The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the Office of Experimental Technology Incentives Program. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations, and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions: Applied Mathematics – Electricity – Mechanics — Heat – Optical Physics – Center for Radiation Research — Lab oratory Astrophysics’ – Cryogenics’ – Electromagnetics’ — Time and Frequency'. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials, the Office of Air and Water Measurement, and the following divisions: Analytical Chemistry — Polymers — Metallurgy – Inorganic Materials — Reactor Radiation - Physical Chemistry. THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services developing and promoting the use of available technology; cooperates with public and private organizations in developing technological standards, codes, and test methods; and provides technical advice services, and information to Government agencies and the public. The Institute consists of the following divisions and centers: Standards Application and Analysis – Electronic Technology - Center for Consumer Product Technology: Product Systems Analysis; Product Engineering - Center for Building Technology: Structures, Materials, and Safety; Building Environment; Technical Evaluation and Application – Center for Fire Research: Fire Science; Fire Safety Engineering. THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment: and serves as the principal focus wthin the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Institute consist of the following divisions: Computer Services — Systems and Software – Computer Systems Engineering – Information Technology. THE OFFICE OF EXPERIMENTAL TECHNOLOGY INCENTIVES PROGRAM seeks to affect public policy and process to facilitate technological change in the private sector by examining and experimenting with Government policies and practices in order to identify and remove Government-related barriers and to correct inherent market imperfections that impede the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS: promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following organizational units: Office of Standard Reference Data – Office of Information Activities – Office of Technical Publications — Library – Office of International Standards — Office of International Relations. 1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234. * Located at Boulder, Colorado 80302. The Journal of Research of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement methodology, and the basic technology underlying standardization. Also included from time to time are survey articles on topics closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete citations to all recent NBS publications in NBS and non-NBS media. Issued six times a year. Annual subscription: domestic $17.00; foreign $21.25. Single copy, $3.00 domestic; $3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry” and Section B "Mathematical Sciences.” U.S. DEPARTMENT OF COMMERCE Juanita M. Kreps, Secretary NATIONAL BUREAU OF STANDARDS Ernest Ambler, Acting Director Order all publications from the Superintendent of Documents U.S. Government Printing Office, Washington, D.C. 20402 The Secretary of Commerce has determined that the publication of this periodical is necessary in the transaction of the public business required by law of this Department. Use of funds for printing this periodical has been approved by the Director of the Office of Management and Budget through June 30, 1981. Institute for Materials Research, National Bureau of Standards, Washington, DC 20234 (June 13, 1977) Irradiation of CC14, CFC1z, and CF2Cl2 in the presence of C2H6 in vessels containing silica sand or fused quartz tubing results in the formation of chlorine-containing products. The formation of these compounds occurs at wavelengths extending up to approximately 400 nm, that is, at wavelengths well beyond the absorption threshold of the chloromethanes in the gas phase. It is suggested that CCl4 adsorbed on silica surfaces photodissociates to yield CClz and CCl, species. The poor material balance obtained in these experiments indicates that several of the chlorine-containing fragments are strongly adsorbed on the surface. At a CCl4 pressure of 13 Pa (0.1 torr), photolysis with 366 nm light in the presence of sand results in the decomposition of one molecule for every 104 photons striking the surface. Under otherwise identical conditions, the photon-induced breakdown of CFCland CF2Cl2 is respectively only 10 percent or 3 percent as efficient. Key words: Chloromethanes, photochemistry; quantum yields; quartz; sand; surface reactions; tropospheric sink. at 1. Introduction the gas phase absorption region when they are adsorbed on surfaces. Most of these earlier studies dealt with materials In a recent study [1]' from this laboratory, it was shown adsorbed on metallic oxides. However, in one recent study that in the gas phase, the photodecomposition cross section of [5] it was shown that irradiation of cis- or trans-1,3-pentaCCl4 at 313 nm was < 3.7 + 0.4 x 10-26 cm2 molecule - 1 diene in Pyrex or quartz vessels leads to isomerization at 300 K. Because sunlight reaching sea level consists mainly of wavelengths for which light absorption does not occur in the wavelengths above 320 nm, this laboratory result suggests gas phase. that in the troposphere gaseous CCl4 will not be dissociated In the present laboratory study, the photodecomposition of by light. The photodissociation cross section of fluorine- CCl4, CFC1z, and CF,Cl, adsorbed on fused quartz and , substituted methanes at the wavelengths which reach the different types of sand is examined. Most of the experiments earth's surface should be even smaller than that for CCl4, were carried out using light of 366 nm, which is well beyond since the absorption spectra of the fluorine-substituted com the gas phase absorption threshold of these substances [2, 6). pounds are shifted to shorter wavelengths [2]. Therefore, The majority of the experimental results have been obtained chloromethanes and fluorochloromethanes released to the with CCl4, which has a higher photodecomposition rate than atmosphere would be expected to diffuse to the stratosphere, the fluorine-substituted compounds, but from the few experiunless some mechanism for their removal other than photode- ments carried out with CFC1z and CF,Cl, it is possible to composition exists. As Molina and Rowland [2] suggested, estimate the importance of their photodecomposition relative these compounds in the stratosphere would absorb high en to that of CCl4. ergy photons and undergo photodissociation to produce chlo- Although, at this writing, no significant trophospheric rine atoms, which in turn would be expected to undergo a removal mechanisms of these chloromethanes have been chain reaction resulting in the removal of ozone molecules. demonstrated, it has been suggested that the existence of a However, there were available two pieces of evidence trophospheric sink cannot be rule out at the present time [7, which indicated that possibly CCl4 when adsorbed on certain 8]. The results reported here do not prove the existence of surfaces does undergo photodecomposition at wavelengths as such a sink, but do suggest that removal mechanisms not long as those which reach the earth's surface. The first such previously considered may be operative. observation was that the cross section for photodecomposition of CCl4 showed an apparent increase by a factor of five when 2. Experimental Detail the measurements were made in quartz rather than Pyrex? vessels. Secondly, measurements of the concentration of CC14 Vessels. Two types of reaction vessels were used for the in the troposphere in the eastern hemisphere as a function of experiments reported here: (1) cylindrical vessels (10 cm latitude from 50 N to 30 S have shown that there is a long, 5 cm diameter) made of Pyrex, and with one half of the pronounced minimum in the CCl4 concentration in the vicin- volume filled with Pyrex or fused quartz tubing (0.2 mm ity of the Sahara desert (latitudes 20 to 30 N) (3). inside diameter, 0.4 mm outer diameter, and 5 cm length); It has been reported before [4, 5] that organic materials (2) 1000 mL Erlenmeyer flasks, the neck of which has been may undergo photodissociation at wavelengths well beyond 2 In order to adequately describe materials and experimental procedures, it was occasionally necessary to identify commercial products by manufacturer's name or label. In no instances does such 2 2 identification imply endorsement by the National Bureau of Standards, nor does it imply that the Figures in brackets indicate the literature references at the end of this paper. particular product or equipment is necessarily the best available for that purpose. * Guest Worker. 1 cut off and sealed with 0.3 cm thick Pyrex window (fig. 1). The bottom of the Erlenmeyer flasks is covered with a layer of sand 0.2 to 0.5 cm thick. For each kind of sand, and for each kind of halomethane (CF2Cl2, CFC1z, and CCl4) a separate reaction vessel was constructed. As a further precaution to avoid contamination of one halomethane by traces of another, separate vacuum lines were used for handling each of the halomethanes. Irradiation. Medium pressure mercury arcs were used in conjunction with various filters. In most experiments, Corning 0-52 filters were used to eliminate the 313 and 334.1 nm mercury resonance lines and to transmit 65 percent of the 366 nm mercury resonance line. A Corning 3-75 filter was used to obtain 404.5 nm radiation in the absence of shorter wavelengths. In the experiments with the Erlenmeyer vessels, the light beam was focused by means of a quartz lens so that it was slightly diverging; the entire surface of the sand at the bottom of the flask was thus exposed to the light beam. |