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Distribution of aromatic hydrocarbons in the ambient air
AflnfJspkrlc Pnnrcd
EnwoMunr
I” Crcx
Vol. 19 50.
II, pp
1911-1919.
1985
CiOWd98lr8S f
BnLdm
DISTRIBUTION
OF AROMATIC HYDROCARBONS AMBIENT AIR
53.00 + 0.00
1985 Pergamon
Reu
Ltd.
IN THE
HANWANTB. SINGH*, LOUIS J. SALAS, BRUCE K. CANTRELL~ and ROBIN M. REDMOND* SRI International.Menlo Park, CA 94025, U.S.A. (First rereiced 20 December 1984 and injnalform
11March
1985)
Abstract-Aromatic hydrocarbons were measured in twelve United States cities during 1979-1984 with the help ofan instrumented mobile laboratory. Approximately 100 measurements were made at each site over a 1-2 week period on a round-theclock basis. Measurements at three sites were repeated to obtain seasonal differences. Although variabilities exist in these measurements, the average distribution of aromatic hydrocarbons in urban air is benzene 2l?;. toluene 36 %, ethylbenzene 9 O/b,m/p-xylene I5 %. o-xylene 7 T,. 3/4-ethyl toluene 4 “,,, 1.2,4-trimethylbenzene 6 “D and 1,3,5-trimethylbenzene 2 7;. Average concentrations in the range of l-9 ppb benzene, l-l 7 ppb toluene, l-5 ppb ethylbenzene, 0.610 ppb m/p-xylene, 0.3-4 ppb Oxylene, 0.2-3 ppb 3/4-ethyltoluene. 0.a ppb 1.2.4 ttimethylbenzene and 0.1-2 ppb 1,3,4-trimethylbenzene have been measured. Maximum concentrations at each site are typically less than 10 times the mean value. For both chemical and meteorological reasons. concentrations of aromatic hydrocarbons are at their highest during night and early morning hours with minimum occurring during late morning and early afternoon. Relative diurnal behavior of aromatic hydrocarbons shows that most are depleted at a rate 5-50 times faster than benzene. Based on data in southern California (site 13). it is estimated that a mean OH concentration of at least 2.6 (kO.6) x IO6 molec cm-’ must prevail during 0730-1330 h even in February. A long-term examination of benzene and toluene data from southern California air suggests that their levels may have declined by a factor of 5-10 over the last two decades. In remote atmospheres benzene is present at a concentration of 0.1-0.2 ppb. and is l-3 times more abundant than toluene. It is estimated that such air masses are only 2-7 days away from their urban source. K~J word index:
Aromatic hydrocarbons, benzene, hydroxyl
ISTRODUCTIOS (AHCs) form a significant other fuel oils (IO-25 %) as well as of automobile exhaust. Ambient data collected from a number of cities around the world show that
Aromatic fraction
AHCs
hydrocarbons
of gasoline
are about
nonmethane
and
25 Tl0(on a carbon
hydrocarbon
basis) of the total
concentration;
although
vary between 20 and 40% (Lonneman, 1979; Nelson and Quigley, 1982; Isidorov er nl., 1983; Sexton and Westberg. 1984). The greatest health risk from exposure to aromatic hydrocarbons is due to benzene, which is a suspected carcinogen. In addition, these hydrocarbons are photochemically reactive and contribute to smog as well as to a number of intermediate products that may be additionally toxic. In this study we present ambient data on the distribution of select AHCs from twelve United States cities based on fifteen short term studies performed on a round-the clock schedule. this fraction
is found
to
EXPERIMESTAL Select AHCs were measured using a gas chromatograph equipped with a flame ionization detector (FID). A 2m Present addresses: *NASA-Ames Research Center. Moffett Field, CA 94035. U.S.A. + U.S. Bureau of Mines, Minneapolis, MN 55417, U.S.A. *City of Tucson, Tucson, AZ 85719, U.S.A.
radical, air pollution.
x 118 in stainless steel column packed with 10 %, N, lV-bis (2Cyanoethyl) formamide on acid washed chromosorb P was employed for the separation of these species. The column was maintained at 65°C FID temperature was 275’C. and a carrier flow (helium gas) rate of 45 ml min-’ was adequate for analysis of ambient aromatic hydrocarbons (Hester and Meyer, 1979). Typically, about 400 ml of air were preconcentrated in a stainless steel trap (6” x l/16”) which was filled with glass wool (4” bed length) and held at liquid argon temperature. Figure 1 shows a typical chromatogram of an urban air sample. The analysis time was about 15 min. Calibrations were performed using 5.00 ppm benzene and toluene primary standards. These standards were couunercially obtained from Scott Marrin (Riverside, CA), but were checked against a benzene permeation tube as well. The FID carbon response was assumed to hold for other aromatic hydrocarbons. This simplification can cause errors of approximately f 5 %. Benzene toluene standard was analyzed at least twice daily. A precision of +30/ (one standard deviation) could be achieved based on the analysis of these standards. The overall accuracy of these measurements is expected to be It I5 %. All measurements were performed on site using an instrumented mobile laboratory. An all-stainless steel manifold was used for the sample inlet. The sample inlet height was typically 3 m above ground. A seven-day-per-week, 24-h-perday measurement schedule was followed during these field studies. Sampling frequency of one air analysis every two hours was maintained. Details of sampling and analysis are available elsewhere (Singh et al., 1983). At each of the twelve sites. approximately 100 samples were analyzed. All sites were selected to represent well mixed open air conditions away from major point sources.
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Houston, TX Mae St. and I-10 Fromage Rd. S--17 March, 1984
2.2f2.1 13.5 0.4
1.03
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64.0 1.6
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8.2 0.0
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0.6f 6.6 0.0
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0.8 & 0.9
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1.4f 1.4
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Il.9 0.0
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37.5 0.9
*These measurements were repeated in an attempt to obtain seasonal dilTerence. t Mean concentration f one standard deviation, $Jviaximum measured concernration. $Minimum measured conccntra~ion.
Is*
14*
28.8 1.0
8.7 f 5.9 16.1 0.3
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16.9f 12.3
Downey, CA 7601 East Imp. Ranch0 Los Amigos Hospital 18-27 Feb. 1984
13,
0.3 1.6* 1.6 6.6 0.0
2.6 + 3.5 17.4 0.0
2.6 k 3.3 15.6 0.0
2.1 f 4.2 16.7 0.0
12
7.4 * 9.3 44.7 0.5
Staten Island, NY 4.4 f 6.6 Wild Av. & Victory Blvd. 34.0 0.1 25 April-l May 1984
0.7 f 0.6 3.9 0.0
0.8 f0.8 5.9 0.0
1.6+ 1.5 14.1 0.2
0.8 f 0.8 7.3 0.1
4.3f4.1 30.6 0.4
1.9& 1.7
11.1
Philadelphia, PA Lycoming and Castor St. 4-22 April 1983
II
0.5 f 0.4 2.0 0.1
0.7 f 0.6 2.8 0.1
1.6f 1.5 7.1 0.2
0.8& 1.2 9.5 0.1
2.6f. 1.8 8.8 0.6
Chicago, IL 79:h St. & Lavandale 20 April-2 May, 1981
10
4.6 + 3.3 14.8 0.8
2.9 0.0
3.8 0.0
10.8 0.1
10.5 0. I
46.3 0.4
Carnegie Mellon Campus 64.6 0.4 7--17 April, 1981
9
0.74 1.0 5.7 0.0
7.2 0.0
1.0* 1.0
15.6 0.0
3.3
0.1 io.2 1.3 0.0
6.7 0.0
0.2 + 0.1
4.0 0.0
0.9 f 0.9
1.6& I.8 7.3 0.0
2.9 + 4.8 29.7 0.0 4.0f
0.5 f 0.3 1.4 0.0
0.2 f 0.2 1.2 0.1
0.8 0.0
0.9 f 0.8 5.4 0.0
0.8 f 0.7 3.3 0.1
24.7 0.0
c
400 mf AMBIENT AIR
SAMPLE
Fig. 1. Chromatogram showing a separation of selected aromatic hydrocarbons from ambient air.
RESULTS
AND DISCUSSIOS
Table 1 summarizes the concentrations measured at all 1Sof the urban sites. Expressed in units of parts per billion (ppb = 10s9 v/v) are the average and standard deviation, and maximum and minimum values measured. Average benzene concentrations were in the l-9 ppb range, although maximum concentrations of 6-65 ppb wereencountered. Except in Pittsburgh (Site 9), average toiuene concentrations (1-I 7 ppb) were higher than benzene. This is not surprising since the toluene content of gasoline as well as automobile exhaust is 3-4 times the benzene content (Mayrsohn et al., 1977). The Pittsburgh data may be anomalous because of possible benzene emissions from coke ovens (Mara and Lee, 1978). Average urban concentrations of ethyl benzene, m/p-xylene, o-xylene, 3/4-ethyl
roluene. 1.2.4-trimethylbenrene and l.3.5-trimsthylbenzene were l-5 ppb. 0.6-10 ppb. 0.34 ppb, 0.2-3 ppb, 0.44 ppb and 0.1-Z ppb. rsspectiv-ely. In almost all cases the maximum concentration measured ‘has less than 10 times the mean value. The standard deviations (arithmetical) are substantial, and reilect both mean and random variabilities. Three field experiments (Sites 13-151 were repeated in winter months (February-March 1954) to understand seasonal differences. The southern California data (Sites 1 and 13) show evidence that AHC levels in winter are higher by 50-lOO”,,. This observation is in agreement with the benzene data colletsd by Shikiya et al. (1984) from the south coast air basin. At Houston (Sites 4 and 14) no perceptible ditfsrences during March and May were observed. In Denver (Sites 6 and 15) the AHC levels were significantly lower in March when compared to May. Part of this dityerence could be due to the fact that extremely adverse weather conditions prevailed during the March tieId study and normal city life may have been interrupted. Long term studies are necessary to reliably understand seasonal differences. Table 2 summarizes the average relative concentration of AHCs normalized to the avsrage benzene concentration. The standard deviation ofthese ratios is modest, and in urban centers reflects a pattern of AHC distribution that contains 21 O0benzene. 369, toluene, 9 ‘I-”ethyl benzene, 22 O,,,xylenes, 4 “.,,ethyl toluenes and 8’5, trimethylbenzenes. When one compares this with the average gasoline composition and average exhaust emissions (Mayrsohn er al., 1977), it is clear that the fraction of benzene and toluene is more than the source fraction, while that of xylenes and higher aromatics is less. An important reason for this difference is that compared to benzene. the other aromatics are S-50 times more reactive in the atmosphere. One of the primary means of atmospheric removal of AHCs is their reaction with the OH radical. Removal by reaction with ozone, or due to photolysis, can be estimated to be much less than 1‘,, of the OH removal rate. The OH reaction rates (cm’ molec- ’ j- ’ at 298 K) have been measured to be 1.2 x 10. ” for benzene (Atkinson et al., 1979; Tully er 31.. 198 1).The relative rate constants compared to benzene (at 295 K) are toluene, 4.8; ethylbenzene, 6.7; p-xylene, 9.2; mxylene. 17.5; o-xylene, 10.0, ethyl toluene, 11.0; 1.2,4trimethylbenzene, 33.0, and 1.3,5-trimethylbenzene, 52.0 (Atkinson ef al.. 1979). Figure 2 shows a clear example of the diurnal behavior of these AHCs as observed at Site 13. This site is chosen both for demonstration purposes. and because fair weather prevailed during the entire period of thtr experiment SO that normal urban activities were not interrupted. Figure 2 clearly shows dramatic diurnal variation with minimum late morning and early afternoon concentrations. Similar diurnal behavior for a variety of organic chemicals was reported earlier (Smgh et ai.. 1982: Wathne. 1983).
1915
Distribution of aromatic hydrocarbons in the ambient air
The afternoon minimum could be due to a number of reasons including (1) chemical removal (2) dilution due to the afternoon increase in mixing depth and (3) reduced emissions. To eliminate the role of dilution, Fig. 3 shows the relative mean diurnal behavior of AHCs as normalized against benzene. These relative afternoon minimums clearly show the faster removal of AHCs, as suggested by their greater relative reactivity towards the OH radical. It is further possible to estimate a lower limit of the mean OH radical concentration. This computation is possible if one assumes that the decline in the AHC/benzene ratio between 0600-0900 h and 1200-l 500 h is due strictly to chemical removal. If there were no emissions after the 0600-0900 period, the 1200-l 500 h concentration ratio would probably be somewhat lower: that is why only a lower limit is computed as follows 1
OH,S
In
[AWo/[Blo , f1j
[AHCl,/[Bl,
where KAHc and K, are the OH rate constants for AHC and benzene, respectively. The initial time (t = 0) is taken to be 0730, while the final time (t = 6 h) is 1330. The minimum OH concentrations required to accomplish the decline in non benzene AHC concentration are 3.1 x lo6 (molec cme3), 3.4 x 106,2.0 X 106, 2.3 x 106, 2.8 x 106, 2.8 x 1O’and 1.9 x lo6 for toluene, ethylbenzene, m/p-xylene, o-xylene, 3/4-ethyl toluene, l,2,4 trimethylbenzene and 1,3,5 trimethylbenzene, respectively. (In the case of m/p-xylene the meta isomer is assumed to be twice as abundant as the para isomer based on data of Mayrsohn et al., 1976.) Thus, a morning average OH concentration of at least 2.6 (f0.6)~ 106moleccm-3 was present at Site 13, even during the month of February. This is not significantly lower than the summertime OH concentration estimates of 2.5 (&- 2.0) x lo6 moleccm- 3 (Calvert, 1976) and 2.9 (+ 1.9) x lo6 moleccm-3 (Singh et al., 1981) for this region. Assuming a mean daytime OH abundance of 2.5 x lo6 moleccm-3 (night-time OH is expected to be negligible in comparison) and the rate constant data provided above, the life time (e-fold) of AHCs in the boundary layer of a typical urban atmosphere can be computed (in units of sunlit hours) to be benzene, 93 h; toluene, 19 h; ethylbenzene, 14 h;pxylene, 10 h; m-xylene, 5 h; o-xylene, 9 h; ethyl toluene, 8 h; 1,2+trimethylbenzene, 3 h and 1,3,5-trimethylbenzene, 2 h. In 10 sunlit hours, 10 7: of benzene, 40 % of toluene and nearly 99% of 1,3,5 trimethylbenzene could be depleted via reaction with the OH radical alone. The increased demand for high octane rating in unleaded fuels has led to an increase in the aromatic content. In the exhaust, however, the catalytic converter can prefentially reduce the aromatic fraction. Typical results (EPA, 1978a) suggest that the AHC fraction is 17 % in the exhaust of cars with catalyst, and 24 % in cars without catalyst. Figure 4 shows average
5
15
10 TIME
OF
20
25’
DAY
0
5
10 TIME
15
25
25
OF DAY
Fig. 2. Diurnal behavior of aromatic hydrocarbons in southern California (Site 13).
benzene and toluene concentrations measured at several sites in southern California. Because of the possibility of strong seasonal trends (Shikiya et al., 19841, data taken during winter months is excluded from Fig. 4. Further, it should be noted that each data point in Fig. 4 represents an average of a large number of samples, The pre-1970 data are taken from Ahshuller and Beltar (1961), Lonneman et al. (1968), and Leonard et at, (1976). Subsequent data are taken from this study, the results summarized in Brodzinsky and Singh (1982), and from Shikiya et al. (1984). Brodzinsky and Singh (1982) study is a comprehensive review of all available volatile organic chemi~l data
for the period of 1970-1980. It isclear from Fig. 4 that benzene and toluene concentrations have declined dramatically during the last two decades. Since the mid-1970s, however, the rate of decline in benzene concentrations appears to have slowed considerably. It is reasonable to assume that the concentrations of other AHCs have declined similarly over the last two decades. In recent years, attempts also have been made to measure concentrations of AHC in clean remote atmospheres. Table 3 summarizes some of these data. It is clear from Table 3 that benzene and tolueneare the dominant species. Unlike the urban atmosphere, the
Distribution of aromatic hydrocarbons in the ambient air
two dominant species, and the source of these aromatic hydrocarbons appears to be anthropogenic activity predominantly from fuel consumption. These chemicals are highly reactive with lifetimes that may vary from a few hours to several days. These species are transported to remote atmospheres and are present in detectable concentrations after a residence time of several days. There is indication that over the last two decades the urban levels of aromatic hydrocarbons
TOLUENE
1.S-
1917
i
have declined
significantly.
Acknowledgements-Research funded by the Environmental Protection Agency (EPA) under cooperative agreements Contract Nos. R805990010 and CR8032802010. We are thankful to many individuals for the use of facilities during the conduct of these field studies. REFERENCES
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-
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0”
COMPUTED
3
6
9
12
CONCENTRATION FOR THE
PERIOD
1s
IN UNITS OF 0730
16
11
2.
OF MOLECULES
on-’
TO 1330 HOURS
Fig. 3. Mean diurnal concentration of aromatic hydrocarbons relative to benzene (ppb/ppb) in southern California (Site 13).
mean toluene/benzene ratio at mid-latitudes (Table 3) is only 0.3-1.0. Assuming an OH radical concentration of6x 105moleccm-3’ m the clean atmosphere (Singh, 1977; Volz et al., 1981)and an exclusive urban source, it is possible to compute the residence time of an air mass needed to change toluene/benzene ratio from 1.7 to 0.3-I .O.From Equation (1) it is estimated that such an air mass would be only 2-7 days away from its source. To date it is not known if significant nonanthropogenie sources of AHCs exist. CONCLUSIONS
Aromatic hydrocarbons can be measured in urban and remote atmospheres. Toluene and benzene are the
Altshuller A. P. and Bellar T. A. (1961) Gas chromatographic analysis of hydrocarbons in the Los Angeles atmosphere. J. Air Pollut. Control. Ass. 13, 81-87. Atkinson R., Darnall K. R., Lloyd A. C., Winer A. M. and Pitts J. N., Jr. (1979) Kinetics and mechanisms of the reaction of the hydroxyl radical with organic compounds in the gas phase. Adv. Photo&em. 11.375-488. Brodzinsky R. and Singh H. B. (1982) Volatile organic chemicals in the atmosphere; and assessment of available data, EPA-600/8-83-027. Final report prepared by SRI International. Calvert J. G. (1976) Hydrocarbon involvement in photochemical smog. Enuir. Sci. Technol. 10, 256-262. Eichmann R., Ketseridis G., Schebeske G., Jaenicke R., Hahn J., Wameck P. and Junge C. (1980) n-Alkane studies in the troposphere-II. Gas and particulate concentrations in the Indian Gcean air. Atmospheric Encironmenr 14, 695-703. EPA (1978a) Air quality criteria for ozone and other hydrocarbons. EPA-m/8-78-004. Greenberg J. P. and Zimmerman P. R. (1984) Nonmethane hydrocarbons in remote tropical, continental, and marine atmospheres. .I. geophys. Res. 89, 4767-4778. Hester N. E. and Meyer R. A. (1979) A sensitive technique for the measurement of benzene and alkylbenzenes in air. Envir. Sci. Technol. 13, 107-109. Isidorov V. A., Zenkevich I. G. and Ioffe B. V. (1983) Methods and results of gas chromatographic-mass spcctrometric determination of volatile organic substances in an urban atmosphere. Atmospheric Environment 17, 1347-1353. Leonard M. J., Fisher E. L., Brunelle M. F. and Dickinson J. E. (1976) Effects of the motor vehicle control program on hydrocarbon concentrations in the central Los Angeles atmosphere. J. Air. Pollut. Control Ass. 26, 359-363. Lonneman W. A. (1979) Ambient hydrocarbon measurements in Houston. Proc. Ozone/Oxidants Interactions with the Total Environment. Houston, TX, 14-17 October, 278-287. Lonneman W. A., Bellar T. A. and Altshuller A. P. (1968) Aromatic hydrocarbon in the atmosphere of the Los Angeles Basin. Enuir. Sci. Technol. 11, 1017-1020. Mara S. J. and Lee S. S. (1978)Assessment of human exposure to atmospheric benzene. EPA-450/3-78-031.
Maynohn H., Kuramoto M., Crabtree J. H., Sothern R. D. and Mano S. H. (1976) Atmospheric hydrocarbon concentrations JunoSeptember, 1975. DTS-76-15. California Air Resources Board. Mayrsohn H., Crabtree J. H., Kuramoto MI, Sothem R. D. and Mano S. H. (1977) Source reconciliation of atmospheric hydrocarbons, 1974. Atmospheric Encironment 11, 189-192.
^ 8
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62
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70
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72
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76
74
78
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I 80
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I 82
84 1984
YEAR
1960
I
Fig. 4. Benzene and toluene trend in the atmosphere of the South Coast Air Basin of California.
Table 3. Midlatitude background concentrations of some aromatic hydrocarbons in remote marine and continental
Chemical Benzene
Tolusne
o/m/p-Xylenes
Location
atmospheres
Average concentration (ppb)
Source
Atlantic air (35” N) Pacific air (45’ N) Niwot Ridge* (40; N) Niwot Ridge (40” N) PaciEc air (47” N) Northern hemisphere Southern hemisphere
0.07 f 0.03 0.23 + 0.04 0.24 0.16*0.08 0.23+0.!1 0.05 0.01
Atlantic air (35’ N) Pacific air (45” N) Niwot Ridge (WN) Niwot Ridge (40”N) Pacific air (47” N) Northern hemisphere Southern hemisphere
0.02 f 0.01 0.22 f 0.04 0.14 0.13~0.17 0.13f0.08 0.02 0.006
251 (4) (5) (5) (5)
Atlantic air (48” N) Pacific air (W-O’ N) Indian Ocean (41’ S)
o.oi-O.02 0.04-0.02 0.003-0.005
(6) (5) (6)
0) Ifi (4) (5) (5) (5) (1)
Source: (I) Penkett (1982); (2) Rasmussen and Khalil(l983); (3) Greenberg and Zimmerman (1984); (4) Roberts et al. (1984); (5) Nutmagul and Cronn (1985); (6) Eichmann et al. (1980).
‘Niwot Ridge is located in the Colorado Rockies.
Distribution of aromattc hydrocarbons in the ambient air Nelson P. F. and Quigley S. M. (1982) Non-methane hydrocarbons in the atmosphere of Sydney, AustTalia. Encir. Sci. Technol. 16, 650-655. Nutmagul W. and Cronn D. R. (1985) Determination of selected atmospheric aromatic hydrocarbons at remote continental and oceanic locations using photoionization tlash-flame ionization detection. J. armos. Chem. (to be submitted). Penkett S. A. (1982) Non-methane organics in the remote troposphere. In Atmospheric Chimisrry (edited by Goldberg E. D.). DD. 329-355. Dahlem Konferenzen Springer, New York. Rasmussen R. A. and Khalil M. A. K. (1983) Atmospheric benzene and toluene. GeophJs. Res. Lerc. 10, 10961099. Roberts J. M.. Fehsenfeld F. C., Liu S. C., Bollinger M. J., Hahn C., Albritton D. L. and Sievers R. E. (1984) Measurements of aromatic hydrocarbon ratios and NO, concentrations in the rural troposphere: estimation of air mass photochemical age and NO, removal rate. Atmospheric Environment 18. 2421-2432
Sexton K. and Westberg H. (1984)Nonmethane hydrocarbon composition of urban and rural atmospheres. Atmospheric Environment 18, 1125-t132. Shikiya J., Tsou G., Kowalski J. and Leh F. (1984) Ambient monitoring of selected halogenated hydrocarbons and benzene in the California South Coast Air Basin. 77th
1919
Annual APCA Meeting, Paper No. 84-1.1.24-29 June, San Francisco. Singh H. B. (1977) Atmospheric halocarbons: evidence in favor of reduced average hydroxyl radical concentration in the troposphere. Geophys. Res. Lat. 4, 101-104. Singh H. B., Martinez J. R., Hendry D. G., Jaffe R. J. and Johnson W. B. (1981) Assessment of the oxidant-forming potential of light saturated hydrocarbons in the atmosphere. Enu. Sci. Technol. 15, 113-l 19. Singh H. B., Salas L. J. and Stiles R. E. (1982) Distribution of selected gaseous organic mutagens and suspect carcinogens in ambient air. Em. Sci. Technol. 16, 872-880. Singh H. B.. Salas L. J., Stiles R. E. and Shigeishi H. (1983) Measurements of hazardous organic chemicals in the ambient atmosphere. Final Report prepared by SRI International. NTIS PB83-156935. Tully F. P., Ravishankara A. R., Thompson R. L., Nicovich J. M.. Shah R. C., Kreutter N. M. and Wine P. H. (1981) Kinetics of the reactions of hydroxyl radical with benzene and toluene. J. phys. Chem. 85, 2262-2269. Volz A., Ehhalt D. H. and Denvent R. G. (1981) Seasonal and latitudinal variations of ‘%O and the tropospheric concentration of OH radicals. J. geophys. Rex 86,5163-5171. Wathne E. M. (1983) Measurements of benzene, toluene and xylenes in urban air. Atmospheric Environment 17, 1713-3722.
EnwoMunr
I” Crcx
Vol. 19 50.
II, pp
1911-1919.
1985
CiOWd98lr8S f
BnLdm
DISTRIBUTION
OF AROMATIC HYDROCARBONS AMBIENT AIR
53.00 + 0.00
1985 Pergamon
Reu
Ltd.
IN THE
HANWANTB. SINGH*, LOUIS J. SALAS, BRUCE K. CANTRELL~ and ROBIN M. REDMOND* SRI International.Menlo Park, CA 94025, U.S.A. (First rereiced 20 December 1984 and injnalform
11March
1985)
Abstract-Aromatic hydrocarbons were measured in twelve United States cities during 1979-1984 with the help ofan instrumented mobile laboratory. Approximately 100 measurements were made at each site over a 1-2 week period on a round-theclock basis. Measurements at three sites were repeated to obtain seasonal differences. Although variabilities exist in these measurements, the average distribution of aromatic hydrocarbons in urban air is benzene 2l?;. toluene 36 %, ethylbenzene 9 O/b,m/p-xylene I5 %. o-xylene 7 T,. 3/4-ethyl toluene 4 “,,, 1.2,4-trimethylbenzene 6 “D and 1,3,5-trimethylbenzene 2 7;. Average concentrations in the range of l-9 ppb benzene, l-l 7 ppb toluene, l-5 ppb ethylbenzene, 0.610 ppb m/p-xylene, 0.3-4 ppb Oxylene, 0.2-3 ppb 3/4-ethyltoluene. 0.a ppb 1.2.4 ttimethylbenzene and 0.1-2 ppb 1,3,4-trimethylbenzene have been measured. Maximum concentrations at each site are typically less than 10 times the mean value. For both chemical and meteorological reasons. concentrations of aromatic hydrocarbons are at their highest during night and early morning hours with minimum occurring during late morning and early afternoon. Relative diurnal behavior of aromatic hydrocarbons shows that most are depleted at a rate 5-50 times faster than benzene. Based on data in southern California (site 13). it is estimated that a mean OH concentration of at least 2.6 (kO.6) x IO6 molec cm-’ must prevail during 0730-1330 h even in February. A long-term examination of benzene and toluene data from southern California air suggests that their levels may have declined by a factor of 5-10 over the last two decades. In remote atmospheres benzene is present at a concentration of 0.1-0.2 ppb. and is l-3 times more abundant than toluene. It is estimated that such air masses are only 2-7 days away from their urban source. K~J word index:
Aromatic hydrocarbons, benzene, hydroxyl
ISTRODUCTIOS (AHCs) form a significant other fuel oils (IO-25 %) as well as of automobile exhaust. Ambient data collected from a number of cities around the world show that
Aromatic fraction
AHCs
hydrocarbons
of gasoline
are about
nonmethane
and
25 Tl0(on a carbon
hydrocarbon
basis) of the total
concentration;
although
vary between 20 and 40% (Lonneman, 1979; Nelson and Quigley, 1982; Isidorov er nl., 1983; Sexton and Westberg. 1984). The greatest health risk from exposure to aromatic hydrocarbons is due to benzene, which is a suspected carcinogen. In addition, these hydrocarbons are photochemically reactive and contribute to smog as well as to a number of intermediate products that may be additionally toxic. In this study we present ambient data on the distribution of select AHCs from twelve United States cities based on fifteen short term studies performed on a round-the clock schedule. this fraction
is found
to
EXPERIMESTAL Select AHCs were measured using a gas chromatograph equipped with a flame ionization detector (FID). A 2m Present addresses: *NASA-Ames Research Center. Moffett Field, CA 94035. U.S.A. + U.S. Bureau of Mines, Minneapolis, MN 55417, U.S.A. *City of Tucson, Tucson, AZ 85719, U.S.A.
radical, air pollution.
x 118 in stainless steel column packed with 10 %, N, lV-bis (2Cyanoethyl) formamide on acid washed chromosorb P was employed for the separation of these species. The column was maintained at 65°C FID temperature was 275’C. and a carrier flow (helium gas) rate of 45 ml min-’ was adequate for analysis of ambient aromatic hydrocarbons (Hester and Meyer, 1979). Typically, about 400 ml of air were preconcentrated in a stainless steel trap (6” x l/16”) which was filled with glass wool (4” bed length) and held at liquid argon temperature. Figure 1 shows a typical chromatogram of an urban air sample. The analysis time was about 15 min. Calibrations were performed using 5.00 ppm benzene and toluene primary standards. These standards were couunercially obtained from Scott Marrin (Riverside, CA), but were checked against a benzene permeation tube as well. The FID carbon response was assumed to hold for other aromatic hydrocarbons. This simplification can cause errors of approximately f 5 %. Benzene toluene standard was analyzed at least twice daily. A precision of +30/ (one standard deviation) could be achieved based on the analysis of these standards. The overall accuracy of these measurements is expected to be It I5 %. All measurements were performed on site using an instrumented mobile laboratory. An all-stainless steel manifold was used for the sample inlet. The sample inlet height was typically 3 m above ground. A seven-day-per-week, 24-h-perday measurement schedule was followed during these field studies. Sampling frequency of one air analysis every two hours was maintained. Details of sampling and analysis are available elsewhere (Singh et al., 1983). At each of the twelve sites. approximately 100 samples were analyzed. All sites were selected to represent well mixed open air conditions away from major point sources.
I911
5
x
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+I -3-o d-‘o’
tl
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fl
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6
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o-o
+I
mqo =‘--d
+I t-4
r-o-
5
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+I
+I
c-49-y j -0
0
7
Denver, CO Marion & E. Slst 24 Mar-l April 1984
Houston, TX Mae St. and I-10 Fromage Rd. S--17 March, 1984
2.2f2.1 13.5 0.4
1.03
40.3
6.1 f 5.8
3.3 f 3.9 25.8 0.4
78.2 0.3
7.3 f9.5
64.0 1.6
1.1 f 3.5 31.5 0.0
8.2 0.0
1.5f 1.6 5.9 0.0 0.4 f 0.7 4.2 0.0
0.6f 6.6 0.0
1.9f2.3 14.8 0.0
0.8 & 0.9
12.3 0.0
1.2 0.0 1.1
1.4f 1.4
16.0 0.0
3.2 f 2.5
Il.9 0.0
3.3 f 3.1
37.5 0.9
*These measurements were repeated in an attempt to obtain seasonal dilTerence. t Mean concentration f one standard deviation, $Jviaximum measured concernration. $Minimum measured conccntra~ion.
Is*
14*
28.8 1.0
8.7 f 5.9 16.1 0.3
4.2 f 3.2
10.2 f 7.8
4.6 rf;3.7
16.9f 12.3
Downey, CA 7601 East Imp. Ranch0 Los Amigos Hospital 18-27 Feb. 1984
13,
0.3 1.6* 1.6 6.6 0.0
2.6 + 3.5 17.4 0.0
2.6 k 3.3 15.6 0.0
2.1 f 4.2 16.7 0.0
12
7.4 * 9.3 44.7 0.5
Staten Island, NY 4.4 f 6.6 Wild Av. & Victory Blvd. 34.0 0.1 25 April-l May 1984
0.7 f 0.6 3.9 0.0
0.8 f0.8 5.9 0.0
1.6+ 1.5 14.1 0.2
0.8 f 0.8 7.3 0.1
4.3f4.1 30.6 0.4
1.9& 1.7
11.1
Philadelphia, PA Lycoming and Castor St. 4-22 April 1983
II
0.5 f 0.4 2.0 0.1
0.7 f 0.6 2.8 0.1
1.6f 1.5 7.1 0.2
0.8& 1.2 9.5 0.1
2.6f. 1.8 8.8 0.6
Chicago, IL 79:h St. & Lavandale 20 April-2 May, 1981
10
4.6 + 3.3 14.8 0.8
2.9 0.0
3.8 0.0
10.8 0.1
10.5 0. I
46.3 0.4
Carnegie Mellon Campus 64.6 0.4 7--17 April, 1981
9
0.74 1.0 5.7 0.0
7.2 0.0
1.0* 1.0
15.6 0.0
3.3
0.1 io.2 1.3 0.0
6.7 0.0
0.2 + 0.1
4.0 0.0
0.9 f 0.9
1.6& I.8 7.3 0.0
2.9 + 4.8 29.7 0.0 4.0f
0.5 f 0.3 1.4 0.0
0.2 f 0.2 1.2 0.1
0.8 0.0
0.9 f 0.8 5.4 0.0
0.8 f 0.7 3.3 0.1
24.7 0.0
c
400 mf AMBIENT AIR
SAMPLE
Fig. 1. Chromatogram showing a separation of selected aromatic hydrocarbons from ambient air.
RESULTS
AND DISCUSSIOS
Table 1 summarizes the concentrations measured at all 1Sof the urban sites. Expressed in units of parts per billion (ppb = 10s9 v/v) are the average and standard deviation, and maximum and minimum values measured. Average benzene concentrations were in the l-9 ppb range, although maximum concentrations of 6-65 ppb wereencountered. Except in Pittsburgh (Site 9), average toiuene concentrations (1-I 7 ppb) were higher than benzene. This is not surprising since the toluene content of gasoline as well as automobile exhaust is 3-4 times the benzene content (Mayrsohn et al., 1977). The Pittsburgh data may be anomalous because of possible benzene emissions from coke ovens (Mara and Lee, 1978). Average urban concentrations of ethyl benzene, m/p-xylene, o-xylene, 3/4-ethyl
roluene. 1.2.4-trimethylbenrene and l.3.5-trimsthylbenzene were l-5 ppb. 0.6-10 ppb. 0.34 ppb, 0.2-3 ppb, 0.44 ppb and 0.1-Z ppb. rsspectiv-ely. In almost all cases the maximum concentration measured ‘has less than 10 times the mean value. The standard deviations (arithmetical) are substantial, and reilect both mean and random variabilities. Three field experiments (Sites 13-151 were repeated in winter months (February-March 1954) to understand seasonal differences. The southern California data (Sites 1 and 13) show evidence that AHC levels in winter are higher by 50-lOO”,,. This observation is in agreement with the benzene data colletsd by Shikiya et al. (1984) from the south coast air basin. At Houston (Sites 4 and 14) no perceptible ditfsrences during March and May were observed. In Denver (Sites 6 and 15) the AHC levels were significantly lower in March when compared to May. Part of this dityerence could be due to the fact that extremely adverse weather conditions prevailed during the March tieId study and normal city life may have been interrupted. Long term studies are necessary to reliably understand seasonal differences. Table 2 summarizes the average relative concentration of AHCs normalized to the avsrage benzene concentration. The standard deviation ofthese ratios is modest, and in urban centers reflects a pattern of AHC distribution that contains 21 O0benzene. 369, toluene, 9 ‘I-”ethyl benzene, 22 O,,,xylenes, 4 “.,,ethyl toluenes and 8’5, trimethylbenzenes. When one compares this with the average gasoline composition and average exhaust emissions (Mayrsohn er al., 1977), it is clear that the fraction of benzene and toluene is more than the source fraction, while that of xylenes and higher aromatics is less. An important reason for this difference is that compared to benzene. the other aromatics are S-50 times more reactive in the atmosphere. One of the primary means of atmospheric removal of AHCs is their reaction with the OH radical. Removal by reaction with ozone, or due to photolysis, can be estimated to be much less than 1‘,, of the OH removal rate. The OH reaction rates (cm’ molec- ’ j- ’ at 298 K) have been measured to be 1.2 x 10. ” for benzene (Atkinson et al., 1979; Tully er 31.. 198 1).The relative rate constants compared to benzene (at 295 K) are toluene, 4.8; ethylbenzene, 6.7; p-xylene, 9.2; mxylene. 17.5; o-xylene, 10.0, ethyl toluene, 11.0; 1.2,4trimethylbenzene, 33.0, and 1.3,5-trimethylbenzene, 52.0 (Atkinson ef al.. 1979). Figure 2 shows a clear example of the diurnal behavior of these AHCs as observed at Site 13. This site is chosen both for demonstration purposes. and because fair weather prevailed during the entire period of thtr experiment SO that normal urban activities were not interrupted. Figure 2 clearly shows dramatic diurnal variation with minimum late morning and early afternoon concentrations. Similar diurnal behavior for a variety of organic chemicals was reported earlier (Smgh et ai.. 1982: Wathne. 1983).
1915
Distribution of aromatic hydrocarbons in the ambient air
The afternoon minimum could be due to a number of reasons including (1) chemical removal (2) dilution due to the afternoon increase in mixing depth and (3) reduced emissions. To eliminate the role of dilution, Fig. 3 shows the relative mean diurnal behavior of AHCs as normalized against benzene. These relative afternoon minimums clearly show the faster removal of AHCs, as suggested by their greater relative reactivity towards the OH radical. It is further possible to estimate a lower limit of the mean OH radical concentration. This computation is possible if one assumes that the decline in the AHC/benzene ratio between 0600-0900 h and 1200-l 500 h is due strictly to chemical removal. If there were no emissions after the 0600-0900 period, the 1200-l 500 h concentration ratio would probably be somewhat lower: that is why only a lower limit is computed as follows 1
OH,S
In
[AWo/[Blo , f1j
[AHCl,/[Bl,
where KAHc and K, are the OH rate constants for AHC and benzene, respectively. The initial time (t = 0) is taken to be 0730, while the final time (t = 6 h) is 1330. The minimum OH concentrations required to accomplish the decline in non benzene AHC concentration are 3.1 x lo6 (molec cme3), 3.4 x 106,2.0 X 106, 2.3 x 106, 2.8 x 106, 2.8 x 1O’and 1.9 x lo6 for toluene, ethylbenzene, m/p-xylene, o-xylene, 3/4-ethyl toluene, l,2,4 trimethylbenzene and 1,3,5 trimethylbenzene, respectively. (In the case of m/p-xylene the meta isomer is assumed to be twice as abundant as the para isomer based on data of Mayrsohn et al., 1976.) Thus, a morning average OH concentration of at least 2.6 (f0.6)~ 106moleccm-3 was present at Site 13, even during the month of February. This is not significantly lower than the summertime OH concentration estimates of 2.5 (&- 2.0) x lo6 moleccm- 3 (Calvert, 1976) and 2.9 (+ 1.9) x lo6 moleccm-3 (Singh et al., 1981) for this region. Assuming a mean daytime OH abundance of 2.5 x lo6 moleccm-3 (night-time OH is expected to be negligible in comparison) and the rate constant data provided above, the life time (e-fold) of AHCs in the boundary layer of a typical urban atmosphere can be computed (in units of sunlit hours) to be benzene, 93 h; toluene, 19 h; ethylbenzene, 14 h;pxylene, 10 h; m-xylene, 5 h; o-xylene, 9 h; ethyl toluene, 8 h; 1,2+trimethylbenzene, 3 h and 1,3,5-trimethylbenzene, 2 h. In 10 sunlit hours, 10 7: of benzene, 40 % of toluene and nearly 99% of 1,3,5 trimethylbenzene could be depleted via reaction with the OH radical alone. The increased demand for high octane rating in unleaded fuels has led to an increase in the aromatic content. In the exhaust, however, the catalytic converter can prefentially reduce the aromatic fraction. Typical results (EPA, 1978a) suggest that the AHC fraction is 17 % in the exhaust of cars with catalyst, and 24 % in cars without catalyst. Figure 4 shows average
5
15
10 TIME
OF
20
25’
DAY
0
5
10 TIME
15
25
25
OF DAY
Fig. 2. Diurnal behavior of aromatic hydrocarbons in southern California (Site 13).
benzene and toluene concentrations measured at several sites in southern California. Because of the possibility of strong seasonal trends (Shikiya et al., 19841, data taken during winter months is excluded from Fig. 4. Further, it should be noted that each data point in Fig. 4 represents an average of a large number of samples, The pre-1970 data are taken from Ahshuller and Beltar (1961), Lonneman et al. (1968), and Leonard et at, (1976). Subsequent data are taken from this study, the results summarized in Brodzinsky and Singh (1982), and from Shikiya et al. (1984). Brodzinsky and Singh (1982) study is a comprehensive review of all available volatile organic chemi~l data
for the period of 1970-1980. It isclear from Fig. 4 that benzene and toluene concentrations have declined dramatically during the last two decades. Since the mid-1970s, however, the rate of decline in benzene concentrations appears to have slowed considerably. It is reasonable to assume that the concentrations of other AHCs have declined similarly over the last two decades. In recent years, attempts also have been made to measure concentrations of AHC in clean remote atmospheres. Table 3 summarizes some of these data. It is clear from Table 3 that benzene and tolueneare the dominant species. Unlike the urban atmosphere, the
Distribution of aromatic hydrocarbons in the ambient air
two dominant species, and the source of these aromatic hydrocarbons appears to be anthropogenic activity predominantly from fuel consumption. These chemicals are highly reactive with lifetimes that may vary from a few hours to several days. These species are transported to remote atmospheres and are present in detectable concentrations after a residence time of several days. There is indication that over the last two decades the urban levels of aromatic hydrocarbons
TOLUENE
1.S-
1917
i
have declined
significantly.
Acknowledgements-Research funded by the Environmental Protection Agency (EPA) under cooperative agreements Contract Nos. R805990010 and CR8032802010. We are thankful to many individuals for the use of facilities during the conduct of these field studies. REFERENCES
0.3,
/ j ! / f j
(
!
!
j
:
[
I
‘I
)
/
;
i
!
ETHYL
0.2
BENZENE
-
t
0
‘MEAN
0”
COMPUTED
3
6
9
12
CONCENTRATION FOR THE
PERIOD
1s
IN UNITS OF 0730
16
11
2.
OF MOLECULES
on-’
TO 1330 HOURS
Fig. 3. Mean diurnal concentration of aromatic hydrocarbons relative to benzene (ppb/ppb) in southern California (Site 13).
mean toluene/benzene ratio at mid-latitudes (Table 3) is only 0.3-1.0. Assuming an OH radical concentration of6x 105moleccm-3’ m the clean atmosphere (Singh, 1977; Volz et al., 1981)and an exclusive urban source, it is possible to compute the residence time of an air mass needed to change toluene/benzene ratio from 1.7 to 0.3-I .O.From Equation (1) it is estimated that such an air mass would be only 2-7 days away from its source. To date it is not known if significant nonanthropogenie sources of AHCs exist. CONCLUSIONS
Aromatic hydrocarbons can be measured in urban and remote atmospheres. Toluene and benzene are the
Altshuller A. P. and Bellar T. A. (1961) Gas chromatographic analysis of hydrocarbons in the Los Angeles atmosphere. J. Air Pollut. Control. Ass. 13, 81-87. Atkinson R., Darnall K. R., Lloyd A. C., Winer A. M. and Pitts J. N., Jr. (1979) Kinetics and mechanisms of the reaction of the hydroxyl radical with organic compounds in the gas phase. Adv. Photo&em. 11.375-488. Brodzinsky R. and Singh H. B. (1982) Volatile organic chemicals in the atmosphere; and assessment of available data, EPA-600/8-83-027. Final report prepared by SRI International. Calvert J. G. (1976) Hydrocarbon involvement in photochemical smog. Enuir. Sci. Technol. 10, 256-262. Eichmann R., Ketseridis G., Schebeske G., Jaenicke R., Hahn J., Wameck P. and Junge C. (1980) n-Alkane studies in the troposphere-II. Gas and particulate concentrations in the Indian Gcean air. Atmospheric Encironmenr 14, 695-703. EPA (1978a) Air quality criteria for ozone and other hydrocarbons. EPA-m/8-78-004. Greenberg J. P. and Zimmerman P. R. (1984) Nonmethane hydrocarbons in remote tropical, continental, and marine atmospheres. .I. geophys. Res. 89, 4767-4778. Hester N. E. and Meyer R. A. (1979) A sensitive technique for the measurement of benzene and alkylbenzenes in air. Envir. Sci. Technol. 13, 107-109. Isidorov V. A., Zenkevich I. G. and Ioffe B. V. (1983) Methods and results of gas chromatographic-mass spcctrometric determination of volatile organic substances in an urban atmosphere. Atmospheric Environment 17, 1347-1353. Leonard M. J., Fisher E. L., Brunelle M. F. and Dickinson J. E. (1976) Effects of the motor vehicle control program on hydrocarbon concentrations in the central Los Angeles atmosphere. J. Air. Pollut. Control Ass. 26, 359-363. Lonneman W. A. (1979) Ambient hydrocarbon measurements in Houston. Proc. Ozone/Oxidants Interactions with the Total Environment. Houston, TX, 14-17 October, 278-287. Lonneman W. A., Bellar T. A. and Altshuller A. P. (1968) Aromatic hydrocarbon in the atmosphere of the Los Angeles Basin. Enuir. Sci. Technol. 11, 1017-1020. Mara S. J. and Lee S. S. (1978)Assessment of human exposure to atmospheric benzene. EPA-450/3-78-031.
Maynohn H., Kuramoto M., Crabtree J. H., Sothern R. D. and Mano S. H. (1976) Atmospheric hydrocarbon concentrations JunoSeptember, 1975. DTS-76-15. California Air Resources Board. Mayrsohn H., Crabtree J. H., Kuramoto MI, Sothem R. D. and Mano S. H. (1977) Source reconciliation of atmospheric hydrocarbons, 1974. Atmospheric Encironment 11, 189-192.
^ 8
loo
-
90
-
80
-
70
-
60
-
50
-
‘lo
-
30
-
20
I-
A
t .A
I B E 2 +
A A A
lo_
A
9-
5
87-
$
6-
A
5
-
4
-
3-
Z-
1
I 60
62
I
I
I 64
I
I 66
I II
I 68
I
I
70
I
I
72
I
I
I
I
76
74
78
I
I 80
I
I 82
84 1984
YEAR
1960
I
Fig. 4. Benzene and toluene trend in the atmosphere of the South Coast Air Basin of California.
Table 3. Midlatitude background concentrations of some aromatic hydrocarbons in remote marine and continental
Chemical Benzene
Tolusne
o/m/p-Xylenes
Location
atmospheres
Average concentration (ppb)
Source
Atlantic air (35” N) Pacific air (45’ N) Niwot Ridge* (40; N) Niwot Ridge (40” N) PaciEc air (47” N) Northern hemisphere Southern hemisphere
0.07 f 0.03 0.23 + 0.04 0.24 0.16*0.08 0.23+0.!1 0.05 0.01
Atlantic air (35’ N) Pacific air (45” N) Niwot Ridge (WN) Niwot Ridge (40”N) Pacific air (47” N) Northern hemisphere Southern hemisphere
0.02 f 0.01 0.22 f 0.04 0.14 0.13~0.17 0.13f0.08 0.02 0.006
251 (4) (5) (5) (5)
Atlantic air (48” N) Pacific air (W-O’ N) Indian Ocean (41’ S)
o.oi-O.02 0.04-0.02 0.003-0.005
(6) (5) (6)
0) Ifi (4) (5) (5) (5) (1)
Source: (I) Penkett (1982); (2) Rasmussen and Khalil(l983); (3) Greenberg and Zimmerman (1984); (4) Roberts et al. (1984); (5) Nutmagul and Cronn (1985); (6) Eichmann et al. (1980).
‘Niwot Ridge is located in the Colorado Rockies.
Distribution of aromattc hydrocarbons in the ambient air Nelson P. F. and Quigley S. M. (1982) Non-methane hydrocarbons in the atmosphere of Sydney, AustTalia. Encir. Sci. Technol. 16, 650-655. Nutmagul W. and Cronn D. R. (1985) Determination of selected atmospheric aromatic hydrocarbons at remote continental and oceanic locations using photoionization tlash-flame ionization detection. J. armos. Chem. (to be submitted). Penkett S. A. (1982) Non-methane organics in the remote troposphere. In Atmospheric Chimisrry (edited by Goldberg E. D.). DD. 329-355. Dahlem Konferenzen Springer, New York. Rasmussen R. A. and Khalil M. A. K. (1983) Atmospheric benzene and toluene. GeophJs. Res. Lerc. 10, 10961099. Roberts J. M.. Fehsenfeld F. C., Liu S. C., Bollinger M. J., Hahn C., Albritton D. L. and Sievers R. E. (1984) Measurements of aromatic hydrocarbon ratios and NO, concentrations in the rural troposphere: estimation of air mass photochemical age and NO, removal rate. Atmospheric Environment 18. 2421-2432
Sexton K. and Westberg H. (1984)Nonmethane hydrocarbon composition of urban and rural atmospheres. Atmospheric Environment 18, 1125-t132. Shikiya J., Tsou G., Kowalski J. and Leh F. (1984) Ambient monitoring of selected halogenated hydrocarbons and benzene in the California South Coast Air Basin. 77th
1919
Annual APCA Meeting, Paper No. 84-1.1.24-29 June, San Francisco. Singh H. B. (1977) Atmospheric halocarbons: evidence in favor of reduced average hydroxyl radical concentration in the troposphere. Geophys. Res. Lat. 4, 101-104. Singh H. B., Martinez J. R., Hendry D. G., Jaffe R. J. and Johnson W. B. (1981) Assessment of the oxidant-forming potential of light saturated hydrocarbons in the atmosphere. Enu. Sci. Technol. 15, 113-l 19. Singh H. B., Salas L. J. and Stiles R. E. (1982) Distribution of selected gaseous organic mutagens and suspect carcinogens in ambient air. Em. Sci. Technol. 16, 872-880. Singh H. B.. Salas L. J., Stiles R. E. and Shigeishi H. (1983) Measurements of hazardous organic chemicals in the ambient atmosphere. Final Report prepared by SRI International. NTIS PB83-156935. Tully F. P., Ravishankara A. R., Thompson R. L., Nicovich J. M.. Shah R. C., Kreutter N. M. and Wine P. H. (1981) Kinetics of the reactions of hydroxyl radical with benzene and toluene. J. phys. Chem. 85, 2262-2269. Volz A., Ehhalt D. H. and Denvent R. G. (1981) Seasonal and latitudinal variations of ‘%O and the tropospheric concentration of OH radicals. J. geophys. Rex 86,5163-5171. Wathne E. M. (1983) Measurements of benzene, toluene and xylenes in urban air. Atmospheric Environment 17, 1713-3722.