CA1049808A - Chemiluminescent method and apparatus for determining the photochemical reactivity of organic pollutants - Google Patents

Abstract

ABSTRACT:

The photochemical reactivity of organic pollutants in gaseous mixtures such as air, mobile engine exhausts, vapors from organic solvents and the like is determined by reacting the pollutants with oxygen atoms to produce chemiluminescence, and then measuring the difference in the intensity of radiation emitted at two separate wavelengths in the OH(A2.SIGMA. - X2.pi.) system.

Description

This invention relates to methods and apparatus for deter-mining the photochemîcal reactivity of organic pollutants in gaseous maxtures such as air, mobile engine exhausts, vapors from organic solvents and the like.
Hydrocarbons and similar organic pollutants which enter into photochemical reactions in the presence of light and air, yielding ozone and oxidants commonly referred to as photochemical smog, are one of the major current sources of air pollution.
Thus, in order to mDnitor and/or control air pollution, instru-mentation that could monitor at spheric air and sources of these pollutants, particularly motor vehicle exhausts, and give an indication of the total photochemical reactivity of the organic pollutants in the mixture being monitored would be desirable.
One of the m~jor problems in determining the photochemical reactivity of these mixtures is that the ability or propensity of organic pollutants to produce smogs varies greatly depending on the rate coefficients of the many reactions involved. For example, methane can be considered as unreactive and several fold higher n-butane than ethylene concentrations are required to produce similar atmospheric effects. Thus, in order to determine the overall photochemical reactivity of a mixture of orga~iC pollutants, i.e., the relative tendency of these pollutants to participate in oxidant or ozone formation in the presence of light and air`, the concentration of each pollutant present should be multiplied by an individual or group reactivity factor.
A system for classifying organics according to their photochemical reactivity has been proposed by Basil Dimitriades , ,.

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in a paper entitled "The Concept of Reactivity and Its Possible Applications in Control", published in the P m ceedings of the Solvent Reactivity Conference, EPA Report 650/3-74-010, pp. 13-22, National En~ironmental Research Center, U.S. Environ~ental Protection Agency, Pesearch Triangle Park, N.C. 27711, (~ove~ber 1974). In this system, organic pollutants which are considered photochemically non-reactive, such as methane, ethane, propane, acetylene and benzene, are grouped together in one class and given a reactivity rating o~ 1Ø Other organics are divided into four classe~ which are given reactivity ratings, determined by averaging previously measured reactivities for members of these classes, of 3.5 to 14.3.
Barbara Krieger, Mazin Malki and Ralph Kummler have suggested, in their article "Chemiluminescent Reactions of Oxygen Atoms With Reactive Hydrocarbon-q, I. 7000-9000A", Environmental Science & Technology, Vol. 6, pp. 742-744 (August 1972~, that photochemically reactive hydrocarbons can be monitored by their chemiluminescent reactions with oxygen atoms, either in the 7000-9000 angstrom range investigated by the authors or in the vicinity of 3000 angstroms. The authors indicate (page 743) that conditions might be found at which the light intensity wouId be proportional to the product of the rate constant for O atom attack timeS the hydrocarbon concentration. However, the authors do not-identify any such conditions nor do they consider reactivity ratings.
One problem with utilizing the techni~ues suggested by Krieger et al is that acetylenej which is generally considered photochemically non-reactive,- reacts with oxygen atoms and emits radiation at most, if not all, of the wavelengths at which ~hotochemically reactive hydrocarbons emit. The intensity oe , .

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the radiation from acetylene is similar to the intensity of radiation from some of the reactive hydrocarbons. Thus, it would be desirable to have a method for determining the photochemical xeactivity of organic pollutants that either does not respond to acetylene or compensates for its presence.
It is an object of this invention to provide methods and apparatus for determining the overall photochemical reactivit~ of organic pollu~ants in a gaseous mixture via chemiluminescent reactions between the pollutants and oxygen atoms.
Another object is to provide methods and apparatus for determining the overall photochemical reactivity of organic pollutants in a gaseous mixture that conpensate for the presence of acetylene.
According to the invention, the gaseous mixture or sample to be analyzed is mixed with oxygen atoms, which react with both photochemically reactive organic pollutants and acetylene in the sample to produce chemiluminescence. The intensit~ - -of radiation emitted at two separate wavelengths in the OH(A ~- X2~r) system is monitored, and first and second signals that are representative of the intensity of radiation emitted at the fir.st and second wavelengths are produced. The second . . ..
signal is subtracted from the first to eliminate the effects of acetyle~e and produce a third signal that is representative of the overall photochemical reactivity of the or~anic pollutants in the sample.
The preferred apparatus for conducting these analyses - includes a tubular reactor, with radiation sensors such as photo-multiplier tubes positioned beside the reactor at substantially the same position along the axis of the reactor and defining , an observation zone within the reactor. The sample and the oxygen atoms are mixed upstream from the obser~ation zone and flow through the observation zone to an exhaust conduit. The sa~ple and/or oxygen atoms are introduced into the reactor at a point from which the sample and the oxygen atoms take about 10 3 to o-l seconds to flow to the center of the observation zone.
Preferably either the sample or the oxygen atoms are introduced through an inlet oonauit that is adjustably mounted so that the distance between the openings in the conduit through which the sample or oxygen ato~s enter the reactor and the center of the observation zone can be adjusted. This varies the time - it takes the sample and oxygen atoms to flow from this introduction point to the center of the observation zone. Ps will be seen below, adjusting the introduction point in this manner, allows one to vary the correlation betwee~ the photochemical reactivity of the hydrocarbons in the mixture and the resulting signal in any desired direction.
Other objects and aavantages of this invention will be apparent from the following detailed description.
Figures 1 and 2 are graphs of the intensity and spectral distr~bution of OH~A ~ - X21T ) radiation emitted in the atomic oxygen ethylene and atomic oxygen acetylene reactions. ~ -Figure 3 is a schematic diagram o~ one embodiment of this -invention.
Figure~ 1 and 2 illustrate spectral emissions from the atomic oxygen/ethylene and atomic oxygen/acetylene reactions in the area of 300 nanometers, or 3000 angstroms~ These spectra and the mechanisms by which they are produced are described -by K. H. Becker, D. Kley and R. J. Norstrom in "OH* Chemilumin-escence in Hydrocarbon Atom Flamesn, 12th Symoposium (International) ,, ' ' :

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on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania (1969). In order to illustrate the peaks in these curves more clearly, they have been drawn as if both compounds produced roughly the same intensity of radiation. Actually, the intensity of the ra2iation from the atomic oxygen/ethylene reaction, under typical conditions, is 4 to 6 times as great as the intensity of radiation produced by the same concentration of acetylene.
As may be seen from these curves, both ethylene and acetylene emit substantial radiation at 308.9 nanometers. Also, the emission from the atomic oxygen/acetylene reaction at 306.4 or 312.2 nanometers is not very different from the intensity of radiation from this reaction at 308.9 nanometers, while the emission from the a~omic oxygen/ethylene reaction at the latter wavelengths is significantly less than the intensity from this reaction at 308.9 nanometers.
If a mixture of acetylene and ethylene is mixed with a sufficient quantity of oxygen atoms to react with both hydro-carbons, the intensity of the emitted radiation at any given wavelength will be the sum of the intensity of the radiation produced by th~ oxygen atom~ethylene reaction at that wavelength and the intonsity of radiation at that wavelength from the oxygen atom/acetylene reaction. Since the intensity of radiation due to acetylene at 306.4 or 312.2 nanometers is substantially the sa~e as the intensity of radiation from the acetylene reaction at 308.9 nanometers, subtracting the total intensity of radiation at one of the latter wavelengths from the total intensity of radiation at 308.9 nanometers produces a signal or measurement largely attributable to the ethylene in the mixture. The effects of acetylene can be eliminated co~pletely , . , , :

from the final signal by electronically adjusting the signals produced at the first and second wavelengths by acetylene so that the effect of acetylene is the same at both wavelengths.
~hus, the concentration of ethylene in a mixture containing both ethylene and acetylene can be determined by measuring the difference in the intensity of radiation at a first wave- -length, such as 308.9 nanometers, at which both ethylene and acetylene produce significant intensity peaks, and a second wavelength, such as 306.4 or 312.2 nanometers, a~t which acetylene produces substantially the same intensity of radiation but ~ -at which the ethylene emission is significantly reduced.
I have discovered that the spectra emitted by typical photochemically reactive hydrocarbons that are commonly present in the atmosphere and/or in motor vehicle engine exhausts are generally similar to the ethylene spectrum illustrated in Figure 1, and that the intensity of radia*ion emitted by these - ;
hydrocarbons can be correlated reasonably well with the reactivity constants proposed for these hydrocarbons by Dimitriades in the report referred to above, i.e., the intensity of radiation producea by the same concentration of different hydrocarbons can be correlated to the reactivity ratings for the groups in which ~hey are classified by Dimitriades. As will be seen below, the relative response from different hydrocarbons can be varied by changing certain operating oonditions. Thus, the methods and instruments of this invention may alsQ be used with other reactivity classifications~
The-intensity o~ radiation produced by individual hydro-carbons is proportional to the first power of the hydrocarbon concentration, within a factor of about 2, over the range of co~centrations that are likely to be encountered in the atomsphere . ., ' '.

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104980~
motor vehicle engine exhausts or the like. The intensities of radiation emitted by individual hydrocarbons are additive, i.e., the total intensity produced by a mixture of hydrocarbons is substantially equal to the sum of the intensities of the radiation emitted by the individual hydrocarbons. Since the same emissions are obtained from saturated and olefinic hydro-carbons, it may be expected that similar results will also be obtained with other compounds having alkyl groups, i.e., with m~t organic vapors. Thus, by measuring the intensity of radiation emitted by organic pollutants in gaseous mixtures such as air, motor vehicle exhaust or organic solvent vapors, I am able to produce a signal that is related to the sums of the percentages of the individuai organic pollutants present times their reactivity constants, or, in other words, to the overall photochemical reactivity of the mixture.
Any two wavelengths where the acetylene peaks are reasonably close and where there is a substantial difference in the intensity produced by photochemically reactive organics can be used in ' ` the methods and apparatus of this invention. As may be seen fr~m the spectra illu trated in Figures 1 and 2, the acetylene -peak at 306.4 nanoD-oters is closer than the 312.2 peak to the intensity at 308.9. However, the difference in the ethylene i intensity is increased by utilizing 312.2 as the second wave-length. In order to obtain the maximum sensi~ivity, I believe $t is generally preerable to utilize the peaks at 308.9 and 312.2. However, other combinations of wavelengths where the difference in the intensity of radiation from photochemically reactive organics is substantia~ly greater than the difference in the intensity of radiation from the atomic o~ygen/acetylene reaction may also be utilized.
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11)49808 Pigure 3 illustrates one form of apparatus that may be utilized to perform these analyses. This system includes a tubular reactor 10 within which the chemiluminescent reaction is conducted. Tw~ photomultiplier tubes 11, 12, positioned beside the reactor at substantially the same location along the axis of the reactor, define an observation zone 13 withi~
the reactor. Preferably, photomultiplier tubss 11, 12 are matched so that they produce approximately the same response to a similar intensity of radiation at any given wa~elength.
Interference filters 15; 16, centered respectively at 308.9 and 312.2 nanoneters, are positioned between the photomultiplier tubes and the observation zoneO Thus, photomultiplier tube 11 monitors radiation at 308.9 nanometers and photomultiplier tube 12 monitors radiation at 312.2 nanometers.
I prefer to utilize interference filters having a half-width of approximately one nanometer, i e., filters that only transmit one-half as much radiation having a wavelength 0.5 nanometers above or below their center or rated wavelength as they transmit at their center wavelengths. Transmission at center is typically 20%, hence, 0.5 nanometers away it is ", . . .
typically about 10%. Of course, the selection of the filters will be influenced by the wavelengths being monitored, the spectral emissions of hydrocarbons in the vicinity of these wavelengths and the impurities in the sam2le~ -The frequency or wavelength of radiatlon transmitted byinterference filters 15, 16 varies with the incident angle of the radiation upon the surface of the filter. Thus, colli-- mators 17, 18 are positioned between the interference filters and the reactor 10 in order to insure that radiation strikes the filters nearly at right angles. Short lengths of metallic ,'-' 9 :

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honeycomb have proved to be effective collimators in this apparatus.
Lenses 19, 20 may be positioned between the collimators 17, 18 and the reactor 10. These lenses increase the signal produced by the photo~ultiplier tubes by a factor of about 3.
In order to shield the observation zone 13 and the photo-multiplier tubes from extraneous radiation, the filters 15, 16, collLmators 17, 18 and lenses 19, 20 are enclosed in suitable housings 21, 22. Preferably, the rem~inder o~ the reactor is constructed of or covered by an opa~ue material.
The photomultiplier tubes 11, 12 are connected to suitable electronic means, such as a differential electrometer 27, which - produces a signal proportional to the difference between the signals produced by the photomultiplier tubes. The signal from the electrometer is supplied to suitable readout means, such as a gauge or recorder 28, which indicate the overall : ' t / photochemical reactivity of the organic pollutants in the sample being supplied to the reactor.
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The sample to be analyzed is supplied through a sample inlet conduit 31, which cbntains a valve 32 that controls the ..
rate at which the sample is supplied. In order to promote -good m~xing between the sample and oxygen ato~s in the reactor, the end of conduit 31 is provided with a plurality of openings 33, through which the sa~ple is admitted to the reactor.
Sample inlet conduit 31 is mounted in a vacuum feed-through 34 in~the end of reactor 10 so that the conduit may be recipro-cated back and forth along the axis of the reactor in order to vary the distance between the openings 33 through which the sample is admitted to the- reactor ana the center of the . ,~;.,- -, . 1 , , :

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, . . . ' : ' ' ' : , . : .: . ' : ' ' ' ' ' observation zone 13, and thereby adjust the correlation between the photochemical- reactivity of the individual organics being analyzed and the signal produced by the instrument. Moving the openings 33 away from the center of the observation zone increases the proportion of highly reactive organics that react before they reach the observation zone and increases the signal produced by less reactive organics in relation to the signal produced by the same concentration of more reactive ones. Con-versely, if the end of the sample inlet conduit is moved closer to the center of the observation zone 13, the more reactive organics are weighed more heavily in the signal produced by the instrument.
Generally, the sample inlet conduit 31 may be positioned so that the sample and oxygen atoms take about 10-3 to 10-seconds to flow from the openings 33 in the end of conduit 31 to the center of the observation zone. Under typical conditions this can be achieved by positioning the end of the inlet conduit about 1 to 10 centimeters upstream from the center of the observation zone. With typical hydrocarbon mixtures, I prefer to position the end of inlet conduit 31 about 5 centimetcrs upstream from the center of the observation zone, so that the sample and oxygen atoms will take about 2x10-2 second~ to flow fr~m the openings to the center of the observation zone.
The weight given to various organics in the signal produced by the instrument can also be adjusted by Varying the concentra-tions of oxygen atoms within the reactor. Decreasing the oxygen ato~ concentration increases the relative response o~
the more reactive organics because they are consumed less rapidly.
The oxygen atoms are supplied to the reactor by passing an oxygen containing gas through an oxygen inlet conduit 36 which, like the sample inlet conduit 31, contains a ~alve 37 that controls the flow to the reactor 10. The o~gen inlet conduit 36 passes through a microwave discharge cavity 38, or other means of producing oxygen atoms, which converts part of the molecular oxygen in conduit 36 to atomic oxygen. Bends - 39 are provided in conduit 36 between the nicrowave discharge cavity 38 and the reactor 10 in order to prevent radiation from the microwave discharge from reaching the photomultiplier tubes.
Of course, the oxygen could be supplied through a centrally located inlet conduit like conduit 31, instead of the Qample, or both the sample and oxygen could be supplied through conduits of this sort. However, the illustrated arrange~ent is believed to be preferable since it provides a relatively unobstructed flow path for the oxygen atoms, which minimizes recombination of these atoms before they reach the observation zone. I
prefer to supply a mixture of oxygen ana an inert gas such as helium or argon to the oxygen inlet conduit 36 in order to reduce the concentration of molecular o~ygen, which can quench the chemiluminescent reactions and~or scavenge reactive intermediates, within the reactor. A mixture of 9%oxygen and 91% helium has been found to increase the signal from the in~trument by a factor of about 5, at 1.2 Torr, over the signal obtained by supplying pure mDlecular o*ygen to conduit 36. Of course, under so~e circumstances it might be desirable to utilize undiluted oxygen for simplicity~ Undiluted oxygen --would also increase the atomic oxygen concentration in the reactor.
The oxygen containing gas supplied through conduit 36 and the sample supplied through conduit 31 flow through the observation zone 13 and through an exhaust valve 42, which ' : .

serves to regulate the pressure in the reactor and to seal the system when it is not in use, and through an exhaust conduit 43 to a vacuum pump 44, which maintains the desired flow rate through and pressure within the reactor. The pressure in the reactor may be read via a vacuum gauge 45 or mar.ometer connected by conduit 46 to the reactor.
In order to avoid spreading the reaction out along the axis of the reactor, which reduces the percentage of emitted radiation which strikes the photomultiplier tubes and thus reduces the signal~ I pxefer to supply ~he sample-at a relatively low rate, e.g., about 0.5 cc tSTP) per second, and to supply the oxygen containlng gas at about 1 cc (STP) per second.
Passing a 9 percent oxygen/91 percent helium mixture through a typLcal microwave discharge cavity at this flow rates produces an oxygen atom concentration of about 1 percent within the reactor. As was mentioned above, this concentration may be varied, e.g., by varying the microwave discharge power, to achieve the desired correlation between photochemical reactivity ~nd the signal produced.
Under these conditions, this instrument will detect concen-trat~ns of photochemically re w tive hydrocarbons as low as 0.2-1 ppm, depending on the reactivity of the compound. The response of the instrument has been found to be proportional to the first power of the hydrocarbon concentration, within a factor of about 2, from the limit of sensitivity to at least 4,000 ppm. The relati~e response, i.e., the ratios between the signals produced by the same concentrations of different hydrocarbons, also remains substantially constant over this range. As was mentioned above, the total intensity of the radiation emitted by a mixture of hydrocarbons, and the signal produced, are substantially equal to the sum of the intensities , ; . . . .
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or signals that would be proauced by the same concentrations of the individual hydrocarbons by themselves.
- Operation The first step in the operation of this instrument is to start the flow of gas through the microwave discharge and strike the discharge. Zero air can advantaoeously be used in the zeroing operation, which consists of cancelling out any difference in the intensity of the radiation from acetylene at the twv wavelengths being monitored and/or variations in the signals produced by the pho~omultiplier tubes. This is accomplished by supplying a stream of air or other inert carrier gas and zeroing both photomultiplier tube signals, then intro-ducing acetylene and balancing the instrument, e.g., by adjusting the gain on one of the phtomultiplier tubes, to produce a zero signal on the recorder. The instrument i8 then calibrated for photochemically reactive organics by introducing a carrier gas containing one or more of these organics anZ adjusting the instrument to produce a signal on the recorder that corresponds to the known concentration of photochemically reactive organics in the calibration sample. Since the response of this instrument is linear, in most cases this instrument can be calibrated with one data point. However, in some ~ases it may be de~irable to utilize calibration samples containing different concentrations of the photochemically reactive organics in order to test the response of the instrument over a range of concentrations.
Fol~owing calibration~ the instrument can be utilized to determine the photochemical reactivit~ of many different orgànics in gaseous mixtures such as atmospheric air, motor `~
vehicle exhausts, ~apors from organic solvents or the like~
The following examples illustrate some of the results obtainable by the use of this invention.
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Example 1 Reactions between various individual hydrocarbons and oxygen atoms were conducted in a sy~tem similar to the system illustrated in Figure 3. The reactor was a Pyrex (T~) tube 60 centLmeters long and 22 millimeters in internal diameter.
The samples, which consisted of mixtures of scientific grade air and of the hydrocarbon being investigated, were supplied thrDugh a centrally located inlet tube at a flow rate of 0.5 cc (STP) per second. The discharge end of the inlet tube was located 4 centimeters upstream from the center of the observation zone. The oxygen atoms were supplied by passing a 9 percent oxygen/91 percent helium mixture through a 2450 MH3 microwave discharge in a 13 mm. O.D. ~ycor (TM) at a flow rate of 1 cc (STP) per second. The oxygen ato~ concentration in the reactor was estimated to be about 5X1014 cc 1. The pressure inside the reactor was 1.2 Torr.
The reaction was monitored by two matched Centronic Model 4242 photomultiplier tubes mounted as illustrated in Figure 3. One nanometer half-width Corion light filters centered at 308.9 and 312.2 nanometers, and 2.5 centimeter long honeycomb collimators with passages 3 mm wide, were placed between the photomultiplier tubes and the reactor. The photomultiplier tube~ were connected~to a differential electrometer, which in turn was connected to a meter and a chart recorder. The instrument was zeroed and calibrated~ as described above, using samples of air containing 1 to 1250 ppm of acetylene and 0.1 to 1250 ppm of ethylene. After calibration with ethylene, samples containing ~arious o~her hydrocarbons were supplied to the reactor at concentrations from 0.1 to 1250 ppm.
The relative responses-produced by 1250 ppm of these hydro-carbons are set forth in Table I. The relative responses at ....... . ... ~ ... ... .....

other concentrations, under the same operating conditions, are substantially the sa~e as those in the table.

Class V. Reactivitya = 14.3Relative Response Ethylene 100 Propylene 42 Butene-l 79 Butene-2 53 Isobutene 38 Propadiene 133 Butadiene 56 Class IV, Reactivit~ = 9.7 Toluene 55 Class III, Reactivity = 6.5 n-Butane 23 n-Heptane 57 Iso-octane 43 . Class I ,- Reactivity_~ 1.0 Ethane 1.3 -- Propane 7-9 Benzene 20 - Acetyle~e 0 Methane~ dO 2 - a) Reactivity classeæ and numbers as suggested by B. DLmitriades -~b) No detectable signals from methane were obtained at ooncentrations up to 1250 ppm, the highest ooncentration investigated.
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Egample 2 Using the same-system and the same general reaction conditions as in Example 1, tests w OE e conducted to see how the relative signals produced by ethylene and n-butane varied - ~ .
' with the inlet nozzle spacing. Mixtures of cientific grade --, air and 1250 ppm of either ethylene or n-butene were supplied - through the centrally located inlet tube at 0.5 cc (STP)/sec.

The ~ariation in the signals from the individual hydrocarbons - and the ratio between the ethylene signal and the n-butanè
- signal with ~he variation in the distance from the end o~ the inlet nozzle to the center o~ the observation zone are given in the following tabie.

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1049~3~8 -Nozzle Ethylene n-Butane Spacing Signal Signal g cm amps x 10-9 amps x 10- P~atio

2 1850 180 10

3 1100 160 6.9

4 1030 270 3.8 700 320 2.2 In this test, made under the sa~e general conditions as Example 1, both the nozzle spacing and the concentration of oxygen atoms inside the reactor were varied. The oxygen atom ratio was varied by a factor of 2 by varying the power to the microwave cavity. The variation in the signals produced by 125 ppm of ethylene and 125 ppm of n-butane and the ratio between these signals with nozzle spacing (x) and relative oxygen con-centration t~ is shown in the following Table.

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Ethylene n-Butane ;- X . lO] signal signal cm M~ MA - Ratio 6 2 23 100 0.23 6 1 34 11 3.1 2 2 20~ 10 20 As may be seen from the~e examples, the nozzle to obser-vation zone distance and~or the atomic oxygen concentration . . ~ .
in the reactor may be ~aried to suit particular mixtures of organics or paxticular times during a photochemical smog episode.
O course, variou~ other modifications in the methods and apparatus described above will be readily apparent to those skilled in the art. For example, a sphericaI reactor or a short cylindrical reactor with photomultiplier tubes positioned at the end of the reactor could be used instead of the illustrated tubular reactor. This invention may also be practiced with one ., , , , . . ............................ ...... , .. ...... ,.. .,_ ,__ _ , , .. . . .

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~ 1049808 photomultiplier tube that alternately views the reaction zone at each of the two wavelengths, e.g., by using two filters in a filter wheel or by using one interference filter and varying the angle of the light passing through it. These and various other modifications may be made within the scope of this inven-tion, which is defined by the following claims.
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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for determining photochemical reactivity of organic pollutants in a gaseous mixture that may contain acetylene comprising:
mixing said gaseous mixture with oxygen atoms, whereby said oxygen atoms react with photochemically reactive organic pollutants and with acetylene if acetylene is present, to produce chemiluminescences;
producing a first signal and a second signal that are representative of the intensity of radiation emitted at a first wavelength and a second wavelength in the OH(A2.SIGMA. - X2.pi.) system;
subtracting said second signal from said first signal to produce a third signal that is representative of the overall photochemical reactivity of the organic pollutants in said mixture.

2. A method according to claim 1 wherein said first wavelength is 308.9 nanometers and said second wavelength is 306.4 or 312.2 nanometers.

3. A method according to claim 2 wherein said second wavelength is 312.2 nanometers.

4. Apparatus for determining the photochemical reactivity of hydrocarbons in a gaseous mixture that may contain acetylene comprising:
a reaction chamber;
means for supplying a gaseous mixture containing one or more photochemically reactive hydrocarbons to said reaction chamber;

means for supplying oxygen atoms to said reaction chamber, whereby said oxygen atoms react with said photochemically reactive hydrocarbons and with acetylene if acetylene is present, to produce chemiluminescence;
means for producing a first signal and a second signal that are representative of the intensity of radiation emitted at a first wavelength and a second wavelength in the OH(A2.SIGMA.-X2.pi.) system;
means for subtracting said second signal from said first signal to produce a third signal that is representative of the overall photochemical reactivity of the organic pollutants in said gaseous mixture.

5. Apparatus according to claim 4 wherein:
said reactor is tubular;
the means for producing said first signal and the means for producing said second signal comprise first and second radiation detectors positioned beside said reactor at substantially the same location along the axis of the reactor and defining an observation zone within said reactor;
said gaseous mixture and said oxygen atoms flow through said observation zone to an exhaust conduit; and the means for supplying said gaseous mixture to said reactor comprises a sample conduit having an opening upstream from said observation zone through which said sample is admitted to the reactor, the means for supplying oxygen atoms to the reactor comprises an oxygen conduit having an opening upstream from said observation zone through which the oxygen atoms are admitted to the reactor, and at least one of said openings is positioned so that said gaseous mixture and said oxygen atoms take about 10-3 to 10-1 seconds to flow from said opening to the center of said observation zone.

6. Apparatus according to claim 5 including means for adjusting the position of one of said conduits to vary the time required by the gaseous mixture and the oxygen atoms to flow from the opening in the adjustable conduit to the center of the observation zone, and thereby vary the ratio between the signals produced by the same concentration of different pollutants.

7. Apparatus according to claim 5 including means for varying the concentration of oxygen atoms within the reactor and thereby vary the ratio between the signals produced by the same concentration of different pollutants.

8. Apparatus according to claim 7 wherein the means for supplying oxygen atoms to the reactor comprises a microwave discharge cavity and means for passing an oxygen containing gas through said cavity, and the means for varying the concentration of oxygen atoms within the reactor comprises means for varying the power to the microwave discharge.