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~ . , - . APPLICATION OF REACTIVITY CRITERIA I N OXIDANT-RELATED EMISSION CONTROL IN THE USA B . Dimitriades and S . B . Joshi*
ABSTRAC T The u6 e oJ the neacti-v .i,ty cA .i teAian in ox,cdan t-n eta ..ted em .i.abion contAot .i .a aeex,amined in the tight 06 hecenZ n.ebeanch 6.indinga on po .Q.eci tAn ,t t4andpotc .t and on ovWa .wt chem .i,at

ic.y . Occunh.ence o 6 po .U.u t an t tA a .nopon.t .i,d ~n.te&

pne.ted to actate that ne .a.c,t

.i .v .i,t,i.2a os onganic.d be mea.acviced underi .Co ngen .itkad.i,a .ti .on and gneaten on.gan.i

.c- to-n.it

icogen-ox.idez A r-ti .o cond-i.aona than pne .v.i.ou .aty t hough t
. Re.aati,v,i,t

.i,ea unden such cond,i.ti.o ►-cb have been me .a.buned by .the Fnv.i )r .onme.n.taZ Pno .tec.ti.on Agency Son beveAat ongan,i,ca and neau .P
. t6 ate - u.ae.d he t e to buppotc,t a new two-ctada ne.ac,ti.v .i,ty c .ea.4ai6ic .a.ti.on ob onganic emi.ba .i.onb . Pn.ob .2em6 a.aaociated az.i.th the smog chumben meaawcement oS such neac t i .v i ti
..e .z and w .lth t

he uae ob h .inetl.c indices ob neaa .t,i.v .i,ty ane d.i.acuaae.d
. INTRODUCTION The reactivity concept was born out of early laboratory studies tha t showed that organic emissions vary .widely with respect to their ability to cause photochemical oxidant formation in the ambient air . As of today, the concept has been in use in control practices-in the U . S . in the form of regulations calling for selective control of organic solvent emissions . The best known of these regulations is the Rule 66 (1) . It was designed for use in Los Angeles, but it was adopted -- 'intact or slightly modified on -- by several air pollution agencies outside California . The Federal Government has issued a drastically different guideline for use of the reactivity concept (2), but has not objected to the use of Rule 66 by the various State and local governmental agencies .Both the Rule 66 and the Federal Government guideline are not entirely consistent with the latest scientific evidence regarding the role of organic emissions in the ambient oxidant problem (3) . In their present form, these regulations impose two limits to organic solvent emissions : a high limit (e .g . 3000 lb/source/day) for the nonreactive emissions and a low limit (e .g . 15 or 40 lb/source/day) for the reactive ones . Enforcement of two such limits means that the thrust of the regulatory measuresis not so much toward emission reduction as it is toward substitution of nonreactive emissions for reactive ones . In fact, in theory at least, these regulations could cause an increase in the amounts of emissions discharged into the a~~here . This possibility is now thought to have some adverse consequences . B . Dimitriades, U .S . Environmental Protection Agency, Research Triangle Park, North Carolin a S . B . Joshi, Northrop Services, Inc ., Research Triangle Park, North Carolina 70 5
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Because of these problems, the United States Environmental Protectio n Agency (USEPA) is in the process of reexamining the use of' the reactivity concept, in the light of the considerable research findings reported in the last 4 to 5 years . The following is a presentation and discussion of some thoughts that evolved out of such a reexamination
. REASSESSMENT OF THE USE AND UTILITY OF THE REACTIVITY CONCEPT RECENT RESEARCH FINDINGS AND IMPLICATION S There are two aspects of the reactivity issue that need to be reexamined, First, there is the question of whether or not application of the reactivity concept has thus far been demonstrably beneficial . The second aspect pertains to the relative roles of the "reactive" and "nonreactive" organics when such organics are'subjected to pollutant transport conditions . These aspects need to be reexamined both separately and in conjunction
. Recent air quality data in the U .S . have indicated that in some localities the air quality improved, in others it deteriorated, and in others it showed no effect in the years since the initiation of emission control . From detailed studies of the impact of emission control on oxi dant air quality '(4,5), it can be deduced that the impact of the reactivity-related solvent control regulations cannot be directly assessed . This is simply because such impact is masked by '14 the overwhelming effects of other factors such as a reduction in organic auto emission (which is indiscriminate rather than reactivity-related), an increase 4 in nitrogen oxides emissions, growth and/or shifting of human activities, etc
. The judgement submitted here is that the impact of any reactivity-related control measure upon air quality can be assessed only indirectly based on laboratory evidence ; direct atmospheric observation is inconclusive
. The second aspect of the reactivity issue, the one that deals with the relative roles of the "reactives" and "unreactives," likewise can be examined only indirectly based on laboratory evidence . Relative contributions of the "reactive" and "unreactive" organics to ambient oxidant and, in general, the relative roles of such pollutants cannot be determined from aerometric data
. In conclusion, laboratory evidence -- that is, smog chamber evidence -- with all its uncertainties arising from its indirect na ture, continues to provide the only basis for reassessing the use and utility of the reactivity concept . For such a reassessment, therefore, it is necessary first that the .~ recent laboratory evidence be presented and *critically examined
. Recent evidence relevant to the issue consists mainly of smog chamber data suggesting that, under the relatively high organic-to-NO ratio and prolonged irradiation conditions prevailing in transported air masses, many of, the though-' to-be unreactive organics could, in fact, cause significant oxidant buildup (6,7) . This is illustrated by the reported data (6,7) and more pointedly by the data of Table 1, obtained from an on-going EPA study . These interpretations, * however, of the recent smog chamber data and associated conclusions have beenquestioned by some investigators on grounds related to the smog chamber background contamination problem . Specifically, it has been claimed that, i n

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general, norireactive organics exhibit enhanced reactivity in smog chamber s because of the presence of a substantial background of hydroxyl ( OH) radicals in such chambers (8) . Furthermore, such OH concentrations, were estimated (from the EPA data on methane ( CH4) disappearance) to be 1 .5 x 10 - 1 .7 x 108 molecule/cm3 ; that is, they were comparable to or higher than the concentrations observed in the real atmosphere (8) . Such claims, although qualitatively correct, do not invalidate the original .~ interpretation of the recent smog chamber data, namely, that under optimu m
~ . conditions many of the thought-to-be unreactive oraanics can in fact cause significant ozone ( 03) buildup . To .explain, consider all possible situation s J, that cause chamber background OH buildup . First, background OH could form from photolysis of nitrogen- compounds desorbing off the chamber walls . Such a source, however, is unlikely to be important relative to the source created by the NO reactant itself in the chamber . Second, and more likely, background OH could ~orm from photochemical reactions of wall-desorbed organics ( and inorganics ) ganics) and the NOx reactant . However, considering that such desorbed organic s are at almost immeasurably low concentrations, it may be deduced that this S source of OH is also unavoidably present ' in the real atmosphere as natura l o background contamination . Alternatively, the bias caused by the background OH

radical problem can be reduced

- - but not eliminated

-- by subtracting from measured reactivity values the background air reactivity . Finally, irrespectiv e i ns' of the chamber background OH problem, it has been established that conditions of

, prolonged irradiation and an optimum organic-to-NOx ratio do enhance the reactivities of the less reactive, organics . In conclusion, the laboratory evidence suggests that in consideration of

. . .,the pollutant transport phenomenon, the existing reactivity classification of organics should be revised so as to reclassify most . of the "unreactive" organics into the single class of "reactives ." The question of precisely where the borderline should be separating the two classes is a crucial ' but extremely complex one, and it cannot be answered definitively based on the evidence presently available unless some simplifying assumptions are made . In answer to this question, these authors offer one procedure for defining the boundary between reactives and unreactives . This procedure is based on use of reactivity data ;obtained in an appropriate chamber under optimum irradiation time and reactant organic-to-NOx ratio conditions . This procedur p is intended to identify as "nonreactive" those organics which under no circumstances will yield oxidant/03 toncentrations exceeding 0 .08 parts per million ( ppm) . This procedure is illustrated in the next section, using the data of Table 1 . Note, however, that th e ' ; .COnditions under which these data were obtained may not be ttle optimum ones . ~dditionai tests using a variety of organic-to-NO
, ratio conditions should be.-4de -- upon the least reactive organics only -- in order to obtain the reata quisite "optimum " values . ught . REVISED REACTIVITY CLASSIFICATIO N t From the data of Table 1, it would appear rational at first glance that the

ndary point separating the reactive organics from the unreactive ones should n the reactivity value for propane . Such a judgment, however, presupposes tha t

ppm/0•2 ppm mixture of propane/NO would yield the same concentration of 03 t he real atmosphere as in EPA's smog chamber . 707

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1

TABLE I . REACTIVITIES OF ORGANICS MEASURED I N TH E EPA SMOG CHAMBERa 1 (11 )
Reactant Maximum 03 Peak-03 time, Average rate of Organicb/ conc ., ppm c-/ hrs organic disappearance, /

% per hrBackground air 0 .03 -Y -- -- Background air + 0 .01 - - 0 .2 ppm NO x Methane 0 31 0 .05 Acetonitri'le, 0 28 0 .02 io--s-r Benzaldehyde 0 3 .1 2 .4 7 ; ~1,1,1-trichloro- 0 4•6 0 .1 ethan e Chloroform 0 2 .4 0 .8 ~-'Benzene 0 .02 1 .5 3 .1 7 ; =-o 9-2Methylenechloridec9<< :tz 0 .03 1 .8 5 .7 /11

- () Ethane 0 .03 3 .0 0 .5 74-6-6 Propane 0 .08 3 .6 2

. 0 7-; . .,. :' .- .--'Acetylene 0 .10 5 .3 6
. 3 =-0Isobutyl acetate 0 .18 4 .3 4 .3 l r Acetone 0 .18 12 .1 0 . 9 -_,n-Butane 0 .23 11 .9 1 . 4 , Methanol 0 .25 12 .3 1 .3 Acetic Acid 0 .26 13 .7 2 . 0 't-Butanol 0 .28 13 .8 1 . 1 =,Methyl ethyl ketone 0 .30 .8 .3 1 . 5 =i-Propanol 0 .32 8 .6 3 .3 Acrylon .itrile . 0 .37 5 .5 5 .3 =1Perch loroethylene 0 .49 1 .8 12 . 8

a / Chamber was a 400-liter Pyrex glass cylinder irradiated externally with 22 B1ac!<light and 7 Sunlamp fluorescent lamps
. b/Initial reactant concentrations were 4 .0 ppm and 0 .2 ppm for the organic and NO ., respectively . Each organic was tested at least in duplicate . Irradiation was continued until 03 peaked out
.L/Reported data represent observed maximum 03 concentrations less the maxim um 03 concentration observed for background air (0 .03 ppm) . 14 d/Corrected for dilution . Perchloroethylene, methylene chloride, and chloro form showed considerably higher rates of disappearance after peaking of 03• ~ e/Value represents highest 03 yield observed in 6 replicated tests randomly dispersed within the reactivity measurement program
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While the above presupposition may or may not be correct

-- it can never b e checked directly
-- the uncertainties involved are not necessarily prohibitive
. Comparison of the smog chamber and natural sunlight radiation data indicates that the light intensity in the EPA chamber is comparable to that of natural Sunlight . Note that small differences in the radiation factor between " smog chamber and real atmosphere are of no serious consequence because, for low reactivity systems, such differences affect mainly the peak-03 time and very -l ittle the maximum 03 concentration . The effect on peak-03 time, however, (assuming l unlimited irradiation time under pollutant transport conditions) should not cause the reactivity measurement results to be drastically different than those in Table 1 . Relevant important factors, other than radiation, are the chamber background contamination , . the chamber wall surface, and dilution
. The contamination problem has already been discussed ; adjusting the reactivity measurement by subtracting the background air reactivity is simplistic but probably the most acceptable solution to this problem . Thus, for example, after such adjustment, the chamber reactivity of methape is zero, essentially in agreement with the photochemical model predictions (8,9) . Furthe rmore, th e

~ . surface of the EPA chamber ( Pyrex . glass) is not expected to alter drastically the dominant steps of the oxidant formation process . Finally, dilution in the smog chamber tests may not be as intensive as in ordinary real atmospheres . However, the difference can be tolerated with the justification that ( a) it is not believed to be extremely large, and (b) it makes the chamber data represent the "most unfavorable" situation, a situation often used in control deliberations
. Summarizing the reasoning in the preceding paragraph, the smog chamber reactivities listed in Table 1 should not be drastically different than those expected to be manifested in the real atmosphere . This means that the suggestion that the propane reactivity is the boundary point separating the "re- .actives" from the "unreactives" would not be grossly wrong . Furthermore, whatever overall error exists is probably in the direction that will produce high reactivity values, that is, a downward misplacement of the boundary point . Fortunately, however, raising the boundary point above the propane reactivity value, e .g ., by as much as 100 % ( i .e' ., to 0 .1b ppm 03), does not seem to produce A much different classification because the 0 .08-0 .16 ppm interval is not expected to be densely "populated" by reactivity values of organics . Thus, for example, all halogenated paraffins are expected to have reacti v,i ti es comparabl e to those of chloroform, methylene chloride, and l,l,l-trichloroethane ( i .e . , below that of propane), whereas all alcohols, k etones, acids and esters are e xpected to be comparable to or more reactive than acetone, methanol, and acetic a cid, (i .e ., much more reactive than propane)
. These expectations, of course, nted to be verified by testing commonly emitted organics in addition to those listed in Table 1 . This raises the next question, concerned with the reactivity 10jsurement method . The reactivity measurement method commonly used thus far is the smog~er method entailing irradiation of the test organic-NOx mixture and mea14refl ent of resultant oxidant yeild . Use of certain' kinetic entities ( such a s of organic precursor disappearance or rate constant for the OH-organic O ;tioi~ as substitutes for or indices of reactivity has also been proposed

general, the chamber data . on oxidant yield are inherently more vali d .1e quire extensive and tedious

- :smog chamber tests covering a wide range of

70 9

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reactant organi c-to-N0 ratio condi ti ons . The kinetic indices of reactivity are valid only to degrees 8epending on their correlation with the oxidant yield data . In the case of the simple, two-class classification scheme proposed here, the most important reactivity data needed are tho se for the least reactive organics, namely, for the organics comparable to or less reactive than propane . For those organics, the oxidant yield reactivity is extremely difficult to measure reliably because such a measurement is sensitive to interferences from background contamination and organic reagent impurities . Measurements of therate of organic reactant disappearance and of the rate- constant for the OHorganic reaction apparently can be made more reliably (10), but the results do not seem to correlate well with the oxidant yield data, at least, judging from the . data in Table 1 . Thus, of the 18 organics listed in Table 1, benzaldehyde, benzene, methylene chloride, and acety3ene showed rates of organic reactant disappearance that were disproportionately high relative to their oxidant yields (Table 1) . It is not known to what degree this inconsistency reflects experimental error or peculiarities ., in the chemistry of these compounds . Clearly,additional research is needed (a) to establish the relative oxidant-yield reactivities of low-reactivity organics more reliably and for a larger variety ofreactant organic-to-NOX ratio conditions, and (b) to further explore the correlations of oxidant yields with rates of organic precursor reactions and wit h OH-organic reaction rate constants for a variety of organic compounds, especiall y halocarbons . Id CONCLUSIONS A 1 . Laboratory evidence continues to . provide the only basis for assessing theutility and recommending the use of the reactivity criterion in the oxidantcontrol'of organic emissions : related 2 . Occurrence of the pollutant transport phenomenon suggests that the existing reactivity classification of organics be revised to consist of two classes, reactives and unreactives, defined as follows : • The unreactive organics are . those which under optimum irradiation and organic-to-NO ratio conditions in an appropriate smog chamber yield no more than 0 .09 ppm of 03- v Reactive organics are all organics except those found by measurement or theoretical estimation to be unreactive .Alternatively, if the recent EPA smog chamber data are to be accepted, unreaa :tive organics are defined as those with reactivities lower than the reactivity of propane . 3 . Reactivities of unreactive .organics -are extremely difficult to measure reliably . Uncertainity problems are caused either by experimental error associated with the smog chamber measurement or by the questionable validity of the kinetic indices of .reactivity
.

710REFERENCES e 1, Los Angeles Air Pollution Control District . Rule 66 .

2, Federal Register, 36, 15502, Aug . 14, 1971, Appendix B
. 3 . Proceedings of the Solvent Reactivity Conference . EPA-650/3-75-010, Nov

. 1974, U .S . Environmental Protection Agency, Research Triangle Park, N .C
. 27711
. 4 . Trijonis, J ., T .K . Peng, G .T . McRae, and L . Lees . Emissions and Air Quality Trends in the South Coast Air Basin . EQL Memorandum No . 17, Environmental Quality Laboratory, California Institute of Technology, Pasadena, Calif , ' Jan . 1976
.

5 . Wayne, L .G ., K .W . Wilson and C .L . Boyd . "Detection and Interpretation of irly, Trends in Oxidant Air Quality ." Final Report to EPA from Pacific Environmental Services, Inc . Contract No . 68-02-1840, July 1976
. of 6 . Altshuller, A .P ., S .L . Kopczynski, D . Wilson and W .A . Lonneman . Photochemical Reactivities of n-Butane and other Paraffinic Hydrocarbons, JAPCA, 19, p . 787, 1969 . ~'~ ally 7 . Heuss, J . "Smog Chamber Simulation of the Los Angeles Atmosphere ." Scientific Seminar on Automotive Pollutants . EPA-600/9-75-003, Feb . 10-12, 1975 . ~ U .S . Environmental Protection Agency, Research Triangle Park, N .C . 27711 . 8 . Weinstock, W ., and T .Y . Chang . " Methane and NonUrban Ozone ." Paper presented at National APCA Mtg in Portland, Oregon, June 29, 1976
. 9 . Dimitriades, B ., M . Dodge, J . Bufalini, K . Demerjian, and A .P . Altshuller, ing Envir
. Science and Technology (In press) 1976 . ~. ; 10 . Darnall, K .R ., A .C . Lloyd, A .M . Winer, and J . Pitts, Jr . "Reactivity Scale

for Atmospheric Hydrocarbons Based on Reaction with Hydroxyl Radical
. " 

### no Envir . Sci & Technology, 10, No . 7, pp . 692-696, July 1976
. :~. ~1 . Reactivities of Organics Under Pollutant Transport Conditions, S .B . Joshi or and B . Dimitriades, EPA report in preparation, 1976

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