Intercomparison of Integrating Nephelometer Measurements Michael G. Ruby and Alan P. Waggoner” Department of Civil Engineering, University of Washington, Seattle, Washington 98195
cient (bRg;termed Rayleigh scattering in honor of its discoverer), and a particle scattering coefficient (bsp). Absorption by gases usually is not important. Absorption by particles may account for -5% of the total extinction in rural areas and has been measured as high as 40% in a polluted urban atmosphere ( 3 ) .In most cases, however, the scattering of light accounts for most of the light extinction. The sum of b~~ and b,, is called the scattering extinction coefficient (bscat). The value of the extinction coefficient in eq 1 depends on the wavelength of light used in making the measurement. Each of the scattering coefficients of gases and particles is proportional to where X is the wavelength of the light and CY is a variable (termed the Angstrom exponent) which is -4 for gases and typically ranges between 0.5 and 2.5 for particles. The value of a for particles is determined by the size distribution of the aerosol, with an CY approaching zero in a situation with many large particle%(such as a fog or dust storm) and an CY of -1.5 common in a polluted urban atmosphere ( 4 ) .
Two distinct models of commercially available integrating nephelometer are currently in use. They have a different spectral response and will report different values of scattering coefficient for the same aerosol. The expected differences for aerosols with different size distributions are calculated, and experimental results are reported. Values of the Rayleigh scattering coefficient for air, Freon-12, and Freon-22 at different elevations are listed to enable operators to properly calibrate their instruments. A calibration procedure is described that allows the instrument to measure the particle scattering coefficient directly and provides increased accuracy, particularly in clean-air areas.
Introduction New U S . federal regulations and the state air-quality implementation plans that will follow require an increased effort to monitor the visibility-degrading aspects of air pollutants. One of the instruments widely used for this purpose is the integrating nephelometer. This paper reports information on the integrating nephelometer that will permit more accurate and more precise collection and reporting of data and compares the performance of two different models of integrating nephelometer. A useful measure of visual air quality is the reduction in the intensity of light, AI, traveling through a short length of sight path, Ax. This is expressed as a simple linear proportion, known as the Beer-Lambert law AIlIo = -b,,tAx (1) where the proportionality constant, bext, is called the extinction coefficient. It is the fractional change in the light intensity per unit length. Since the fraction is unitless, bextcarries only’ the units of inverse length (e.g., m-l). A reported value of bext alone will carry all of the information content of eq 1. An international commission has recently recommended ( 1 ) the use of the symbol ue for the extinction coefficient. Persons working with light scattering measurements should become familiar with this notation and increasingly use it in their own work. As the new notation is as yet unfamiliar to most, this paper will utilize the traditional symbol bext. Another common measure of visual air quality is the visual range, the farthest distance at which a human observer can distinguish the contrast of a large black object against the horizon sky. If a number of simplifying assumptions are made, it is possible to estimate the average extinction coefficient from the visual range, L,, with the Koschmeider equation
where C/C* is the threshold contrast ratio of the object viewed and its background. If CIC* is 0.02 (2% contrast), then -In CIC* is 3.9. The Koschmeider equation then is often written
to provide an estimate of the visual range (rigorously, the “meteorological range”) from the locally measured extinction coefficient. Light extinction is due to the absorption and scattering of light by the gases and particles in the air. Thus, the extinction coefficient is the Sum of a gas absorption coefficient (bag),a particle absorption coefficient (hap), a gas scattering coeffi0013-936X/81/09 15-0109$01.OO/O
@
Measurement of Scattering Coefficient The extinction coefficient may be determined by measuring the fractional reduction in the intensity of a light beam over a fixed path length with a transmissometer. Since this is precisely what is defined by eq 1, bext can be calculated directly. As they require the measurement of a small difference , instruments are most in two large numbers (AI= I - I O ) such frequently used where extinction is large, as in a smokestack or for fog measurements at airports. An alternate approach is to measure the component parts of the extinction coefficient individually. The nephelometer can be used to determine the Rayleigh or particle scattering coefficient by measuring the amount of light scattered out of the beam, which is the same as the reduction of the light in the beam when absorption is small. As its name implies, the integrating nephelometer integrates over scattering angle and theoretically measures the light scattered in all directions. In practice, the integrating nephelometer does not measure the light scattered at small angles. This “truncation error” has been analyzed in detail by several authors ( 5 ) .Typically, it generates uncertainties in the measurement of b,, of f5%. Since both b,, and bRg vary with the wavelength of light, the value measured by the integrating nephelometer will depend on the spectral sensitivity of the specific instrument. Currently, two different models of commercially available integrating nephelometers are used in ambient sampling networks. In some instances, both models are used in the same network, and the data have been compared and combined without regard to their difference. Meteorology Research, Inc., produced and distributed the 1550 model series (including the Model 2050) integrating nephelometer from 1969 to 1979. This model utilizes a pulsed xenon flashtube to illuminate the sample volume. From 1973 to June, 1979, Meteorology Research, Inc., produced the 1560 model series integrating nephelometer which utilizes a constantly lighted quartz halogen lamp and a different detector phototube and light filter. In June, 1979, the model series designation was changed to 1590 to mark several modifications that were introduced. Optically, however, the Models 1560 and 1590 are identical. As a result of the changes in the lamp, the phototube, and the filter, the spectral sensitivities of the 1550 model and the 1560/90 models are different, and the response of the two types to the same aerosol will be different.
1981 American Chemical Society
Volume 15, Number 1, January 1981
109
FIIter,
460
500 660 Wavelength (nm)
1550
700
Flgure 1. Nominal relative spectral response of the EG & G FX-6 light source, the Kodak Wratten No. 2A filter, and the type S detector pho-
totube of the MRI 1550 model series integrating nephelometer.
I
400
500 600 Wavelength (nm)
700
Figure 2. Nominal relative spectral response of the 2900 K light source, the Kodak Wratten No. 58 filter, and the type S-4 detector phototube of the MRI 1560 and 1590 model series integrating nephelometers.
The nominal relative spectral responses of the light source, the light filter, and the detector phototube for the 1550 and 1560/90 models are shown in Figures 1 and 2. The resulting total spectral sensitivities of the integrating nephelometers are shown in Figure 3. The response of the 1550 is broad and shifted toward the blue end of the spectrum, effectively centering on 475 nm. The photon-counting electronics of the 1560/90 models permits the use of a narrow-band green light filter with the resulting response curve centered a t 525 nm. The 1560/90 models have been characterized as approximating the spectral response of the human eye (the photopic response). The photopic response curve included in Figure 3 shows that this is not the case. The use of a broad-band light filter in an integrating nephelometer can increase the uncertainty in the calibration of the instrument. The actual spectral response of the lamp and phototube will vary from one unit to another and may differ significantly from the nominal response curves shown in Figures 1 and 2. With a narrow-band light filter the variability in the lamp and phototube causes much less uncertainty in the relative spectral sensitivity of the instrument. Not only is the characteristic wavelength of the instrument better defined, but the nominal calibration values reported below are much closer to the values which might be calculated for the individual instrument. The wavelength dependence of light scattering by an aerosol will result in different values of the scattering coefficient being measured by instruments with different spectral sensitivity. As described above, b,, is approximately linearly proportional where a typically ranges from 0.5 to 2.5. If th'is waveto kU, length variation is coupled to the spectral response curves 110
Environmental Science & Technology
15+0/90
Wavelength (nm)
Figure 3. Nominal relative spectral sensitivity of the MRI Model 1550 and Model 1560/90 integrating nephelometers and the human eye
(photopicresponse). shown in Figure 3, it is possible to calculate the relative magnitude of the particle scattering coefficient of an aerosol that would be measured by each instrument. Table IV reports the theoretical ratio of a measurement of particle scattering coefficient made by the Model 1550 and Model 1560/90 integrating nephelometers for four different values of a. Since the actual value of a for an ambient aerosol is seldom known and the fraction of b,,,, which is due to particles varies with the mass concent,ration, there would be little justification for utilizing these factors to routinely convert a reading by one model to an equivalent reading by the other. If it is assumed that the absorption component of light extinction is small, then the bext = bscat. This allows a measured bscat to be used in the Koschmeider equation (eq 2a) to estimate the local visual range. However, there are significant ambiguities involved in making such a calculation. Since the Koschmeider equation assumes a human observer, the difference between the spectral response of the human eye and the particular integrating nephelometer which was used to measure bscat must be taken into account. The necessary correction will depend on both the size distribution of the aerosol and the fraction of bscat which is due to particles. Over a reasonable range of expected values of a and bscat, the correction factor for bscat may vary from 0.85 to 0.55 when data from the Model 1550 are used and from 0.95 to 0.85 when data from the Models 1560/90 are utilized. Obviously, the use of a single correction factor could yield misleading results. A detailed discussion of these problems has been presented by Harrison (6). An alternate approach would be to modify the spectral response of the integrating nephelometer to match a photopic response and use these values in the Koschmeider equation. This can be (and has been) done by substituting other light filters for those presently used. However, this is not recommended. The particular filters which accomplish this (the Kodak Wratten No. 106 for the 1550 model and the Kodak Wratten No. 9 for the 1560/90 models) are broadband and create all of the resultant problems of uncertainty in instrument calibration discussed above. Further, the usefulness of bscat (or for that matter contrast, C and C*) data weighted according to a photopic response is quite limited. The human eye responds to relative color, brightness, and detail (7). A distant object with the same photopic brightness as the horizon may or may not be visible depending on the relative colors. In addition, in clean-air areas the conversion from bscat to visual range is questionable, as the simple Koschmeider equation assumes that the illumination is uniform between the viewer and the distant object and beyond and that the locally measured bext is equal to the average be,t. In clean-air areas, objects which can be seen near the visual-range limit
~~~~~
~
Table 1. Value of the Rayleigh Scattering Coefficient of Clean Air Weighted According to the Theoretical Spectral Response of Two Models of Integrating Nephelometer at 25 OC (X10m5m-l) Model 1550
Models 1560/90
2.13 1.99
1.31 1.21
4000
1.85
1.13
6000
1.72
1.06
8000
1.61
0.98
elevatlon, 11
08 2000
a
1 std atrn.
Table II. Value of the Rayleigh Scattering Coefficient of Freon-12 Weighted According to the Spectral Response of Two Models of Integrating Nephelometer at 25 OC (X10-5 m-l) elevation, ft Oa
a
Model 1550
Models 1580190
33.67
20.63
2000
31.36
19.22
4000
29.21
17.90
6000
27.21
16.67
8000
25.34
15.53
1 std atrn.
Table 111. Value of the Rayleigh Scattering Coefficient of Freon-22 Weighted According to the Spectral Response of Two Models of Integrating Nephelometer at 25 OC (X10-5 m-l) elevatlon, ft
08 2000 4000 6000 8000 a
Model 1550
Models 1560190
16.41
10.06
15.28
9.37
14.23
8.72
13.26
8.12
12.35
7.57
1 std atrn.
must be large and elevated because of the curvature of the earth, implying that much of the sight path will pass through thinner and cleaner air at higher altitudes. Thus the use of local bscatdata to estimate a visual range must be done with caution and only for acknowledged approximate estimates of the actual visual range.
Instrument Calibration Any instrument requires calibration to two points, preferably near zero and near the upper end of the range to be measured. The two points most commonly used for nephelometer calibration are the Rayleigh scattering coefficients of particle-free air and the refrigerant gas Freon-12 (CC12FZ). The wavelength dependence of scattering and the different spectral sensitivities of the instruments require different reference values to be used for air and Freon-12 in the two nephelometer models. The manufacturer suggests adjusting the 1550 series nephelometer to read 2.3 X 10-5 m-1 when filled with filtered air and 36.0 X 10-5 m-l when filled with Freon-12. For the 1560/90 series, the manufacturer's calibration values for air and Freon-12 are 1.5 X 10-5 m-l and 23.5 X 10-5 m-l, respectively. In each instance, the manufacturer's recommended setting assumes that the calibration gas is a t 0 "C and 1 atm.
The value of the scattering coefficient of gases varies with temperature and pressure and is inversely proportional to the fourth power of the wavelength of light (A-4). The singlewavelength Rayleigh scattering coefficients calculated by Penndorf (8) may be adjusted to other temperatures and pressures by using the ideal gas law. These may then be coupled to the nominal spectral sensitivity of the individual integrating nephelometers presented in Figure 3 to obtain the total bRg that each should measure. The theoretical values for the two model series of integrating nephelometer at 25 "C are shown in Table I. For comparison, the value of bRg for a phom-l. topic response (at 25 "C and 1atm) is 1.11X Harrison (9) has measured the ratio of the Rayleigh scattering coefficients of Freon-12 and air in an integrating nephelometer a t 250 wavelengths between 350 and 750 nm. The ratio was found to be a constant 15.78. The bR$lr for the two nephelometer models may be multiplied by this ratio to yield a bRgF-I2,as listed in Table 11. Because of the problems associated with the continued release of Freon-12 to the atmosphere, the IMOS Task Force ( 1 0 ) has suggested the use of Freon-22 (CHClF2) as a substitute which is possibly less harmful. Table I11 presents values calculated from the Rayleigh scattering coefficient of bRgF-22, ratio of 7.69 measured by Harrison (9). The following calibration procedure was established in order to operate the integrating nephelometer as an instrument for measuring only b,,. First, the integrating nephelometer is filled with filtered air and adjusted to an output of zero. The span calibration is then established by filling the sample volume with Freon-12 or Freon-22 and adjusting to the value appropriate to the elevation and the actual temperature of the calibration gas. After the span calibration is established, the zero may be offset to any positive value to assure a positive output on clean air. (This is especially important if the instrument is to be used in an airplane.) Finally, this zero calibration value is measured by again filling the instrument with filtered air. The particle scattering coefficient is measured above this base of the Rayleigh scattering of the air. The appropriate setting for the span calibration can be determined from the values given in Tables 1-111. Because the output has been adjusted to zero with filtered air in the first step of the calibration procedure, the correct span calibration setting is found by subtracting the corresponding value in Table I from the appropriate value in Table I1 or 111. If the atmospheric pressure is known, a direct conversion can be made from the 1 std atm value using the factor P/1 (atm), rather than relying on the elevation approximations given. Similarly, it would be desirable to adjust the calibration values to the actual temperatures in the sampling volume. During several calibration runs in conjunction with the measurements reported here, the span gas temperature in the sampling volume was observed to stablilize -8 "C above room temperature in the Model 1560 instrument and -1 "C below room temperature in the Model 1550. If the temperature is known, this adjustment may be made by using the factor 298/(T("C) 273). Because many instrument operators have used the manufacturer's calibration values without realizing the need to adjust them for temperature and atmospheric pressure, it may be advisable to specify that a temperatureand/or elevation-adjusted calibration was utilized. For clean-air locations with b,, below 5 X m-l, instrument zero drift is the major limit on measurement accuracy. In such areas it is best to establish the zero calibration every hour. A once-a-day zero check often will fail to observe a significant intraday drift and can generate a false confidence in the instrument stability. An automated system to accomplish the zero calibration can be constructed from a timer switch, a solenoid valve, a filter, and a blower (of -200-L/min
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capacity). I t is important (1)to select a blower that can develop sufficient head to overpressure the instrument and completely fill it with clean air (-0.3 m water column) and (2) to keep the temperature of the clean air approximately the same as the temperature of the sample. With such a system m-l. it is possible to measure b,, a t levels of 0.3 x The span drift is small in both models of integrating nephelometer. An internal mechanical span calibrator is available in each model of integrating nephelometer. The ratio between the span calibration value read from the internal calibrator and Freon has been observed to be relatively stable. Thus the internal calibrator may be used for daily span checks, and recalibration with Freon may be restricted to a schedule of once every three months, to minimize the release of fluorocarbons to the atmosphere. The use of an air preheater in the sample line (MRI Model 471) is necessary under some circumstances. For example, if the instrument is operated in an air-conditioned room in a hot, humid climate, a fog can be condensed in the sampling volume. Some operators may elect to use a preheater under other circumstances solely to ensure that the humidity of the sample is below 60%. Below this level there is relatively little change in b,, as humidity changes. This permits a measurement that is much less dependent on meteorological variables and emphasizes the contributions of the aerosol. An increase in the air temperature by 8-9 "C is sufficient to reduce the relative humidity of the sample air below 60% under almost all conditions. If the air preheater is used, it is important that it be regulated so that it does not provide excessive heat to the sample and alter the aerosol in uncertain ways. In some circumstances it may be sufficient to make a single adjustment to the applied voltage across the preheater such that an increase in temperature of no more than 10 O C is observed between the ambient air and the inlet to the nephelometer. Under other circumstances it may be desirable to install a controller, built up from two temperature transducers, an IC control, and a triac, to maintain a constant temperature difference.
Experimental Verification In order to verify the theoretical ratio of the particle scattering coefficient, bsp, of the two MRI integrating nephelometer models reported in Table IV, we conducted a comparative test during March and April, 1980, on the University of Washington campus in Seattle, Washington. In addition to the MRI Model 1550 and Model 1561 integrating nephelometers tested, a laboratory integrating nephelometer which can make simultaneous measurements a t three separate, narrow-band wavelengths was operated on the same air sample stream in order to determine the value of a of the ambient aerosol a t any moment. A four-wavelength version of this instrument has been described by Waggoner, Ahlquist, and Charlson ( 1 1 ) .Because some operators are using the air preheater to lower the relative humidity of the sample air, the test was run both with and without the preheater. The instruments were operated in a heated room which was maintained a t 21-22 "C. Temperature transducers were used to monitor the air temperature a t various locations in the test equipment. During the period of the tests, Seattle experienced relatively clean air, with several storms and high-pressure systems moving onshore from the North Pacific. The average b,, measured during the period was 2.9 X 10-5 m-l, and the m-l (both maximum hourly value observed was 12.80 X as measured by the 1561 model, with temperature-adjusted calibration). There were no periods of persistent fog or coincident high relative humidity and high particulate matter concentration during the tests. The value of a for the sampled aerosol can be estimated 112
Environmental Science & Technology
Table IV. Ratio of Particle Scattering Coefficients Measured by Two Models of Integrating Nephelometer Model 1550 Model 1560/90
calcd measured a
wavelength dependence of aerosol
A-0.5
A-1
A-1.5
A-2
1.06
1.12
a
a
1.18 1.18 f 0.05
1.26 1.24 f 0.05
Insufficient data
a Relative frequency of the wavelength dependence of the Angstrom exponent (a)observed in Seattle, Washington, from March 8 to April 4, 1980. Figure 4.
from the values of b,, reported by each of the three channels of the three-wavelength integrating nephelometer and the weighted center wavelength of each channel by fitting a power curve to the data. The frequency distribution of the observations is shown in Figure 4. I t is characterized by a broad maximum between a = 2.1 and 2.3. Distributions from measurements made with and without the air preheater were not statistically different, While this distribution may be representative of a young, low-concentration, urban air mass, it is not suggested that it will be typical of other urban areas. As mentioned above, a value of a of 1.5 has been measured in a more polluted urban area ( 4 ) . The ratio of the measurements of b,, obtained from the Model 1550 and Model 1561 integrating nephelometers was estimated a t a = 1.5 and 2.0 by least-squares linear regression of the data in the immediate neighborhood of these points. The results and probable errors (fincludes 50%of the data) are listed in Table IV. Results from measurements made with and without the air preheater were not statistically different. These results are based on a calibration of the instruments which took into account the actual temperature of the calibration gas. If the calibration had been performed with the values suggested by the manufacturer, without adjusting them to the actual gas temperature, the ratio for a = 2.0, for example, would have been 1.21 f 0.05. Measurements also were made comparing the particle scattering coefficient reported by two Model 1561 integrating nephelometers, one with and one without the air preheater. When the preheater was operated at 120 V ac, it was found to deliver sufficient heat to increase the air temperature by -37 "C. (When it was operated ahead of a Model 1550 integrating nephelometer with its stock air blower, a temperature increase of 21 "C was observed.) Under these conditions the b,, measured in the instrument without the preheater in the sample line averaged more than 1.2 times the b,, measured by the instrument operated with the preheater, over a wide range of relative humidities. I t was possible to obtain a 10 "C increase in the temperature
of the air with only 40 V ac across the preheater. When operated in this manner, the two instruments reported statistically indistinguishable values of b,, a t ambient relative humidities below 60%.At higher relative humidities the ratio of the values measured by the instrument without the preheater to the values measured by the instrument with the preheater increased significantly. This reduction in light scattering by reducing the relative humidity of an aerosol has been described and measured previously by Covert, Charlson, and Ahlquist (12).
S u m m a r y and Conclusions The two models of commercially available integrating nephelometer currently in use will report different values of scattering coefficient for the same aerosol. Since the ratio between the measured values will depend on the size distribution of the aerosol, which is variable from place to place and day to day and seldom known, it is not possible to prescribe conversion factors that could be used to precisely convert the measurements by one model into values that would be measured by the other. Because of this it is essential to clearly specify either the spectral response of the instrument or the manufacturer’s model number when reporting data from an integrating nephelometer. The potential uncertainties associated with the conversion of the scattering coefficient to a visual range (in addition to the limited‘circumstances in which the simple Koschmeider equation is valid) leads us to suggest that data from an integrating nephelometer should always be measured and reported as b,, and not as bscat or as a visual range. Acknowledgment We acknowledge many helpful conversations with N. C.
Ahlquist, R. J. Charlson, and T. V. Larson during the development of this paper.
Literature Cited (1) International Association on Meteorology and Atmospheric Physics, Radiation Commission, “Terminology and Units of Radiation Quantities and Measurements”; National Center for Atmospheric Research: Boulder, CO, 1978. (2) Middleton, W. E. K. “Vision Through the Atmosphere”; University of Toronto Press: Toronto, Ontario, Canada, 1952. (3) Weiss, R. W.; Waggoner, A. P.; Charlson, R. J.; Thorsell, D. L.; Hall, J. S.; Riley, L. A. “Studies of the Optical, Physical, and Chemical Properties of Light Absorbing Aerosols,” in “Proceedings: Carbonaceous Particles in the Atmosphere” (CONF 7803101); Novakov, T., Ed.; Lawrence Berkeley Laboratory: Berkeley, CA, 1979; p 257. (4) Charlson, R. J.; Covert, D. S.; Tokiwa, Y.; Mueller, P. K. J. Colloid Interface Sci. 1972,39, 260. (5) Rabnioff, R. A.; Herman, B. M. J. Appl. Meteorol. 1973, 12, 184. (6) Harrison, A. W. Atmos. Enuiron 1979,13, 645. (7) Henry, R. C. “Psychophysics and Visibility Values”, in “Proceedings of the Workshop in Visibility Values”; Fox, D., Loomis, R. J., Green, T. C., Eds.; U S . Department of Agriculture, Forest Service, 1979; p 74. See also Land, E. M. Sci. Am. 1977,237 (6), 108. (8) Penndorf, R. J. Opt. Soc. Am. 1957,47, 176. (9) Harrison, A. W. Can. J. Phys. 1977,527, 1898. (10) Inadvertant Modification of the Stratosphere (IMOS) Task Force, “Fluorocarbons and the Environment”; U.S. Council on Environmental Quality, 1975. (11) Waggoner, A. P.; Ahlquist, N. C.; Charlson, R. J. “Recent Developments in Nephelometers”, in “Atmospheric Aerosols: Their Optical Properties and Effects” (NASA CP-2004); U.S. National Atmospheric and Space Administration: Langley Research Center, 1977; p TuA4-1. (12) CoGert, D. A.; Charlson, R. J.; Ahlquist, N. C. J. Appl..Meteorol. 1972,1I, 968.
Received for review June 30,1980. Accepted October 20,1980. This work was supported in part by U.S. Environmental Protection Agency Grant No. CR807376010.
Assessment of the Oxidant-Forming Potential of Light Saturated Hydrocarbons in the Atmosphere Hanwant B. Singh,* J. Raul Martinez, Dale G. Hendry, Raphael J. Jaffe, and Warren B. Johnson SRI International, Menlo Park, California 94025
Smog-chamber and field data were analyzed to assess the oxidant-forming potential of light saturated hydrocarbons (LSHCs), which consist of Cx-C6 alkanes, in urban, suburban, and rural atmospheres. Empirical, mechanistic, and computer-simulation approaches were used to estimate that LSHCs could produce from 25 to 125% as much oxidant as alkenes. The broad range of the estimate stems partly from uncertainty in the average daytime atmospheric abundance of the HO radical, which was estimated to vary from 0.5 X lo6 to 10 X IO6 molecules/cm3 (mean daytime HO concentration = 2.9 f 1.9 X lo6 molecules/cm3). The relative ability of LSHCs to produce oxidants is expected to be higher under “rural or transport” conditions when compared to “urban or no-transport” conditions. When present in equal carbon abundance, LSHCs are significantly less effective in oxidant formation than alkenes, an advantage partially offset by the dominant atmospheric abundance of LSHCs.
0013-936X/81/0915-0113$01.00/0
@ 1981 American Chemical Society
Introduction The control of photochemical air pollution has been defined to mean the achievement of a 120-ppb hourly ozone standard not to be exceeded more than once a year on the average. Current control strategies depend on hydrocarbon (HC) abatement as the primary means of controlling photochemical air pollution. Because HCs differ in their ability to produce oxidant‘, a strategy based on the control of those HCs that manifest themselves most strongly in smog formation would constitute a potentially superior technical approach that could also be cost-effective. Based on such thinking, the principle of “substitution” was devised, which states that the emission of more reactive HCs are controlled by substituting them with less reactive ones. In California such a substitution principle has been applied for more than a decade in the form of “Rule 66” ( 1 ) . In recent years, the recognition of the rural photochemical air pollution problem has led to additional smog-chamber Volume 15, Number 1, January 1981 113
