loss-rate curves for the two different sea-state-roughness regimes, one in which there is little or no whitecapping and one which displays extensive capping. A further complication is that the greater turbulence during capping may result in the formation of oil-in-water and water-inoil emulsions. In conclusion, data have been presented for the change in specific hydrocarbon concentrations in small oil slicks undergoing weathering in typical open sea conditions. It is suggested that the relative rates of dissolution and evaporation can be estimated by spiking the oil with an aromatic such as cumene. Unequivocal determinations of dissolution rates can only be achieved by measuring dissolved hydrocarbon concentrations to levels as low as about 0.1 mg/l in the water under the slick. This was attempted with only partial success in the present study. It is suggested that future studies should employ the same technique but with more sensitive water sampling and analytical techniques. An approximate interpretive model has been developed to quantify evaporation and dissolution rates from a slick. The acceleration of weathering due to the onset of whitecapping and the variation in weathering in different parts of the same slick have also been observed experimentally. It is hoped that experimental studies such as this may yield observations and data which will be used in the development of more complex and realistic models of the physical, and ultimately biological, behavior and effects of oil spills on the oceans.
Acknowledgments Our thanks to C. L. Smith for reviewing an early draft of the manuscript and to W. Y. Shiu for assistance with analyses.
Literature Cited (1) Moore, S. F., Dwyer, R. L., Katz, A. M., Mass. Inst. Tech. Sea Grant ReDt. 73-6. 1973. (2) James, W. P., et al., Texas A&M Univ. Sea Grant Prof. Rep. 73-201, College Station, Tex., 1973. (3) Kinney, P. J., Button, D. K., Schell, D. M., Proc. Joint Conf. Prevention and Control of Oil Spills, Amer. Petrol. Inst., New York, N.Y., 15-17, 1969. (4) Smith, C. L., MacIntyre, W. G., ibid., Washington, D.C., 1971. (Bj-Skadier,H . O., Mikolaj, P . G., ibid., p 475, 1973. (6) Blumer, M., Enuiron. A f f a i r s , 1 ( l ) ,54 (1971). (7) McAuliffe, C . , J . Phys. Chern., 70, 1267-75 (1966). (8) Mackay, D., Wolkoff, A. W., Enuiron. S e i . Technol., 7, 611-14 (1973). (9) Zwolinski, B. J., Wilhoit, R. C., “Handbook of Vapor Pressures and Heats of Vaporization of Hydrocarbons and Related Compounds.” Amer. Petrol. Inst. Project 44, Washington, D.C., 1971. (10) McAuliffe, C., Science, 158,478-9 (1969). (11) Leinonen. P . J., Mackay, D., Can. J . Chern. Eng., Sl, 230(1973). (12) Mackay, D., Matsugu, R. S., ibid., pp 434-9. (13) Liss, P . S., Slater, P. G., N a t u r e , 247, 181-4 (1974).
Received f o r reuieu M a y 14, 1974. Accepted N o u e m b e r 4, 1974. Work supported by Erindale College iUnzniuersity of Toronto) and Seabulk International Inc.
Cumulative Chemical Light Meter G. D. Dixon* and D. H. Davies Westinghouse Research Laboratories, Pittsburgh, Pa. 15235 J. D. Voytko Westinghouse Environmental Systems, Monroeville, Pa. 15146
A cumulative light meter has been devised based on the light-induced degradation of aqueous polymer solutions. The viscosity of the solution can be related to the total amount of incident light absorbed. The system possesses a number of significant advantages over competitive actinometers. It is very cheap, needs no extra equipment or skilled personnel, and can be simply modified to respond to any desired spectral region.
Quantification of total incident light provides information about the health of an ecosystem by relating its typical primary productivity to the amount of available energy. Pitts and co-workers ( I ) have developed a chemical actinometer, suitable for such field studies, which consists of a thin film of polymethyl methacrylate in which is dissolved o-nitrobenzaldehyde. Radiation, between 280 and 410 nm, causes photoisomerization to produce o-nitrosobenzoic acid with a quantum efficiency of 0.5. Another system (2) is based upon the photodimerization of anthracene. Since the anthracene is soluble in benzene, but the dianthracene is not, spectrophotometry can be used to measure the amount of anthracene remaining. 234
Environmental Science & Technology
We have developed a light-monitoring device based upon the photochemical degradation of a polymer. The degradation causes a decrease in the molecular weight of the polymer which results in a decrease in the viscosity of solutions containing the polymer. If one now carries out the degradation of a polymer solution in a transparent tube, one can follow the progress of the degradation by measuring the viscosity of the solution using the GardnerHoldt method of time of rise of a bubble in the solution. The viscosity can then be related to the quantity of light which has fallen on the tube, which means that the tubes can be examined in the field if necessary, a stopwatch being the only apparatus needed. It is well established that the spectral sensitivity region needed for the photopolymerization of organic monomers can be altered by using reactive dyes. We have extended this principle to cause the degradation of polymers. By using dyes, one can then use radiation in the visible part of the spectrum to initiate these reactions. The polymer chosen was hydroxyethyl cellulose (HEC) which is susceptible to photodegradation. It is also water soluble and does not precipitate out of solution a t high temperatures. Further, the system does not suffer from an “inner filter” effect often found in actinometer systems. This is because the polymeric cellulose ether fragments have absorption characteristics similar t o the original polymer.
Experimental The HEC used was obtained from Dow Chemical, Cellosize QP 4400. This has an approximate molecular weight of 50,000. Solutilons (1-2 wt %) were made up using boiled, distilled water, and the sample tubes were sterilized before use. a-Chlorophyll was used as obtained, from Pfaltz and Bauer, Inc., to provide the desired spectral characteristics. The solutions were irradiated in Gardner-Holdt viscosit y tubes which are made from Pyrex glass. The tubes are 11 cm long and of 1-cm diameter. They were filled with solution to within 1 cm of the top. Each tube was sealed with a cork which left a standard sized gap of 0.8 cm. The tube was then inverted and the time taken for the bubble of air to reach the top of the tube was measured with a stopwatch. The time of bubble rise is proportional to the viscosity of the solution. These tubes are transparent down to 400 nm then transmission drops off until no light passes through below 310 nm. The polymer and water are also transparent and colorless in this range. Hence, the absorption characteristics of the solutions were set by the presence of additives. I t was our intention to shield the solutions from ultraviolet radiation so that we did not require a uv component in our test light source. This allowed us to use a tungsteniodine incandescent lamp, which gave 4000 ft-candles at a distance of 30 cm, where the air temperature was 30°C. Using this intense lamp allowed us to carry out a 3-month life test in less 1,han 1 month. The spectral energy distribution (SED) of the lamp is given in Figure 1, together with the SED for sunlight plus daylight for comparison. Light intensity, in ft-candles, was measured with a Gossen Tri-Lux meter (P. Gossen & Co., GmbH, 8520 Erlangen, Germany). The tubes were maintained a t constant temperatures throughout the irradiation periods. This was achieved by inserting a circulating water bath, made from Plexiglas, between the samples and the light source to remove most of the infrared. The tubes were further cooled with a fan. Viscosity measurements, made a t regular intervals, were taken either after allowing the tubes to cool to room temperature or a t ambient temperature, and a correction made for the difference. From these measurements, we were able to prepare a calibration curve of viscosity vs. the amount of iIradiation to which the tube had been exposed. An example is given in Figure 2. This solution contained approximately 2 wt % of HEC and 0.001% a-chlorophyll. The tubes were irradiated a t 30°C with 4000 ftcandles. Illumination and viscosity measurements were made a t room (25%) and ambient (30°C) temperatures. The useful life of these particular tubes is roughly 1 X lo6 ft-candle hr, but the shape of the curve, and hence the lifetime of the actinometer, depends upon the molecular weight and the {concentration of HEC. These characteristics can also be easily modified through the selection of dyes, as described below. Results and Discussion This system appears to have satisfactory absorption characteristics for use as a light meter in the evaluation of photosynthesis and, by the addition of dyes, the light meter can be made to absorb in any desired region. The absorptivities of the various components in the system examined are shown in Figure 3. Although the polymer absorbs in the near-uv range, as does the glass and the chlorophyll, ,the lamp has only a very small percentage of its output below 500 nm. Thus, it appears reasonable to assume that only a small amount of reaction is occurring in this region. This leads to speculation that the reaction
that does occur arises from some interaction between the polymer and chlorophyll excited by absorption a t the red end of the spectrum. Our system has been designed to provide an appreciable drop in viscosity for a period of 300 hr of intense irradiation. This is equivalent to about 1 month of summertime exposure in Arizona. The published data on sunlight show some variation. Instantaneous, midday sunlight plus daylight on a clear day a t Cleveland, OH, have been estimated to be 8500 f t candle ( 3 ) , while Dore (2), in his published work on an anthracene chemical light meter, stated that a typical
i
I
400
500 600 Wavelength, nm
700
Figure 1. Spectral energy distributions of tungsten/iodine incandescent lamp and sun plus daylight
5,Z v i t ~ ~ H E C a n d O . W l % C h l o r o p h ~ l l
>
10
20
30 40 50 60 70 80 90 Cumulative Radiation, /t candle hrs x
1W 110
Figure 2. Calibration curve for HEC irradiated at 30'C. Viscosities measured at 25' and 30°C
.'vavelenglh, nm
Absorption regions of a Gardner-Holdt tube and aqueous solutions of HEC and chlorophyll Figure 3.
Volume 9 , Number 3, March 1975 235
day's illumination could vary from 3000-50,000 ft-candle hr . We measured the peak intensity of bright noon sunlight in Pittsburgh during July and obtained 6000 ft-candles. An hour later, the value had dropped to 3000 ft-candles due to partial cloud cover. This gave an average midday value of 4500 ft-candles. We can, therefore, correlate our solarium as equivalent to average daylight plus sunlight intensity in midsummer a t Pittsburgh latitudes. Allowing for the hour-to-hour variation in intensity, based on diurnal variation curves published by Koller ( 4 ) , we integrated the sunlight radiation over a 14-hr daylight period to obtain a total value of 42,000 ft-candle hr, or an average of 3030 ft-candles. Aqueous solutions of colorless polymers necessarily absorb white light equally a t all wavelengths. This can be used to advantage because the addition of a dye can affect the rate of degradation in a specific spectral range. An example of this is eosin, a red dye, which increased the sensitivity of the actinometer remarkably in the range 520430 nm. The eosin is activated by light to form a leuco dye which then initiates free radical attack on the polymer chain. Alternatively, a dye can be used to reduce the rate of degradation. Such a dye would have no photooxidative ability and would behave as an internal filter. Thus, the sensitivity of the light meter can be adjusted to order, both with respect to reaction rate and to spectral sensitivity. Further, the reactive dyes serve only as energy transfer agents so that they do not change chemically or optically during an exposure. This, of course, eliminates the use of photobleachable dyes for this purpose. A detailed analysis of the chemistry of cellulose-dye interactions and of effects on the viscosity of solutions is given elsewhere ( 5 ) . A combination of inert filter dyes (to control spectral sensitivity) with a photoreducible dye (absorbing in the desired region) gives a system that can be modified in its rate of response for a chosen spectral sensitivity. The combination of HEC and a-chlorophyll is useful. This system has a slow "dark" reaction, and the degradation proceeded in the bulk of the material rather than a t the tube surface facing the light source. The sensitivity t o temperature, present in all organic actinometers, was also investigated. The effect of tube temperature on the viscosity has been indicated in Figure 2. This effect is readily compensated for when the ambient temperature is known. The effect of temperature on the rate of degradation is more serious. Figure 4 shows the increase in the rate of degradation caused by increasing the temperature from 35-40°C. However, one standard deviation for these tubes is about 3% so that there is no significant difference between the two rates. Since plants are exposed to continuously varying temperatures, an experiment was designed to systematically vary the temperature between 25" and 45"C, with control samples held between 25" and 30°C. To evaluate separately the photo and thermal effects, duplicate samples, in darkened tubes, were subjected to the same thermal cycles. Figure 5 shows the relative rates of degradation for the irradiated (lower line) and dark (upper line) tubes, in the two temperature ranges. There apears to be no significant increase in the amount of degradation due to the extra time a t elevated temperatures, over the lifetime of the devices. The viscosity measurements in Figure 5 have not been corrected for variations in room temperature. The difference between the upper and lower curves is a measure of the photodegradation alone and Figure 6 is a calibration curve for these samples in which the reduction 236
Environmental Science & Technology
in viscosity, due to light alone, is plotted against the total amount of irradiation. The actinometer appears to possess reciprocity, an essential requirement for any cumulative light meter. One sample, with a bubble rise time of 120 sec, was irradiated for 100 hr a t 2000 ft-candles intensity, between 25" and 45"C, causing a 15.5% decrease in the viscosity. A second
20 30 40 Cumulative Radiation It candle hrs
54
10
Figure
4.
x
Decrease in viscosity of HEC solutions irradiated at
35" and 40°C
Reaclion time, Hours
1w
2w
3w
4w
20 30 40 50 W 70 80 Cumulative Radiation, (1. candle h r s X l
W
10
r 4
Figure 5. Total radiative and thermal degradation of H E C in different temperature ranges
r
I
I
a
7
t e n p r a t u r e range 25-45'C
:t e i p e r a l u r e
13
20
382
40
c L T u I a t i + e ?aoiation
50
60
'I c a i d l e - r r i x
Figure 6. Photodegradation of HEC
range Z5-30'C 70
80
sample with a bubble rise time of 92 sec, was also irradiated for 100 hr, but a t 4000 ft-candles intensity and 30°C. This caused a 29.3% decrease in the viscosity. As expected, doubling the light intensity doubles the rate of degradation; the small difference being explained by errors in measuring the light intensities. This means that the device will undergo the same percentage decrease in viscosity for the same number of photons absorbed, whether they arrive consecutively or simultaneously. A final word is needed on the units used to present the data. Historically, pyroheliometers have been used to measure radiation from the sun. This technique measures energy and has the units of microwatts per square centimeter. Our light meter has been calibrated in the units foot-candle hours, as used in the lighting industry, from which one can obtain an approximate “number of days exposure.” The conversion from energy to light units is difficult and appears to have received little attention. The problem of conversion arises from the changes in the spectral energy distribution of sunlight, which means that no single conversion unit can be applied. Obviously, the best method is to measure both quantities simultaneously under the test conditions with a range of sun angles and cloud cover. This way one can
obtain a direct comparison between light and energy units for a given light meter. This problem is the subject of further investigation. Conclusion The light-measuring device described offers the possibility of comparative, cumulative analysis of the total radiative impact on photosynthesis. Its principal advantage lies in its low cost coupled with the absence of any need for complex measuring equipment or skilled techniques. This allows the use of a large number of simultaneous tests in the field at minimal cost.
Literature Cited (1) Pitts, J. N., Cowell, G. W., Burley, D . R., Enuiron. Sci. T e c h nol., 2,435 (1968). (2) Dore, W. G., Ecology, 39, 151 (1958). (3) Luckiesh, M., “Germicide, Erythemal and Infrared Energy,” D. Van Nostrand Co., New York, N.Y ., 1946. (4) Koller, L., “Ultraviolet Radiation,” Chap. 4, “Solar Radiation,” Wiley & Sons, New York, N.Y ., 1946. (5) Davies, D. H., Dixon, G. D., J . A p p l . Poly Sci., 16, 2449 (1972).
Received f o r reuieu: S e p t e m b e r 28, 1973. Accepted October 1, 1974. Presented a t t h e 11th Informal Conference on Photochemistry, Nashuille, T e n n .
Gas Phase Kinetic Study of Relative Rates of Reaction of Selected Aromatic Compounds with Hydroxyl Radicals in an Environmental Chamber George J. Doyle, Alan C. Lloyd, K. R. Darnall, Arthur M. Winer, and James N. Pitts, Jr.* Statewide Air Pollution Research Center, University of California, Riverside, Calif. 92502
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A 6000-liter ‘environmental chamber has been used to determine the relative rates of disappearance of eight aromatic compounds in air under simulated atmospheric conditions of temperature, pressure, concentrations, light intensity, and other trace contaminants (NO,, CO, hydrocarbons, water). Evidence is presented to support the hypothesis that, in the early stages of reaction under these conditions, the OH radical is the species largely responsible for the chemical transformation of the aromatics (as well as other relatively unreactive hydrocarbons such as n-butane). Under this assumption and using published
rate constants for the reaction, OH n-butane, rate constants are calculated for reaction of OH with toluene, p - , m-, and o-xylene, 1,2,3-, 1,2,4-, and 1,3,5-trimethylbenzene with values (at 304 f 1°K) ranging from (2.5 f 0.9) X 109 1. mol-1 sec-1 for toluene to (3.1 f 0.4) X 1010 1. mol-1 sec-l for 1,3,5-trimethylbenzene.An upper limit of 2.3 X 109 1. mol-1 sec-1 was determined for benzene. The rate constants for reaction of OH with m-xylene and the trimethylbenzenes are at least as large as that for OH propylene, confirming that substituted aromatics may play a significant role in atmospheric chemistry.
In recent years, a substantial body of kinetic data has been obtained for atmospherically important (1-4) reactions of the hydroxyl radical (OH) which has now been detected in ambient air for the first time ( 5 ) . These data have been summarized in recent major articles ( 6 4 , reviews (9-12), and compilations (13, 14). Investigations have been,made of the reactions of OH with alkanes, alkenes, and inorganic molecular and radical species (11). A critical need remains, however, for kinetic and mechanistic data for the reactions of OH with aromatic compounds in the gas phase (15-17). This is apparent, not only from the standpoint of the need to develop validated kinetic mechanisms of photochemical smog formation (6-8, 1820), but also with respect to the increased use of aromatic compounds in uinleaded gasoline (21) and the resultant increase in ambient concentration of aromatics (22, 23) over
those already present due to the use of solvents ( 2 4 ) . Recent studies in this laboratory have provided absolute rate constant data for O(3P) atom reactions with a series of aromatic hydrocarbons ( 2 5 ) . We report here the measurement, in an environmental (“smog”) chamber, of rates of disappearance of eight aromatic compounds which, under the assumptions detailed below, we ascribe to reaction with OH. Traditionally, smog chambers have been employed in photochemical investigations with highly applied goals such as generating a technical data base upon which to base control strategy decisions (26-28). This work demonstrates that, a t least in some cases, specifically under carefully controlled operating conditions and with precise chemical analyses, basic kinetic data can be derived from a smog chamber study for a variety of compounds under
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