial support from the Atmospheric Sciences Section of the National Science Foundation, the Division of Biomedical and Environmental Research of the U.S. Department of Energy, and the U.S. Army Research Office en-
abled my research to be undertaken over the years, and I am very grateful.
References (1) Gribbin, J. Our Changing Universe: The New Astronomy; Dutton: New York, 1976. (2) Castleman, A. W., Jr. Space Sci. Rev. 1974,15, 547. (3) Solomon, S. et al. Nature 1986, 321, 755. (4) Wood, W. F!; Castleman, A. W., Jr.; lhng, N. J. Aerosol Sci. 1975, 6, 367. (5) Castleman, A. W., Jr. et al. Int. J. Chem. Kinet. 1975,1, 629. (6) Castleman, A . W., Jr.; Tang, I. N. J . Photochem. 1976177 6, 349. (7) Nagase, S.; Hashimoto, S.; Akimoto, H. J . Phys. Chem. 1988,92, 641. (8) Holland, F! M.; Castleman, A. W., Jr. Chem. Phys. Lett. 1978,56, 51 1. (9) Wang, X . et al. Rate Constant of the Gas Phase Reaction of SO3 with HzO, personal communication. (10) Sievert, R.; Castleman, A. W., Jr. J . Phys. Chem. 1984, 88, 3329. (11) Junge, C. E.; Chagnon, C. W.; Manson, J. E. J . Meteorol. 1961, 18, 81. (12) Castleman, A . W., Jr.; Munkelwitz, H. R.; Manowitz, B. Nature 1973, 244, 345. (13) Castleman, A. W., Jr.; Munkelwitz, H. R.; Manowitz, B. Tellus 1974, 26, 222. (14) Kay, B. D.; Castleman, A . W., Jr. J . Chem. Phys. 1983, 78,4297. (15) . , Castleman. A. W.. Jr. In Phvsics and Chemistry ‘of Upper Atmosphkres; McCormac, B. M., Ed. Reidel: Dordrecht, 1973; pp. 143-57. (16) Castleman, A. W., Jr. In Advances in Colloid and Interface Science Nucleation; Zettlemoyer, A., Ed.; Elsevier: Oxford. U.K.. 1979: DD. 73-128. (17) Castleman,’ A. W.-,*Jr.; Keesee, R. G. Aerosol Sci. Tech. 1983,2, 145. (18) Castleman, A. W., Jr.; Holland, F! M.; Keesee, R. G.J. Chem. Phys. 1978, 68, 1760.
(19) Lee: N.; Keesee, R. G. Castleman, A. W., Jr. J . Colloid Interface Sci. 1980, 75. 555. (20) Castleman, A. W., Jr. J . Aerosol Sci. 1982,13, 73.
(21) Castleman, A. W., Jr. In Heterogeneous Atmospheric Chemistry; Schryer, D. R., Ed.: American GeoDhvsical Union: Washington, DC, 19821 pfi. 13-27. Holland, F! M.; Castleman, A. W., Jr. J . Phys. Chem. 1982,86, 4181. Castleman, A. W., Jr.; Keesee, R. G.In The Stratospheric Aerosol Layer; Whitten, R.C., Ed. Springer-Verlag: Berlin, 1982; Vol. 28, pp. 69-92. Upschulte, B. L. et al. Chem. Phys. Lett. 1984,111, 389. Kay, B. D.; Hermann, V.; Castleman, A. W., Jr. Chem. Phys. 1986,102, 407. Kay, B. D.; Hofmonn-Sievert, R.; Castleman, A. W., Jr. Chem. Phys. 1986, 102.407. Keesee, R. G . et al. Aerosol Sci. Technol. 1987, 6, 7 1 . Crutzen. F! J.: Arnold. E Nature 1986. 324, 651. ’ Molina, M. J . et al. Science 1987, 238, 1253. Tolbert, M. A. et al. Science 1987, 238, 1258. Rowland, E S. et al. J . Phys. Chem. 1986,90, 1985. Tbng, K. -K. et al. Nature 1986, 322, 811. McElroy, M. B. Nature 1986,321, 759. Castleman, A . W., Jr.; Keesee, R. G.In Swarms of Ions and Electrons in Gases; Lindinger, W.; Mark, T. D.; Howorka, E , Eds.; Springer-Verlag: New York, 1984; pp. 167-93. Keesee, R. G . ; Castleman, A. W., Jr.; J . Phvs. Chem. Ref. Data 1986.15, 1011. Viggiano, A. A . et al. J . Geophys. Res. 1982,87, 7340.
A. Welford Castleman, J x , received his B. Ch.E. from Rensselaer Polytechnic Institute in 1957 and his Ph.D. degree at the Polytechnic Institute of New York in 1969. At Pennsylvania State University, he is actively investigating the dynamics of formation, laser photoionization, dissociation and spectroscopy, and reactions and boriding of gas-phase clusters.
Environmental applications of genetically engineered organisms By Luther Val Giddings The new biotechnologies are now producing numerous new products in human diagnostics and pharmaceuticals. The next wave of innovation is likely to come in the environmental application of genetically engineered organisms. The ability of geneticists to move genes between microbes, plants, and animals
offers a means of creating powerful tools for many circumstances. Applications are likely to be most dramatic in agriculture, but they will also be seen in new methods for dealng with some existing environmental problems, The Congressional Office of Technology Assessment recently studied this issue and released a report (1) that supports those who are enthusiatic at the
0013-936X/88/0922-1267$01.50/0 0 1988 American Chemical Society
advent of this new technology and reassures those who find grounds for caution and skepticism in the history of application of new technologies. New biotechnologies for environmental applications hold great promise. With the recombinant DNA techniques that have been developed since the discovery of restriction enzymes in 1973, it is now possible-at least in princiEnviron. Sci. Technol., Vol. 22,No.
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