current research Hydrocarbon Composition of the Atmosphere of the Los Angeles Basin-1 967 Aubrey P. Altshuller,' William A. Lonneman, Frank D. Sutterfield, and Stanley L. Kopczynski Division of Chemistry and Physics, Research & Monitoring Office, Environmental Protection Agency, Research Triangle Park, N.C. 27709
Several investigators have measured aliphatic hydrocarbons (Altshuller and Bellar, 1963; Gordon et al., 1968; Neligan, 1962; Neligan et al., 1965; Stephens and Burleson, 1967,1969; Stephens et al., 1967). Data on aromatic hydrocarbons are sparse, coming chiefly from a 1966 study in Los Angeles (Lonneman et al., 1968). The study reported here, conducted September-November 1967, involved for the first time the concurrent analysis of a large number of samples for aliphatic and aromatic hydrocarbons. The samples were collected for a given day either in downtown Los Angeles or in Azusa and were analyzed in a mobile laboratory facility of the Division of Chemistry and Physics. The results obtained make it possible to examine associations among both the aliphatic and aromatic hydrocarbons on a diurnal basis, and also to relate data from the two sampling locations. The hydrocarbon concentrations are also related to oxidant concentrations.
S
everal hundred samples, collected in downtown Los Angeles and Azusa from September through November 1967, were analyzed for individual aliphatic and aromatic hydrocarbons. Results are presented for groups of, and individual, hydrocarbons collected on an hourly average basis. Based on the shape of diurnal curves and ratios of hourly average concentrations of various hydrocarbons-propylene, butene, pentane, toluene, and xylene, most of the aliphatic and aromatic hydrocarbons showed the same variations through most of the morning hours. Thus, atmospheric concentrations of these hydrocarbons, as well as ethylene and acetylene, are largely associated with motor vehicle emissions. Variations in ratios later in the day were considered consistent with the differences in rates of reactions of these individual hydrocarbons. Changes in the concentrations of methane, ethane, propane, and isobutane during the day were not consistent with those of the hydrocarbons just discussed. Based on the present study as well as previous work, it is clear that most of the concentration levels of these particular paraffinic hydrocarbons are not associated with vehicular emissions or with fuel losses. The results for n-butane also suggest that some part of the atmospheric concentration levels for this hydrocarbon also may not be associated with vehicular sources. Differences in the variations in concentration with time of day of several of the Cs and Cl0 aromatic hydrocarbons compared to toluene and the xylenes suggest the possibility of some distinctions in source emission characteristics for various aromatic hydrocarbons. The average ratios of aliphatic hydrocarbons at downtown To whom correspondence should be addressed.
Los Angeles to Azusa and of aromatic hydrocarbons at downtown Los Angeles to Azusa was 1.81 and 2.21, respectively. Between 6 and 9 A.M. the corresponding ratios were 2.02 and 2.28 ppm. Despite the appreciably lower hydrocarbon concentrations at Azusa throughout the day, oxidant concentrations were as high or higher in Azusa as downtown Los Angeles. The difference in functional relationship between oxidant and hydrocarbon at the two locations is attributed to the longer solar irradiation period for the air masses on the trajectories to Azusa. The relationship between hydrocarbon and oxidant concentration levels is complicated in the Los Angeles basin by the occurrence of appreciative concentration levels of ethane, propane, and isobutane associated with background sources of hydrocarbon emissions. Experimental Atmospheric samples were collected in Tedlar bags by means of an automatic time sequential air sampler constructed by the California Air Resources Laboratory, whose personnel also transferred samples from Azusa toethe analytical facilities in downtown Los Angeles. Gordon et al. (1968) have reported on analyses for aliphatic hydrocarbons collected at the same locations. The trapping procedure for the aromatic hydrocarbons was identical to that reported previously (Bellar et al., 1963; Lonneman et al., 1968). The trapping procedure used prior to the analysis of aliphatic hydrocarbons was similar to that for the aromatic analysis with the following modifications: The helium flow rate to the chromatograph was 46.1 cc/min instead of 66.7 cc/min. The sample loop volume was 100 cc instead of 72 CC. The trapped sample was warmed to room temperature by immersing it into a Dewar flask rather than vaporizing it by use of a high-current power supply. The trap packing material was 10% Carbowax 1540 on gas chrom Z instead of glass beads. Light aliphatic hydrocarbons were analyzed directly on silica gel by use of a 5-cc sample volume. Aliphatic hydrocarbons were separated on a 15-ft-long l/s-inch 0.d. stainless steel column packed with 15 % dibutyl maleate on acid-washed chromosorb G and two 1-ft-long '/*-in. 0.d. stainless steel columns packed with 10% bismethoxyethyl adipate on 60-80 gas chrom Z . The 1-ft columns were used at each end of the dibutyl maleate column to prevent liquid-phase bleed-off from reaching the flame ionization detector. The analytical column temperature was maintained at 25°C. Aromatic hydrocarbons were separated on a 300-ft 0.06-in. i.d. copper open tubular column coated with metabis(M-phenoxyphen0xy)benzene and Apiezon L. The column temperature was maintained at 70°C. Ethane was not resolved Volume 5, Number 10, October 1971 1009
from ethylene nor acetylene from propane on the dibutyl maleate column. These light aliphatic hydrocarbons were analyzed directly without a concentration step on a 28-ft '/*-in. stainless steel column packed with 60-80 mesh silicon gel. The column temperature was 25OC and the helium carrier gas flow rate was 55 cc/min. Results for these four hydrocarbons as separate components were available on only 20 to 2 5 z of the samples from the two locations, primarily those obtained during the morning hours. Methane was determined by use of a modified flame ionization analyzer with an activated carbon precolumn to hold back hydrocarbons other than methane (Altshuller et al., 1966). A total of 120 analyses was obtained from samples collected for 14 days in downtown Los Angeles, and 100 analyses were
obtained from samples collected for 12 days in Azusa from 0500 to 1500 hr between Sept. 19 and Nov. 17, 1967. In addition, samples were analyzed throughout two 24-hr periods in downtown Los Angeles. Results for Various Groupings of Hydrocarbons. Concentrations of the aliphatic hydrocarbons and alkylbenzenes individually measured by gas chromatography have been summed up separately both as parts per million by volume and as carbon parts per million. These measurements are presented in Figures 1 and 2 on an average hourly basis for downtown Los Angeles and Azusa. The curves clearly indicate the morning traffic maxima at both locations. The summed alkylbenzene values show a more pronounced traffic peak than do the summed aliphatic hydrocarbon values. The cause of these differences will become evident when the diurnal curves for the individual hydrocarbons are considered. A slight afternoon peak also is evident at Azusa. This peak occurs too early to be readily associated with returning traffic after working hours. It is more likely associated with diluted but still slightly more contaminated air from other locations where hydrocarbon levels during morning traffic were much higher than in Azusa. Summaries of average hourly concentrations, both as ppm by volume and as ppm carbon, are given for aliphatic hydrocarbons and alkylbenzenes measured in downtown Los Angeles and Azusa (Table I). The hydrocarbon concentrations measured in downtown Los Angeles were almost twice as high as those in Azusa. The proportion of aliphatic hydrocarbons to total hydrocarbons is higher at Azusa than in downtown Los Angeles. In both locations alkylbenzenes decrease by midafternoon to 45 to 50% of morning traffic peak values. Aliphatic hydrocarbons in downtown Los Angeles decrease by midafternoon to 60% of morning traffic peak levels, while at Azusa they decrease only to 70% of morning traffic peak. On a 1; 1; /2 I .L-;" I ppm-by-volume basis, alkylbenzenes average 15 to 25 % of the F -- am TIME OF DAY ( m ) + total hydrocarbon measured at both locations. On a ppmFigure 1. Diurnal variations in average hourly aliphatic hydrocarbon carbon basis, they contribute from 27 to 43% of the total concentrations in downtown Los Angeles and Azusa carbon measured at both locations, with about 40% of the total carbon occurring as alkylbenzene during traffic peak hours. The carbon numbers of the hydrocarbons measured average slightly higher in samples from downtown Los Angeles
4
d I- b ;
1,
'
.;!Q
_ I _
0.60
-
0 TOTAL HYDROCARBONS
ALIPHATIC HYDROCARBONS
A ALKYLBENZENES
0.50
-
f
P
P
a i
0.1-
$
0.30
-
0.20
-
0.10
-
; L
0
I
I
I
6
7
8
a.m.
I
I
I
1
1
1
I
5
10
I1
12
13
14 Q.m.
15 -
TIME OF DAY (PSTI
Figure 2. Diurnal variations in average hourly alkylbenzene concentrations in downtown Los Angeles and Azusa 1010 Environmental Science & Technology
-m .ap -
p.m.
-
a.m. _.)
TIME OF DAY (PST)
Figure 3. Average hourly concentrations of aliphatic hydrocarbons, alkylbenzenes, and total hydrocarbons (excluding methane) in downtown Los Angeles, Oct. 1617,1967
Table I. Summary of Average Hourly Concentrations of Aliphatic Hydrocarbons and Alkylbenzenes in Downtown Los Angeles and Azusa
Time
Ppm by volume Alkylbenzenes
Aliphatics5
5-6 6-7 7-8 8-9 9-1 0 10-1 1 11-12 12-13 13-14 15-16 Av
0.278 0.332 0.442 0.439 0.319 0.292 0.278 0.255 0.252 0.250 0.316
0.063 0.082 0.121 0.127 0.096 0,082 0.072 0.061 0,055 0.054 0.084
5-6 6-7 7-8 8-9 9-1 0 10-1 1 11-12 12-13 13-14 15-16 Av
0.181 0.193 0.225 0.184 0.181 0.146 0.146 0.142 0.172 0.156 0.175
0.031 0.042 0.054 0,048 0.042 0.036 0.030 0.024 0.027 0,026 0,039
Sum Aliphatics Los Angeles*+ 0.341 0.88 0.414 1.03 0.563 1.41 0.566 1.38 0.415 1.02 0.374 0.95 0.350 0.90 0.316 0.86 0.307 0.85 0,304 0.81 0.400 1.02 Azusa4e 0.212 0.56 0.57 0.235 0.279 0.69 0.232 0.59 0.223 0.58 0.182 0.47 0.176 0.46 0.166 0.46 0,199 0.52 0.182 0.50 0.214 0.55
Carbon, ppm Alkylbenzenes
Sum
Av carbon no.
0.49 0.65 0.96 1.01 0.77 0.64 0.58 0.50 0.43 0.44 0.68
1.37 1.63 2.37 2.39 1.79 1.59 1.48 1.36 1.28 1.25 1.70
4.0 4.05 4.2 4.2 4.3 4.25 4.25 4.3 4.2 4.1 4.25
0.25 0.34 0.44 0.39 0.36 0.30 0.24 0.19 0.20 0.19 0.31
0.81 0.91 1.13 0.98 0.94 0.77 0.70 0.65 0.72 0.69 0.31
3.8 3.9 4.05 4.2 4.2 4.0 4.0 4.0 3.6 3.8 4.0
a Excluding methane. b Average carbon no. for aliphatic hydrocarbons, 3.3. c Average carbon no. for alkylbenzenes, 8.1. d Average carbon no. for aliphatic hydrocarbons, 3.15. e Average carbon no. for alkylbenzenes, 7.95.
than in those from Azusa. The average carbon numbers also tend to be slightly higher during traffic peak hours and for several hours thereafter. On Oct. 16 to 17, 1967, samples collected throughout the 24-hr period in downtown Los Angeles were measured by gas chromatography. These measurements of aliphatic hydrocarbons, alkylbenzenes, and total hydrocarbons in ppm by volume are shown in Figure 3. The morning traffic peak was even more pronounced than on the average (Figure l), decreasing by late morning and early afternoon to one quarter of the peak value. A late afternoon traffic peak was followed by a
decrease in concentration and a second large peak late in the evening, even more intense than the morning peak. By very early in the morning of the next day, the hydrocarbon concentrations had dropped below the early afternoon minimum of Oct. 16. The alkylbenzenes made up 25 to 3 0 x of the total hydrocarbons measured as ppm by volume. Measured as carbon ppm, the aliphatic hydrocarbons and alkylbenzenes contributed equally to the total concentration during this 24-hr period. The ratios of summed aliphatic hydrocarbons and summed alkylbenzenes measured at downtown Los Angeles to those
Table 11. Ratios of Hydrocarbon Concentrations at Two Locations (Los AngelesiAzusa) Ratio of aliphatic Ratio of Time (PST) hydrocarbonsa alkylbenzenes
Table 111. Ratios of Aliphatic Hydrocarbonsa to Alkylbenzenes at Various Times
0
5-6 6- 7 7- 8 8-9 9-10 10-11 11-12 12-13 13-14 15-16
1 1 1 2 1 2 1 1 1 1
Overall av
1 81
Excluding methane.
54 72 96 39 76 00
90 80 47 60
2 1 2 2 2 2 2 2 2 2
Downtown
03 95 24 65 28 28 40 54 04 08
2 21 a
Time (PST)
Los Angeles
5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-13 13-14 15-16
4.4 4.0 3.6 3.6 3.3 3.6 4.0 4.2 4.5 4.6
Azusa 5.8 4.6 4.2 3.8 4.3 4.1 4.7 5.9 6.4 6.0
Overall av
3.8
4.5
Excluding methane. Volume 5, Number 10, October 1971
1011
The ratios of aliphatic hydrocarbons to alkylbenzenes measured as ppm by volume at the two sampling locations are given in Table 111. These ratios vary from as low as 3.3 to 1 in downtown Los Angeles to as high as 6.4 to 1 in Azusa. The ratios of aliphatic hydrocarbons to alkylbenzenes are consistently higher at Azusa than in downtown Los Angeles. The ratios decrease during the morning traffic hours and then increase again at both locations. These results can be interpreted as showing that vehicular emissions contain a higher proportion of alkylbenzenes than do the other sources of hydrocarbon emissions. Another grouping of hydrocarbons into paraffins, acetylene, olefins, and alkylbenzenes is given in Table IV. The paraffins (excluding methane) and the alkylbenzenes constitute two thirds to four fifths of the hydrocarbons analyzed, depending on hour of day and location. The percentage of olefins and alkylbenzenes reaches 41 % in downtown Los Angeles and 33 in Azusa during traffic peak hours, but these more reactive hydrocarbons make up a smaller percentage of the hydrocarbons analyzed both earlier and later in the day. Comparison
measured at Azusa are given in Table 11. The ratios for alkylbenzenes are consistently higher than the ratios for aliphatic hydrocarbons. This difference also can be explained by consideration of data on individual hydrocarbons. The higher traffic densities in downtown Los Angeles again are manifest in appreciable increases in these ratios between 7 and 9 A.M. for both classes of hydrocarbons. I
I
1.0,
1
1
I
1
I
I
I
I
I
o METHANE, DOWNTOWN LOS ANGELES 0 METHANE, AZUSA
s
1.0
70
c
a.m.
p.m.
I
4
I
I
I
I
I
I
I
I
I AZUSA
TIME OF DAY (PST)
Figure 4. Diurnal variations in average hourly methane concentrations in downtown Los Angeles and Azusa 1w
I
I
I
I
I
I
I
I
I
14
15
DOWNTOWN LOS ANGELES B ETHANE
--
5
6
.
7
8
a.m.
9
10
11
12
13
QAI
'5
16
L
--.-
6
7
8
9
a.m.
TIME OF DAY (PIT)
10
-
11
12
13
14
pp.m. TIME OF DAY (PST)
Figure 5. Diurnal variations in average hourly concentrations of ethane, propane, and isohutane in downtown Los Angeles
15 16 7
Figure 6. Diurnal variations in average hourly concentrations of ethane, propane, and isobutane in Azusa
Table IV. Average Hourly Concentrations (Ppb by Volume) and Percentages of Classes of Hydrocarbons a t Various Times a t Los Angeles Basin Locations (1967) Hydrocarbon type Location 5-6 7-8 8-9 12-13
Paraffins Acetylene Olefins Alkylbenzenes
DO LA^ DOLA
DOLA
DOLA
Overall av Paraffins Acetylene Olefins Alkylbenzenes
55 9 17 19
x
341 Azusa Azusa Azusa Azusa
Overall av a
189 32 57 63
Downtown Los Angeles.
1012 Environmental Science & Technology
128 24 29 31 212
254 77 111 121
45 14 20 21
z
563 61 11 14 14
.
169 21 35 54 279
258 74 107 127
46 % 13 19 22
566 61 8 12 19
137 18 29 48 232
194 25 36 61
x
61 8 12 19
316 59 8 12 21
97 20 25 24 166
58 12 15 15
v-___-
a.m. TIME OF DAY (PST)
-
14
I5
16
Figure 9. Diurnal variations in average hourly concentrations of toluene, xylene, ethyltoluene, sec-butylbenzene, and 1,3,5-trimethylbenzene in downtown Los Angeles
Ql -r Q
71 8' 9__ ' 1;
a.m.
b
111
TIME OF DAY (PST)
Ik
p.m.
s: I
I
I
I
I
30 I
I
I
I
I
I
6
7
8
;--
1
I 9
a.m.
I
I
l
l
I
I
I
I
10
11
12
13
TIME OF DAY (PSTI
'
I
I
l
I
I
I
I
I
I
AZUSA
Figure 7. Diurnal variations in average hourly concentrations of ributane, isopentane, ri-pentane, propylene, and butene in downtown Los Angeles
0 TOLUENE H XYLENES 0-0 SEC.BUTYLBENZENE+ 1,3,5 .TRIMETHYLBENZENE T ETHYLTOLUENES
I
I
I
7
8
I
I
I
9 10 I1 a.m. TIME OF DAY (PSTI
I
I
12
13
-+-
I
I
I4
15
p.m.-*
Figure 10. Diurnal variations in average hourly concentrations of toluene, xylene, ethyltoluene, sec-butylbenzene, and 1,3,5-trimethylbenzene in Azusa
I
14 IS 16 p m . -_I
Figure 8. Diurnal variations in average hourly concentrations of butane, isopentane, +pentane, propylene, and butene in Azusa
I 6
11-
Table V. Light H y d r o c a r b o n Analyses in
Los Angeles Basin-
19670
of the hydrocarbon compositions in downtown Los Angeles and in Azusa shows significant differences in the two locations. These results strongly suggest that not only the concentration but also the composition of hydrocarbons can vary from location to location in the Los Angeles basin. Individual Hydrocarbons. The concentrations of some individual aliphatic hydrocarbons and alkylbenzenes are plotted as a function of time in Figures 4-10. Ethane, ethylene, acetylene, and propane concentrations are listed separately in Table V. Examination of the figures shows that the curves for methane, ethane, propane, and isobutane differ markedly from those for the other aliphatic hydrocarbons and the alkylbenzenes. Curves for propane in downtown Los Angeles and
(Concentrations in Ppb by Volume)
Hydrocarbon Ethane Ethylene Acetylene Propane Ethane Ethylene Acetylene Propane
Location
5-6
Tirne(PST) 7-8 8-9
DO LA^
76 39 32 31 61 20 24 28
79 77 66 28 46 21 21 21
DOLA DOLA DOLA
Azusa Azusa Azusa Azusa
82 74 76 32 47 18 22 21
~~
12-13
43 25 31 32 49 20 23 26
Analyses made between third week in September and fourth week in October 1967. b Downtown Los Angeles. Volume 5, Number 10, October 1971 1013
Hydrocarbon
Table VI. Aliphatic Hydrocarbon and Alkylbenzene Concentrations (Ppb by Vol) Level below which Overall average of Level exceeded by 10% of values occur values 10% of values DOLAa Azusa DOLA Azusa DOLA Azusa Aliphatic hydrocarbons
Methane Ethane ethylene Acetylene propane Propylene n-Butane Isobutane 1-Butene isobutene 2-Butene 1,3-Butadiene n-Pentane Isopentane 1-Pentane 2-methyl-1-but ene Nonane decane
+
+
1700 40 30 3 20
+
+ +
2 1 1 8 12
1500 34 18 1 9 3 1 1 1 4 7
2
1
5
2400 102 76 10.5 46 12 5.5 2 2 21 35
2200 65 43 4 21 7 3.5 1 1 10 16
3500 180 120 21 80 20 10
3000 100 65 8 35 12
5 5
35 56
2 2 16 26
3
8
6
14 2.5 2 3 3 3
50 9 10 11 21 8 15
23 4 4 6 10 6 7
6
23
13
5
3 5
Alk ylbenzenes Toluene Ethylbenzene p-Xylene o-Xylene m-Xylene Propylbenzenes Ethyltoluenes Other C Yand Clo alk ylbenzenes a
10 1 2 2 4 2 3
6 1 1 1 2 1 1
30 5 6.5 12 4.5 7.5
3
1
12
5
5.5
Downtown Los Angeles.
for methane and ethane in Azusa show no increase during peak traffic hours. Curves for methane and ethane in downtown Los Angeles and isobutane at both locations show only a very weak influence of peak traffic. The ratios of methane, ethane, and propane to other hydrocarbons also are much higher than are observed in automotive emissions. Clearly these three aliphatic hydrocarbons arise largely from nonautomotive emission sources. Ethane and propane contribute appreciably to the aliphatic hydrocarbons, and the paraffin grouping previously discussed, particularly at the Azusa sampling location. The contribution of these two hydrocarbons to the summed aliphatic hydrocarbons is responsible for the traffic peak for this group being less pronounced than the peak for the summed alkylbenzenes. The large amounts of these two hydrocarbons among the aliphatic hydrocarbons at Azusa also can be associated with the lower ratios in downtown Los Angeles to Azusa of aliphatic hydrocarbons compared with the ratios in downtown Los Angeles to Azusa for the alkylbenzenes (Table 11). Concentration curves for the other aliphatic hydrocarbons plotted, including n-butane, isopentane, n-pentane, propylene, the butenes, and the alkylbenzenes, are all very similar. The morning traffic peak is clearly evident in all of these curves. A shoulder appears on a number of the curves after the traffic peak at Azusa, probably a contribution from a more contaminated area not far away. In addition, the consistent early afternoon increases in concentration in the curves for Azusa indicate contributions from a more remote and more heavily contaminated location. The ratios of concentrations of a large number of pairs of hydrocarbons were computed from the hourly average concen1014 Environmental Science & Technology
tration values computed for the time period between 5 A.M. and 3 P.M. Ratios of such hydrocarbons as propylene, 1butene and isobutene, n-pentane, isopentane, toluene, and the xylenes were constant within + 5 to +lox from 5 A.M. to 10 A.M. and even throughout the day. Such a result verifies the close similarity in their emission sources, mainly motor vehicle emissions. Ratios of isobutane and n-butane to the hydrocarbons listed above were not constant but fluctuated throughout the day in such a way as to suggest some differences in the emission sources for butanes. Ratios of concentrations of butanes, pentanes, or toluene to propylene or 1-butene and isobutene started to increase late in the morning and increased into the afternoon because of the more rapid deletion of the faster reactions of the olefins. The ratios of toluene to the xylenes were nearly constant throughout the 5 A.M. to 3 P.M. period at downtown Los Angeles and Azusa. However, the ratios of concentrations of toluene to Cgand Cloalkylbenzenes varied considerably at both sites. These variations appeared to occur because of some differences in sources of emissions rather than differences in rates of reaction. The diurnal curves for individual hydrocarbons in downtown Los Angeles show that the midafternoon concentrations are about half of the morning traffic peak concentrations for slowly reacting paraffinic hydrocarbons. Faster reacting hydrocarbons such as propylene, 1-butene and isobutene, 1,3butadiene, xylenes, and 1,2,4-trimethylbenzene decrease by about a factor of three between morning traffic peak hours and midafternoon. The even faster reacting olefins, such as the 2-butenes, decrease by at least a factor of four from the morning traffic peak hours to midafternoon. As a result, the olefins, aside from ethylene, constitute only a few percent of the total
hydrocarbon composition by afternoon in locations such as downtown Los Angeles. Based on the rates of nitrogen oxide-induced photooxidation for butanes and pentanes compared to butene mixtures (Altshuller, 1966; Altshuller et al., 1970), practically all of the decrease in the paraffins during the day must be attributed to dilution owing to an increase in mixing volume rather than to chemical reaction. Since the butenes decrease by a factor of four, half of this decrease in concentration must be attributed to dilution and an equal amount to chemical reaction. The overall average concentrations, the values exceeded by the upper 10 % of individual concentrations, and the values in the lower 10% of concentrations are listed for individual hydrocarbons in Table VI. The upper 10% of values range from 1.5 to 2.5 times higher than the average concentrations. The upper 10 % also are usually 3 to 5 times, and in a few cases 7 or 8 times, higher than the concentrations represented by the lower 10% of values. These values represent the day-by-day variations that occurred during the study. The sum of hydrocarbons (except methane) determined by gas chromatography was related to oxidant concentrations in both downtown Los Angeles and Azusa. The hydrocarbon concentrations, in ppm by volume, are both the 6 to 8 A.M. average levels and the levels that occurred during the hours of maximum oxidant level. Use of the early morning values involves the assumption that the air mass is reasonably homogeneous, so that the hourly oxidant maximum can be related to traffic-peak hydrocarbons. Use of the hydrocarbon values obtained during the hours of maximum oxidant concentrations assumes that these hydrocarbon levels are proportional to traffic-peak hydrocarbon concentrations. The hydrocarbon concentrations from the periods of maximum hourly oxidant should represent the traffic-peak levels modified by further emissions of hydrocarbons, by physical dilution, and by chemical reaction. The hydrocarbon concentrations were averaged for each of the days with maximum oxidant levels ranging from 0.10-0.19 ppm, 0.20-0.29 ppm, and 0.30-39 ppm ; the individual maximum oxidant concentrations in each interval also were averaged. These values are plotted in Figure 11. The oxidant concentrations are extrapolated on the assumption that oxidant will be zero when the hydrocarbons, except methane, are zero. It is evident that there is a different functional relationship between oxidant and hydrocarbons (except methane) at the two locations. While the reason for this type of result can be associated with reactant concentration or composition, it is more likely to be associated with the longer irradiation period for the air masses on the trajectories to Azusa.
0.1, 0
I
I
I
I
I
I
I
I
0.3
0.2
1
1-
"u\.
A HC (HOURLY MAX. OXIDANT). OXIDANT AZUSA
HYDROCARBON CONCENTRATION, ppm by volume
Figure 11. Relationship of maximum hourly oxidant concentrations to hydrocarbon concentrations (excluding methane) in downtown Los Angeles and Azusa
Discussion Results from gas chromatographic analyses for a number of aliphatic hydrocarbons during the same period in 1967 at the same sampling locations were reported previously (Gordon et al., 1968). The present measurements as averaged by hour would not be expected to be identical because the samples were taken on different days. Neverthdess, it is interesting that the average values from the two studies are comparable. For the downtown Los Angeles samples, half of the values of hourly average concentrations of the various individual hydrocarbons agree within 20% and four out of five of the values agree within 30%. For Azusa samples, half of the values of hourly average concentrations agree within 30 % and four out of five agree within 50 %. The aromatics from benzene to 10-carbon alkylbenzenes were measured in the fall of 1966 in downtown Los Angeles (Lonneman et al., 1968). Their overall average of 0.091 pprn for 7-carbon to 10-carbon alkylbenzenes compared well with the present overall average of 0.083 ppm. The upper 10% of values in 1966 exceeded 0.19 ppm whereas in 1967 the upper 10% exceeded 0.15 ppm. Although a large number of hydrocarbons are present in polluted atmospheres, most of these substances contribute little to the total hydrocarbon concentration. Excluding methane, about 80 of the remaining hydrocarbon concentration throughout the day (and even a greater percentage in the afternoon) could be accounted for by 10 hydrocarbons. These hydrocarbons, in order of decreasing average concentration in downtown Los Angeles, are ethane, ethylene, acetylene, nbutane, isopentane, propane, toluene, n-pentane, m-xylene, and isobutane. The same 10 hydrocarbons contributed equally to the total hydrocarbon levels in Azusa, although the ranking of individual hydrocarbons varied somewhat. Of these 10 hydrocarbons, ethane, propane, and isobutane are of very low reactivity, as is acetylene, which is closely associated with vehicular exhaust emissions. Then n-butane, n-pentane, and isopentane are associated with vehicular and petroleum emissions and contribute particularly to ozone formation. The remaining three hydrocarbons, ethylene, toluene, and m-xylene contribute to eye irritation potential as well as to other adverse air pollution manifestations. Ethylene constitutes over half of the olefin fraction. Toluene plus m-xylene constitutes about half of the alkylbenzene fraction. Below these 10 hydrocarbons in concentration are grouped a number of low-to-moderate reactivity paraffinic hydrocarbons, the alkylbenzenes, and propylene. It is possible that concentrations of a few of the highly reactive olefins are being underestimated because these substances react appreciably soon after they are emitted. However, measurements obtained later in the year when the traffic peak occurred at a time of low ultraviolet light intensity did not indicate large increases in concentration of such rapid reactors. The small quantities of rapidly reacting hydrocarbons may well contribute significantly to the rapid conversion of nitric oxide to nitrogen dioxide. They also include many hydrocarbons which tend to form peroxyacyl nitrates (as does mxylene). Even if these rapid reactors were absent entirely, however, all of the products and effects would still be present, although the course of reaction would be slowed somewhat. Because of uncertainties in associating hydrocarbon levels with subsequent oxidant formation, results such as those plotted in Figure 11 must be used with care. If the linear extrapolation of oxidant level down to 0.10 ppm is valid, 0.10 ppm levels of oxidant may be related to traffic-peak hydrocarbon levels of almost 0.3 pprn by volume (excluding Volume 5, Number 10, October 1971 1015
methane). However, oxidant at the 0.10 ppm level also can be associated with concurrently present hydrocarbon levels of less than 0.1 ppm by volume. These relationships are based on too few results to justify generalizations for Los Angeles and such results certainly should not be extrapolated to other cities. In 1962, Neligan reported a greater proportion of the lower molecular weight paraffinic hydrocarbons in atmospheric samples than in automobile exhaust emissions. His conclusion was based on 16 samples collected between 7 and 9 A.M. in downtown Los Angeles. The analytical procedure utilized did not separate ethane from ethylene. Altshuller and Bellar in 1963 confirmed Neligan’s results on the basis of a larger number of samples collected at the same sampling location analyzed directly (without concentration of samples) between 5 :30 A.M. and 1O:OO P.M. Altshuller and Bellar used a technique which separated ethane from propane. The excess of methane, ethane, and propane was attributed to natural gas losses. Several years later Stephens and Burleson (1967) reconfirmed these earlier results based on samples collected in Riverside, Calif. Stephens and Burleson suggested that natural gas losses alone could not explain the observation of the propane to ethane in the atmospheric samples. They suggest gasoline evaporation losses combined with natural gas losses, but Stephens and Burleson also pointed to emissions of light paraffinic hydrocarbons from oil fields as another possible contributor. In a subsequent paper Stephens, et al. (1967) indicated that gasoline vaporization alone definitely could not account for the greater part of the propane excess in ambient air. They analyzed a group of ambient air samples in an oil field area. The analyses did indicate that where air trajectories travel over oil fields, a part of the excess particularly in propane can be attributed to oil field vapors. In a later paper Stephens and Burleson (1969) confirmed the excess of light hydrocarbons on samples collected at several locations in the continental U.S. and Hawaii. Further attempts to explain light paraffinic hydrocarbon levels in Southern California in this 1969 paper still tend toward attributing the ethane and 30 to 40% of the propane to natural gas. The work of Gordon et al. (1968) and the present work based on sampling in 1967 provide the most complete set of available analyses. Diurnal curves on individual hydrocarbons could be developed for the first time. In the present work, such diurnal curves indicate that not only methane, ethane, and propane, but also isobutane, show markedly different diurnal variations than the rest of the aliphatic and aromatic hydrocarbons analyzed. Based on ratios of hourly values of various hydrocarbons to the n-butane, some of the n-butane also appears to arise from different sources other than the pentanes or fuel hydrocarbons. Aside from natural gas losses and gasoline evaporation, losses of light hydrocarbon gases by diffusion through soil from petroleum deposits must be considered. Based on the results listed in Table V, ethane plus propane on a carbon ppm basis can frequently be in the 0.2 to 0.3 carbon range, Addition of a portion of the isobutane and a
1016 Environmental Science & Technology
small amount of the n-butane could increase the average values to around 0.3 carbon ppm with the higher 10% of values attaining concentrations of 0.5 carbon ppm or more. Therefore, based on the considerations discussed above, complete control of automotive emissions, other combustion sources, and organic solvents in the Los Angeles basin still is not likely to reduce hydrocarbons to the level of air quality for hydrocarbons desired (Schuck et al., 1970). If most of these paraffinic hydrocarbons were associated with natural gas leakage and petroleum gas leakage, then these particular hydrocarbons could not be controlled. Ozone formation has been shown to be produced even by ethane and propane in the presence of nitrogen oxides (Altshuller et al., 1969). However, ozone has been produced only at very high ratios of hydrocarbons to nitrogen oxides. If all other hydrocarbons except ethane and propane were completely eliminated and nitrogen oxides reduced by 90%, background ozone levels might be formed, but these levels cannot be estimated without additional irradiation chamber results. Acknowledgment Frank Bonamassa and other members of the technical staff of the California Air Resources Board Laboratory, Los Angeles, Calif., aided materially in initiating the study and in providing sampling capability in this study. Literature Cited Altshuller, A. P., Int. J . Air WaterPollut.,10,713 (1966). Altshuller, A. P., Bellar, T. A., J . Air Pollut. Contr. Ass., 13, 81-7 (1963). Altshuller, A. P., Ortman, G. C., Saltzman, B. E., ibid., 16, 87-91 (1966). Altshuller, A. P., Kopczynski, S. L., Wilson, D., Lonneman, W., Sutterfield, F. D., ibid., 19,787 (1969). Altshuller, A. P., Kopczynski, S. L., Lonneman, W. A., Sutterfield, F. D., ENVIRON. SCI.TECHNOL., 4,503 (1970). Bellar, T. A,, Brown, M. F., Sigsby, J. E., Jr., Anal. Chem., 35, 1924-7 (1963). Gordon, R. J.. Mayrsohn, H., Ingels, R. M., ENVIRON. SCI. TECHNOL., 2; 1117-20 (1968): Lonneman, W. A,, Bellar, T. A., Altshuller, A. P., ibid., pp 1017-20. Neligan, R. E., Arch. Enuiron. Health, 5, 581-91 (1962). Neligan. R. E.. Leonard. M. J.. Brvan. R. J.. Division of Water, Air, and Waste Chemistry Preprint, 150th Meeting, ACS, Atlantic City, N.J., September 1965. Stephens, E. R., Burleson, F. R., J. Air Pollut. Contr. Ass., 17, 147-53 (1967). Stephens, E. R., Burleson, F. R., 62nd National Meeting Air Pollution Control Assoc., Paper 69-122, New York, N.Y., June 1969. Stephens, E. R., Darley, E. F., Burleson, F. R., Mid Year Meeting, American Petroleum Institute, Division of Refining, Los Angeles, Calif., May 16, 1967. Schuck, E. A., Altshuller, A. P., Barth, D. S., Morgan, G. B., J . Air Pollut. Contr. Ass., 20, 297 (1970). -
,
Receiced for review August 13, 1970. Accepted June 8 , 1971.
