The discontinuities in the plots of n-eicosane and n-eicosene adsorptions vs. concentration in Figure 1 may indicate that a major change occurs in the adsorption process above a concentration of 125 pg/L. Up to this point the percent removal from solution of both hydrocarbons is invariant at 38% of the initial amounts added. Above 125 p g b , percent removal suddenly increases to 71% at 150 pg/L and then gradually decreases at higher concentrations. This may signal a change from a monolayer adsorption process to a multilayer one. However, a more likely possibility is that the accommodation capacity of the simulated seawater solution is exceeded above 125 pglL. Peake and Hodgson (9) measured the accommodation of n-eicosane to be 350 yg/L at 20 “C in distilled water. Sutton and Calder (6) found the solubility of n-eicosane in seawater to be about 40% of the value in distilled water at the same temperature. Therefore, if the same reduction applies to accommodation, the maximum accommodation capacity of our solutions may have been between 125-150 pg of neicosane or n-eicosene per liter. Above this level, hydrocarbon particles may have adhered to the container walls, collected at the water surface, or become attached to smectite particles in such a way to produce the discontinuous results observed in Figures 1 and 2. Adsorptions of aromatic hydrocarbons given in Table I are below detection or lower than those of the other hydrocarbons listed. Only anthracene has substantial adsorption. These observations agree with those of Meyers and Quinn ( I ) and probably reflect the higher solution and accommodation levels of aromatic hydrocarbons suggested by Boylan and Tripp ( I 1 ). Because it has been shown that aromatic hydrocarbons form stable complexes with smectite clay in nonaqueous systems ( I 2 ) ,these results indicate the importance of solvation effects of water in adsorption of dissolved or micellar materials. Evidently, the combined polarity of water molecules is greater than the polarity of the clay surface; consequently, complex formation is not readily accomplished. Instead solvation of the aromatic hydrocarbons results in substantially higher solubilities of these compounds ( 1 3 ) than found for saturated hydrocarbons (5-8). These experiments provide information that allows speculation about the possible interaction of petroleum hydro-
carbons with mineral particles in natural waters. Assuming that solubility and accommodation are related, it is likely that less soluble components of petroleum will be preferentially taken up by settling particles and carried to the underlying sediments. More soluble components will remain in the water column. Furthermore, additional fractionation could occur through removal of slightly soluble hydrocarbons from resuspended sedimented material as shown by Meyers and Quinn ( I ) . Therefore, while association onto settling particles and incorporation into bottom sediments may effectively cleanse water of many petroleum components, processes such as resuspension of settled particles could lead to release of hydrocarbons back into the water and thus be a source of chronic pollution long after the original source of petroleum has been removed. Acknowledgment We thank J. G. Quinn, J. K. Rosenfeld, and C. Sutton for reviewing this manuscript, and J. G. Oas for technical assistance. Literature Cited (1) Meyers, P. A., Quinn, J. G., Nature, 244,23-4 (1973). (2) Blumer, M., Sass, J., Science, 176,1120-2 (1972). ( 3 ) Meyers, P. A,, Quinn, J. G., Geochim. Cosmochim. Acta, 35, 628-32 (1971). (4) Meyers, P. A., Quinn, J. G., ibid., 37,1745-59 (1973). (5) McAuliffe, C. D., Science, 163,478-9 (1969). ( 6 ) Sutton, C., Calder, J. A., Enuiron. Sei. Technol., 8 , 654-7 (1974). ( 7 ) Button, D. K., Geochim. Cosmochim. Acta, 40,435-40 (1976). ( 8 ) Boehm, P. D., Quinn, J. G., ibid., 37, 2459-77 (1973). (9) Peake, E., Hodgson, G. W., J . Am. Oil Chem. Soc., 43,215-22 (1966). (10) Fieser, L. F., Fieser, M., “Advanced Organic Chemistry”, p 1012, Reinhold, New York, N.Y., 1961. (11) Boylan, D. B., Tripp, B. W., Nature, 230,44-7 (1971). (12) Doner. H. E.. Mortland. M. M., Science, 166.1406-07 (1969). (13) Eganhouse, R. P., Calder, J. A,, Geochim. Cosmochim. Acta, 40, 555-61 (1976).
Receiued for review Nouember 21,1977. Accepted February21,1978. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work.
Optimum Control Strategy for Photochemical Oxidants Robert W. Bilger Department of Mechanical Engineering, University of Sydney, Sydney, N.S.W. 2006, Australia
The use of isopleth diagrams relating ozone formation to precursor concentrations for determining control strategy is discussed. Apical lines that give a worst NMHC-to-NO, ratio dividing the diagram into NMHC and NO, control regions are currently not soundly based and are not applicable to strategies where substantial control of oxidant is required. For substantial control, hydrocarbon control is the most effective over most of the area of interest. Precursor background levels and consideration of costs and administrative feasibility are included.
In recent years there has been a substantial international controversy about the best route to the control of photochemical oxidants. Photochemical oxidants are secondary pollutants formed from primary emissions of hydrocarbons and oxides of ni0013-936X/78/0912-0937$01.00/0 @ 1978 American
Chemical Society
trogen under the action of strong sunlight, and their control involves the choice of whether to control hydrocarbon emissions, nitrogen oxide emissions, or both. In 1971 the USA chose the hydrocarbon route with the promulgation of the Appendix J method ( I ) . In Japan a very stringent standard for NO2 (0.02 ppm for a 24-h average) was introduced in 1973 with the implication that this would also control photochemical oxidants ( 2 ) .In Australia initial control policy has been to reduce hydrocarbons while imposing sufficient control on oxides of nitrogen so that atmospheric concentrations will not increase ( 2 ) .In Europe Guicherit et al. (3, 4 ) had inferred from the available data that the NO, route is the best. This has been taken up by Daly ( 5 )in questioning Australian strategies. In 1976 the USA (6) adopted an alternative approach using the isopleth diagram relating ozone formed to precursor concentrations, thus allowing NO, control where appropriate and allowing the effect of NO, control to be taken into account where the determination of the degree Volume 12, Number 8, August 1978
937
of hydrocarbon control is sought. Recent findings of the importance of multiday irradiation effect and long-range transport are posing entirely new questions for control strategists (7). Much of the early controversy arose from the divergences in the available data. Indeed, in 1974 the differences between U S . aerometric data, Japanese mobile smog chamber data, and Los Angeles smog chamber data were so large that the Sydney Oxidant Study was started to obtain appropriate data for Sydney (8).Today, these differences are largely reconciled. The Dodge (9) isopleth diagram, produced from a computer model originally developed by Hecht and Seinfeld (IO) to fit smog chamber data, fits both the U.S. aerometric data and the Sydney Oxidant Study data very well when allowance is made for the inevitable dilution that occurs in the atmosphere (11). Although this fit may be somewhat fortuitous, since quite crude assumptions are involved in the dilution and kinetic models, this diagram is being used as a basis for control strategy decision-making (6) Figure 1 shows the isopleth diagram obtained from the Dodge model when a 3 to 2 dilution of the ozone produced is incorporated. Even with this reconciliation of the base data, divergences of strategy still arise from different interpretations of the use of isopleth diagrams. In this paper the use of isopleth diagrams such as Figure 1 for determining oxidant control strategy is discussed. Questions such as multiday irradiation effects, long-range transport, whether ozone is an adequate surrogate for the objectional oxidants, and whether other species should be simultaneously considered are not considered here. The inherent fallacy in the commonly used criterion for determining the boundary between NO, control and HC control is demonstrated, and a more rational basis for the determination of control strategy is developed. This rational basis includes the effects of cost of control and of background concentrations of the precursors.
Now if y = a ( x ) is the locus of points where ( b y l d x ) , = -1, then y = a ( x ) truly divides the diagram according to
which is a mathematical expression of the statement by Dodge quoted above. The implication is that her “apex points” are given by (byldx), = -1. Such points obviously depend on the relative scaling of the x and y axes. (Choice of the apex points as those being closest to the origin leads to the criterion ( b y l d x ) , = - x / y which compounds the scaling problem.) This scaling problem can be avoided by using the criterion for apex points of ( b y l b x ) , = - y / x or ( b In y l b In x), = -1. The corresponding apical line y = Al(x) divides the diagram according to
(”)bxlx
(”)d y l y
x
fory
$ Al(x)
(3)
Thus, the apical line divides the diagram into regions where fractional reductions in hydrocarbon yield greater or less reduction in ozone than corresponding fractional reductions in nitrogen oxides. For example, a 10%reduction in hydrocarbons will yield a greater reduction in ozone than a 10%reduction in nitrogen oxides when the point is above the apical line and vice versa when below. This appears to be a much more satisfactory criterion. The criterion described in Relation 3 above is expressed in terms of the ambient concentrations of the pollutants. A more satisfactory criterion may be one in terms of fractional reductions o f emissions. Since emissions and concentrations of precursors are linearly related, the criterion can be expressed:
0.6
Criteria for Marginal Reductions In discussing the results of her model, Dodge (9) says: “A line corresponding to a NMHC-to-NO, ratio of 5.6 is drawn through the apex point of these isopleths. This line divides the figure into a region that is predominantly NO,-rich and one that is predominantly HC-rich. In the low HCINO, region to the left of the line, a reduction in HC levels will result in the greatest decrease in 0 3 yields. In contrast, throughout most of the high HCIN, region to the right of the line, a reduction in NO, levels will lead to the greatest decrease in 0 3 formation.”
Y
0.4
0.2
0.1 E
a
P
Guicherit et al. ( 4 ) made a similar statement about their isopleth diagram and obtained an apical NMHC-to-NO, ratio of 5.3 as the dividing line. The fallacy of these results lies in the fact that the apical NMHC-to-HC ratio for any given set of isopleths depends on the axes scaling of the diagram, Le., it is an artifact of how the diagram is drawn. The mathematical background to the fallacy is as follows. Isopleths of ozone concentration z are functions of NMHC concentration x and NO, concentration y :
I X
0
z 0.04
0.0:
z = Z(X,Y)
From the chain rule of differentiation
it follows that
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Environmental Science & Technology
0.01 0.2 ~
0.4
0.6
1.0
2
4
6
where x b and Y b are the background concentrations of nonmethane hydrocarbons and nitrogen oxides, respectively. The ) given by (dyyldx), apex points on the apical line y = A ~ ( xare = - (y - y b ) / ( X - x b ) . Figure 1shows this h e drawn on the isopleth diagram of the diluted Dodge model with x b and Y b arbitrarily chosen as 0.08 and 0.002 ppm, respectively. These values are near the middle of the range found in the literature (4, 22). From Figure 1, the criterion of Relation 4 is a t an almost constant NMHC-to-NO, concentration ratio of 9.0. This is significantly higher than the values of 5.6 found by Dodge (9) and 5.3 found by Guicherit et al. ( 4 ) .This result is not sensitive to the dilution assumption, but is somewhat sensitive to the choice of background concentrations. The criterion of Relation 4 is only valid for determining control strategy if fractional reductions in hydrocarbons and NO, are equally costly and feasible. The relative cost and feasibility of hydrocarbon and NO, emission controls are a t this stage far from well quantified. However, it is generally agreed that hydrocarbon control is less costly and more feasible than NO, control. If it is assumed that hydrocarbon control is twice as easy economically and administratively as NO, control, the appropriate marginal criterion is when ( d y / & ~= ) ~-I/2(y - Y b ) / ( X - x b ) . The locus of these points is also shown in Figure 1. NMHC-to-HC ratios vary from 10 a t 30 pphm ozone to 38 a t 10 pphm ozone. While the ambit of hydrocarbon control is increased, NO, control is indicated for a significant range of‘ precursor concentrations of interest.
Criteria for Substantial Reductions The discussion above has been in terms of marginal (Le., small) reductions in precursor concentrations to yield small reductions in ozone. Two features of the isopleth diagram
become very significant when substantial control is required: (i) If, for example, hydrocarbon control is chosen, then this control method becomes more and more attractive as the controls are tightened. The same is true if NO, control is chosen. (ii) The asymmetry of the diagram is such that the rate a t which (i) occurs is very much greater for hydrocarbons than it is for NO,. The upshot of this is that hydrocarbon control becomes very much more attractive for substantial control requirements. In Figure 2 the degree of control needed to achieve an ozone concentration of 8 pphm is shown for hydrocarbon control alone or for NO, control alone. The NO, control lines are terminated where the control requirement is greater than for NMHC. The NMHC control lines are arbitrarily terminated where the reduction factor is the square of that required for NO, control. (Some models of the cost/feasibility function yield this result for hydrocarbon control being marginally twice as “easy” as NO, control.) Hydrocarbon control is favored for most areas of interest. The “NMHC control twice as easy as NO, control” boundary lies along the NMHC/NO, = 50 line which is seldom exceeded in practice. A further result of this analysis is that it is not necessarily the highest ozone concentration “points” that are the most difficult to control. Thus, a “point” with NMHC = 3 ppm, NO, = 0.06 ppm, 0 3 = 16 pphm is more difficult to control than a point with NMHC = 3 ppm, NO, = 0.18 ppm, O3 = 30 pphm, even though the ozone level reached is only half as great. The result is quite sensitive to the shape of the isopleths a t low NMHC-to-NO, ratios, an area where different models show different results.
Discussion
v
0.01 0.2
N0,CONTROL I
0.4
I
0.6
I
1.0 NMHC
I
I
2
4
I I6
- ppm
Figure 2. Isopleth diagram showing degree of control requirement and boundaries of control strategy regions for control to 8 pphm ozone Degree of control required in percent: - - NMHC; - - NO,. For instance, if situation is at point A (1.8 ppm NMHC, 0.06 ppm NO,, 16 pphm 03). then one can achieve 0.08 ppm 03 by 9 0 % control of NMHC or 80% control of NO,. Boundary for control strategy regions: - NMHC and NO, equally easy; NMHC control twice as easy as NO, control
..
.
The above results have been derived by using only the diluted Dodge model (9, l l ). It would be interesting to find how much they would be altered by using other computer models, smog chamber results, or aerometric data. Although this has not been done, indications are that similar results will be found particularly for the apical and region boundary lines. For both marginal and substantial control, these depend mainly on the shape of the curves in the neighborhood of NMCH-to-NO, ratios near 9 where all of the data are in reasonable agreement. Certainly, the use of diluted or undiluted Dodge models makes little difference to the boundary lines. The “degree of control” lines are obviously much more sensitive to the shape of the “wings” of the 8 pphm isopleth and its absolute scale position. These points need to be checked. It is evident, however, that what is important from the control point of view in comparing models is not so much the fit of the isopleths as the fit of the region boundary lines. The sensitivity of the results to the levels of the background precursors also needs to be determined. Once again it is estimated that the effect will be small on the location of the region boundary lines and much more important for the scaling of the “degree of control” lines. For substantial control to an ozone standard higher than 8 pphm, the region boundary line will lie between that for 8 pphm and the marginal control apical line.
--
Conclusion The use of apical lines, particularly those derived by Dodge (9)and Guicherit et al. ( 4 ) ,is not appropriate where significant reductions in ozone are required. For such requirements a hydrocarbon control strategy is clearly indicated. Volume 12, Number 8, August 1978
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Literature Cited (1) Fed. Regist., 36,115486 (Aug. 14,1971). (2) Japanese Govt., “A Case History of Oxidants and Their Precur-
sors in the Atmosphere of Japan”, National Rep. to OECD, Dec. 1973. (3) Guicherit, R., “The Occurrence of PhotochemicalOxidants and Their Precursors in Western Europe”, Proc. Smog 76 Symp.: Occurrence and Control of PhotochemicalOxidant Pollution, Clean Air SOC.of Aust. and N.Z.. DR I11 1-111 25. Macauarie Universitv. ”, Sydney, Australia, Feb. 171i9, 1976. (4) Guicherit. R., Blokziil, P. J., Classe. C. J., “Some Notes on the Abatement of Photochkmical Ozone Production”,TNO Research Institute for Environmental Hygiene, Delft, The Netherlands, unpublished paper, 1977. (5) Daly, N. T., “Photochemical Pollution in Australian Airsheds”, Occasional Paper No. 6, Bureau of Transport Economics, Dept. of Transport, Aust. Govt., Apr. 1977. (6) Dimitriades,B., “An Alternative to the Appendix-J Method for Calculating Oxidant and NO2 Related Control Requirements”, Int. Conf. on Photochemical Oxidant Pollution and Its Control, pp 871-9, EPA-600/3-77-001,Raleigh, N.C., 1976.
(7) Dimitriades, B., “Oxidant Control Strategy: Recent Developments”, ibid., pp 1143-54. ( 8 ) Iverach,D., “Planning for Oxidant Control in Sydney”,Proc. Int. Clean Air Conf., Clean Air SOC.of Aust. and N.Z., Rotorua, N.Z.,
Feb. 17-21, 1975.
(9) Dodge, M. C., “Combined Use of Modelling Techniques and Smog
Chamber Data to Derive Oxidant-Precursor Relationships”,Int. Conf. on Photochemical Oxidant Pollution and Its Control, pp 881-9, EPA-600/3-77-001,Raleigh, N.C., 1976. (10) Hecht, T. A., Seinfeld, J. H., Environ. Sci. Technol., 6, 47 (1972).
(11) Post, K., Bilger, R. W., Atmos. Environ., submitted for publi-
cation.
(12) Chatfield,R., Rasmussen, R. A., “An Assessment of the Continental Lower Tropospheric Ozone Budget”, Int. Conf. on Photochemical Oxidant Pollution and Its Control, pp 121-36, EPA-
600/3-77-001,Raleigh, N.C., 1976. Received for review November 3,1977. Accepted March 7,1978. Work supported by the State Pollution Control Commission of Neu: South Wales and the Australian Research Grants Committee and appears in preprint form as Charles Kolling Research Laboratory Technical Note ER-23.
Coherent Microscopy and Matched Spatial Filtering for Real-Time Recognition of Diatom Species S. K. Case, S. P. Almeida, W. J. Dallas, J. M. Fournier, and K. Pritz Physics Department, Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061
John Cairns, Jr., K. L. Dickson”, and P. A. Pryfogle Center for Environmental Studies and Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061
A method of image enhancement and real-time input of 3-D, microscopic objects (diatoms) into an optical pattern recognition system is described. The system is designed to identify and count diatoms as a means of water pollution assessment. The method consists of directing a low-power laser beam into a microscope to produce a magnified coherent image of the specimen. The enlarged image is passed through optical preprocessing filters to improve the image quality. The enhanced signal is then imaged to a plane that contains a matched spatial filter used for specimen identification. The system does not require the use of time-consuming photographic processes or expensive incoherent-to-coherent liqht, transducers. Examples of results are given and discussed.
Much attention has been given to the use of optical processing for the recognition of various objects (1-4). Our current research uses optical matched spatial filtering to identify and count diatoms. Because diatoms have a rigid frustule and the variation in size among the diatoms of a particular species is within tolerable limits (5-7), matched spatial filtering (MSF) is a good technique for the recognition and enumeration of these objects. This method can be used as a means of automatically determining diatom community structure. This is important since both changes in kinds of diatom species present and changes in the species abundance relationships in a community can provide valuable information for assessing the effects of water pollutants ( 8 , 9 ) . Toward this end, an optical and electronic recognition system is being developed (7, 10). In this paper, the problem of direct input from a microscope to an optical processor is 940
Environmental Science & Technology
addressed. This advance bypasses the time-consuming method of photographic inputs and, in addition, is capable of relaying more information about the diatoms to the processor. A description of the operation of a Fourier transform matched spatial filter processor is given and is followed by a detailed description of the implementation of the microscope input to an optical processor. Matched Spatial Filtering. The taxonomy of diatoms is based primarily on the recognition of certain physical patterns that are apparent in the cleared frustule of the diatom cell. These patterns are visually matched against some reference ’patterns by\a taxonomist in order to identify the diatom. Diatom forms could also be matched against reference forms by an optical recognition system (Figure 1).Here a test object is imaged onto a mask that is a photographic transparency of a reference object. The mask constitutes an optical filter. If the shape of the test object exactly matches the shape of the reference object (a spatially matched filter), then all of the light originating from the test object will pass through the mask and reach the detector. If the shape of the test object differs from that of the reference object, some of the incident light will be blocked by the mask and the output signal will be changed. With this system, the similarity (match) between the test object and the reference object can be measured. It can easily be appreciated, however, that with this system (Figure l),there will be problems with rotation, scaling (size), and position of the test object. The rotation problem can be solved either by rotating the test object or by making rotation-averaged masks that contain superimposed, rotated images of the reference object. Similar averaging techniques can be used, if necessary, to compensate for size variations of
0013-936X/78/0912-0940$01 .OO/O @ 1978 American Chemical Society
