Reactivities of Complex Hydrocarbon Mixtures Stanley L. Kopctynski, Richard L. Kuntz, and Joseph J. Bufalini" Chemistry and Physics Laboratory, Environmental Protection Agency, National Environmental Research Center, Research Triangle Park, N.C. 2771 1
Complex hydrocarbon mixtures in the presence of NO, were reacted for several hours in a large irradiation chamber. The hydrocarbons were reacted a t concentrations of 5,, 10, and 20 ppmC in mixes containing l/2 or 1 pprn NO,. Rates of oxidation of nitric oxide, nitrogen dioxide dosage, hydrocarbon consumption, eye irritation, and yields of peroxyacetyl nitrate, formaldehyde, and oxidant were measured. The results suggested that if aromatics are substituted for olefins in a hydrocarbon mixture, a decrease in both oxidant and PAN dosages occurs. Eye irritation increased with increasing aromatics. These reactivity parameters decreased by replacing aromatics with paraffins.
Present control strategies for oxidant and related air quality standards are concerned with the role of hydrocarbons in facilitating oxidant control. However, such strategies are based upon assumptions that the present hydrocarbon composition will be maintained and perhaps the present HC /NO, ratio as well. Previous laboratory chamber studies have shown that different hydrocarbons as well as different HC/NO, ratios, show wide variations in oxidant formation (1-3). Studies have shown that not only is the HC/NO, ratio altered with the new vehicles, but that the hydrocarbon composition is altered when exhaust is catalytically treated ( 4 ) . If automotive hydrocarbon emissions are to be catalytically removed, lead-free gasoline is required. A reduction in lead content in fuel could result in fuel compositional changes-i.e., increases in aromatics or increase in substitution in paraffins if the octane number is to be maintained. Changes in fuel composition have been associated with changes in automotive exhaust and subsequently the reactivity of auto exhaust ( 2 , 5 ) . Methods have been proposed for calculating the photochemical reactivities of hydrocarbon emissions (2, 3, 6). Although these methods give qualitative directions in reactivities, the magnitude of the effect can best be determined in smog chambers with real hydrocarbon mixtures. This study was undertaken to delineate what effect hydrocarbon compositional changes would have on smog manifestations. Both the hydrocarbon distribution-i.e., percent aromatic, olefinic, and paraffinic contents as well as the NO, effect have been investigated.
would contain the equivalent of 2.08 ppm (v/v) of compound while an olefinic mix would contain 3.45 ppm (v/v). An aromatic mix would contain 1.20 pprn (v/v) of compound. When the mixture was studied separately-Le., just the paraffins or the olefins-the same five paraffins, six olefins, and five aromatics were studied. Thus, an olefin mixture would contain all of the olefins shown in Table I without the paraffins and the aromatics. The ultraviolet radiations of the hydrocarbon-NO, mixtures were carried out in a 335-ft3 aluminum chamber equipped with polyvinyl fluoride film windows (8). This chamber was equipped with fluorescent blacklights and sunlights located externally. The k d value for the chamber was 0.4 min-' (9).The light intensity was periodically measured by a Dark Ray Meter (Ultraviolet Product Co.). When low meter readings were obtained, one quarter of the light bulbs would be replaced. A further check of the light intensity was done by running a hydrocarbon (propylene) periodically. Some of our recent modeling studies of the NO2 photodissociation in nitrogen suggests that the irradiation of NO2 in nitrogen for short periods (30 sec to 1 ~ _ _ _ _ _ _ _ _ _
Table I. Simulated Los Angeles Atmospheric Mix Compound
Ppm of C
Paraffins and acetylene Acetylene lsopentane n-Pentane 2-Methylpentane 2,4-Dimethylpentane 2,2,4-Trim et h y I pentane Aromatics Toluene m-Xylene n-Propylbenzene Secondary butyl benzene 1,2,4-Trimethylbenzene Olefins Butene-1 cis-2-Butene 2-Methyl-1-butene 2-Methyl-2-butene Ethylene Propylene
0.53 0.86 1.43 0.51 0.48 0.61
0.98 0.72 0.54 0.60 1.11 0.16
0.17 0.13 0.16 0.72 0.29
Experimental The experimental techniques employed in this study were those in vogue between 1969-71-the period in which this study was performed. The hydrocarbon mixture employed is shown in Table I. I t was composed of acetylene, five paraffins, six olefins, and five aromatics. The proportions were such that the early morning traffic peak a t Los Angeles was simulated (7). Table I1 shows the comparison of the simulated chamber mix to the atmospheric samples collected in Los Angeles. The average hydrocarbon in the paraffin mix is a Cs (C = 4.8); for the olefins a C3 (C = 2.9), and for the aromatics a CS (C = 8.3). Therefore, 10-ppm C of a paraffinic mix 648
Environmental Science & Technology
Table II. Composition of Chamber Mix and Atmospheric Samples A v no. of carbon
Percent cornpn (carbon basis) Nominal chamber mix
Z Z Z
+
Paraffins acetylene Olefins Aromatics
atoms/compd
Dolaa Dola Dola 7-8 2nd St. Nominal 7-8 a.m., Tunnel, chamber a.m., 1968 1970 mix 1968
Dola 2nd St. Tunnel, 1970
45
57
49
4.8
4.7
5.1
15 40
11 32
13
2.9 8.3
3.4
3.1
8.2
8.1
Dola = d o w n t o w n Los Angeles.
38
min), followed by measuring the NO2 after turning off the lights is unsatisfactory as a measure of light intensity. This arises from dark reactions oxidizing NO back to NO2 in the gas phase. Our modeling efforts with 5-ppm NO2 and a K a @value of 0.15 min-' ( K a @= kdh.5) show that a 15% error on the low side is made in the k d value if NO2 is measured as much as 5 min after the lights are turned off (an operation usually employed with wet chemical methods). In this study, the kd value was measured from t = 0 to t = 5 min every minute on a continuous basis. The errors introduced by this technique should be minimized. However, since slow colorimetric wet chemical techniques were used in this study, the exact kd value may be slightly higher than the report value of 0.4 min-'. Unfortunately, the chamber of this study is no longer in existence and the true kd value cannot be remeasured with the use of faster responding chemiluminescent NO2 detectors. The chamber was preheated with infrared lamps before the start of the irradiations and operated a t 32 f l 0 C a t a relative humidity of approximately 33%. Chamber air was prepared by introducing ambient air cleaned by passage through activiated charcoal and particulate filters. The gaseous reactants were charged directly into the chamber by calibrated syringes. Known quantities of liquid reactants were introduced into the chamber by vaporizing into a heated glass line while flushing into the chamber by either air or nitrogen. Nitrogen dioxide was analyzed colorimetrically ( 1 0 ) .Nitric oxide was analyzed as nitrogen dioxide after oxidation with potassium dichromate paper (11 ). Oxidant was determined manually by the colorimetric 1% neutral potassium iodide method (12). Interferences from PAN (peroxyacetyl nitrate) and NO2 were considered and corrections were introduced when oxidant was recorded. A 48% response of PAN and a 15% response of nitrogen dioxide relative to ozone were obtained with the KI reagent. No corrections were made for any other oxidants with this reagent. However, since other oxidants usually require a long period to react with the KI reagent, their interferences would be minimized ( 1 3 ) .Nitrogen pentoxide was not tested for interference. The light olefins were analyzed by means of a gas chromatograph equipped with a dibenzyl ether column on silica gel. The paraffins and butenes on a capillary column coated with squalene. The oxygenates and aromatics were separated by a 1,2,3-tris(2-cyanoethoxy)propanecolumn. PAN and methyl nitrate were separated on a borosilicate column packed with carbowax 500 on Gas ChromZ and were measured on an electron capture detector. Attempts to analyze for peroxybenzoyl nitrate by analogous techniques were unsuccessful. Formaldehyde was analyzed by means of the chromotropic acid method as applied to atmospheric systems ( 1 4 ) .
Results and Discussion The reactivities of a large number of hydrocarbons in terms of oxidant formation, NO oxidation, and hydrocarbon oxidation, have been determined by a number of investigators (15-18). Reactivity assignments are valuable from the standpoint that selective controls of hydrocarbons can be considered for air pollution control strategies. In Table I11 are shown the molar reactivity measurements of the test mixtures. This table shows that on a carbon basis almost every reactivity parameter is greatest with the olefin mix. On a ppm-molar basis (Table IV) the reactivity parameters are greatest with the aromatic mixes, reflecting, in part, the higher molecular weight of the aromatics. A linear summation method ( 1 9 ) was attempted in order to calculate reactivity of a mixture on the basis of reactivi-
Table 111. Reactivity Measures for Individual Classes of Hydrocarbons 10 Pprn of C 0.50 p p m of NO,
+
Atmospheric mix
______ Obsvd
5 - H r oxidant dos- 138 age, p p m x min Oxidant maxi67(190)cL mum pphm 5 - H r P A N dosage, 37 p p m X min 5 - H r NOp dosage, 35 p p m X min Time to oxidize 42 0.40 p p m of NO, min HC consumed in 3 . 7 5 h r , p p m of C Formaldehyde 0.45 yield, p p m 0.6 Eye irritation i n d ex
Calcd
78
41
Paraffin mix
24
Aromatic mix
Olefin mix
118
134
37(> 300)'L 50(109)'' 63(60)'L
29
