Environ. Sci. Technol. 1993,27,326-331
(39) Wieland, E. The weathering of sparingly soluble minerals-a coordination chemical approach for describing their dissolution kinetics. Ph.D. Thesis No. 8532, ETH, Zurich, Switzerland, 1988 (in German). (40) Jepson, W. P.; Jeffs, D. G.; Ferris, A. P. J. Colloid Interface Sci. 1976, 55, 454-461. (41) Chernoberezhskii, Y. M. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum: New York, 1982; Vol. 12, Chapter 5 . (42) Arnold, P. W. In The Chemistry of Soil Constituents; Greenland, D. J., Hayes, M. H. B., Eds.; Wiley: New York, 1978; Chapter 4. (43) Kreyszig, E. Statistical Methods and their Application; Vandenhoek & Rupprecht: Gottingen, Germany, 1979 (in German). (44) Gesellschaft Deutscher Chemiker, Deutsches Institut fur Normung German Standard Methods for the Analysis of Water, Wastewater,and Sewage Sludge;VCH Weinheim, Germany, 1988; Chapter E9 (in German). (45) Schwarzenbach, R. P.; Stierli, R.; Folsom, B. R.; Zeyer, J. Environ. Sci. Technol. 1988, 22, 83-92. (46) Roberts, P. V.; Goltz, M. N.; Mackay, D. M. Water Resour. Res. 1986, 22, 2047-2058. (47) Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991,25, 1237-1247. (48) Wu, S. C.; Gschwend, P. M. Environ. Sci. Technol. 1986, 20, 717-725. (49) Weiss, A. In Organic Geochemistry;Eglington, E., Murphy, J., Eds.; Springer: Berlin, 1969; Chapter 31, pp 737-781. (50) Langmuir, J. J . Am. Chem. SOC.1918,40, 1361-1403. (51) Voice, T. C.; Weber, W. J., Jr. Environ. Sci, Technol. 1985, 19, 789-796. (52) Gschwend, P. M.; Wu, S. Environ. Sci. Technol. 1985,19, 90-96. (53) Kung, K. S.; McBride, M. B. Enuiron. Sci. Technol. 1991, 25, 702-709. (54) Westall, J. C.; et al. Environ. Sci. Technol. 1985,19,193-198. (55) Jafvert, C. T.; Westall, J. C.; Grieder, E.; Schwarzenbach, R. P. Environ. Sci. Technol. 1990, 24, 1795-1803. (56) McBride, M. B. Clays Clay Miner. 1976, 24, 88-92. (57) Kummert, R. The surface complexation of organic acids on gamma-aluminumoxide and their relevance for natural waters. Ph.D. Thesis, No. 6371, ETH Zurich, Switzerland, 1975 (in German). (58) Machesky, M. L.; Bischoff, B. L.; Anderson, M. A. Environ. Sci. Technol. 1989, 23, 580-587.
(59) Sposito, G. The Surface Chemistry of Soils; Oxford University Press: New York, 1984. (60) Brown, G.; Newman, A. C. D.; Rayner, J. H.; Weir, A. H. In The Chemistry of Soil Constituents; Greenland, D. J., Hayes, M. H. B., Eds.; Wiley: New York, 1978; Chapter 2.
(61) Parrini, V. P. Russ. Chem. Rev. 1962, 31, 408-417. (62) Foster, R. E. Organic Charge Transfer Complexes;Academic Press: New York, 1969. (63) Sposito, G.; Prost, R. Chem. Rev. 1982, 82, 553. (64) Parker, J. C. In Soil Physical Chemistry;Sparks, D. L., Ed.; CRC Press: Boca Raton, FL, 1986; Chapter 6. (65) McBride, M. B. In Minerals in Soil Environments;Dixon, J. B., Weed, B. B., Eds.; Soil Science Society of America: Madison, WI, 1989; Book Series No. 1, Chapter 2. (66) Stumm, W.; Morgan, J. J. Aquatic Chemistry;Wiley: New York, 1981. (67) Barker, J. F.; Tessmann, J. S.; Plotz, P. E.; Reinhard, M. J . Contam. Hydrol. 1986, I , 171-189. (68) Freeze, R. A.; Cherry, J. A. Groundwater;Prentice-Hall: Englewood Cliffs, NJ, 1979. (69) Backhus, D. A. Ph.D. Thesis, Massachusetts Institute of Technology, 1990. (70) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525-616. (71) Fujita, T. In Progress in Physical Organic Chemistry;Taft, R. W., Ed.; Wiley: New York, 1983; Vol. 14, pp 75-113. (72) Banerjee, S.; Howard, P. H. Environ. Sci. Technol. 1988, 22, 839-841. (73) Serjeant, E. P.; Dempsey, B. Ionisation Constants of Organic Acids in Aqueous Solution;Pergamon: Oxford, U.K., 1979. (74) Parks, G. A. Chem. Rev. 1965, 65, 177-198. (75) Davis, S. H. R. Ph.D. Thesis, California Institute of Technology, 1985. (76) Schindler, P. W.; Stumm, W. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley: New York, 1987; Chapter 4. (77) Spalding, R. F.; Fulton, J. W. J. Contam. Hydrol. 1988,2, 139-153. (78) Pennington, J. C.; Patrick, W. H., Jr. J. Environ. Qual.1990, 19, 559-567.
Received for review April 20,1992. Revised manuscript received October 8, 1992. Accepted October 13, 1992. This work was partially funded by the Swiss National Science Foundation (Project 20-5623.88).
Ambient Levels of Peroxy-n-Butyryl Nitrate at a Southern California Mountain Forest Smog Receptor Location Daniel Grosjean,' Edwln L. Wlillams, 11, and Erlc Grosjean DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003
Ambient levels of the peroxyacyl nitrate [RC(O)OONO,] peroxy-n-butyryl nitrate (PnBN, R = n-C,H,) have been measured at a southern California mountain forest location impacted by urban photochemical smog. The highest level recorded was 2.3 ppb; 24-h averages were in the range 0.04-0.51 ppb. Diurnal variations of PnBN exhibited midafternoon maxima and coincided with those of the other peroxyacyl nitrates PAN (R = CH3) and PPN (R = C,H,). The average PnBN/PAN ratio was 0.077 f 0.002. This ratio and its diurnal variations are discussed in terms of formation and removal processes including in-situ photochemical production, thermal stability, and loss by chemical reactions. The relative photochemical production rates of PnBN and PAN were estimated from an inventory of their respective hydrocarbon precursors. Introduction Peroxyacyl nitrates [RC(O)OONO,] continue to receive 326 Environ. Sci. Technol., Vol. 27, No. 2, 1993
attention for their role in long-range transport of reactive nitrogen in the atmosphere (1-4) and for their adverse effects on human health and ecosystems (5-8). The two most abundant peroxyacyl nitrates in the atmosphere are PAN (R = CH3) and PPN (R = C,H,). Ambient levels of PAN have been measured many times (9). Quantitative measurements of PPN have been recently reported (10, 11). Information regarding ambient levels of higher molecular weight peroxyacyl nitrates such as peroxy-n-butyryl nitrate (PnBN, R = n-C3H,) is very limited. PnBN has been characterized in the classical laboratory studies of Stephens (15),has been recently observed in ambient air (10,11),but to our knowledge has not been the object of quantitative measurements. Yet, this information is of importance for two reasons. First, PnBN and other higher molecular weight peroxyacyl nitrates may be even more phytotoxic than PAN (8). Second, higher molecular weight peroxyacyl nitrates may account for some of the "missing"
0013-936X/93/0927-0326$04.00/0
0 1993 American Chemical Society
reactive organic nitrogen, i.e., the reactive odd nitrogen that is not accounted for by NO2,nitric acid, PAN, PPN, and alkyl nitrates (12, 13). In this study, we have carried out detailed measurements of the ambient levels of PnBN at a southern California mountain forest location. This location is impacted by photochemical pollution as a result of transport from the urban Los Angeles area (14). Measurements of PnBN were carried out during the 1989 photochemical smog season and involved round-the-clock sampling in order to characterize diurnal variations. Ambient levels of PnBN are compared to those of PAN and PPN measured during the same period and at the same location (10). A systematic comparison is made of ambient levels of PnBN relative to those of PAN. The results are discussed with respect to formation and removal processes for PAN and PnBN during polluted air mass transport from the urban Los Angeles area to the mountain forest smog receptor location.
Experimental Methods Ambient levels of PnBN were measured at Tanbark Flat (SanDimas Experimental Forest, USDA Forest Service), elevation 800 m, located 35 km northeast of Los Angeles in the San Gabriel Mountains, a mountain range bordering the northeast side of the Los Angeles urban area. The surrounding vegetation is mostly chaparral and Coulter pine. PnBN was measured on site by electron capture gas chromatography (EC-GC) using a SRI Model 8610 gas chromatograph and a Valco Model 140 BN EC detector as described previously for PAN and PPN (10, 11, 16). The column used was a 70 cm X 3 mm Teflon-lined stainless steel column packed with 10% Carbowax 400 on Chromosorb P, acid washed, and treated with dimethyldichlorosilane. The column and detector temperatures were 36 and 60 "C, respectively. The carrier gas was ultrahigh-purity nitrogen. The column flow rate was 58 mL/min. Ambient air was sampled 4 m above the ground and was continuously pumped through a 25-mm-diameter, 1.2-pm pore-size Teflon filter, a 6.4 m X 6 mm Teflon sampling line, and a 6.7-mL stainless steel loop housed in the GC oven. The inlet Teflon filter was replaced frequently (typically every 3 days) to minimize possible loss of PAN on the particulate matter deposit. Laboratory experiments carried out with ppb levels of alkyl nitrates and peroxyacyl nitrates in purified air showed negligible loss through a Teflon filter inlet and 5-8-m-long Teflon sampling lines (2.2) than that for authentic samples of PnBN. The remainder, i.e., 96.7% of the total number of observations, yielded retention time ratios of 1.9-2.2. Within experimental precision, these ratios matched those obtained for authentic samples of PnBN. Ambient Levels of PnBN. Ambient levels of PnBN were measured at Tanbark Flat from August 8 to October 15, 1989. Some 2900 observations were made, of which about 2100 were above our detection limit of 0.06 ppb. Individual values of ambient PnBN are summarized in Figure 2 as a frequency distribution. The highest levels
2.5 1.0 - 2.5 0.9 - 1.0 0.8
- 0.9
2.0
-
0.7 0.8
-
0.6 0.7
n
1.5
0 . 5 . 0.6
0
i
2‘
0.4
$
- 0.5
8 B
-
0.3 0.4
0
1.0
0.2 - 0.3 0.1 - 0.2
0.5
0
500
1000
1500
Number of Observations
0.0
Flgure 2. Frequency dlstributlon of amblent levels of PnBN, Tanbark
817
Fiat, CA, Aug 8-0ct 16, 1989.
8/21
914
9/18
1012
10116
Date (1989)
recorded were 2.3 and 1.8 ppb on October 13 and October 12,respectively. For comparison, the highest levels of PAN and PPN were >16 and 5.1 ppb, respectively, during the same period and at the same location (IO). Twentyfour-hour averages, which are relevant to vegetation exposure to phytotoxic pollutants such as PnBN, were in the range 0.04-0.51ppb. Seasonal Variations. Figure 3 shows a time series plot of the daily maxima and of the 24-h-averaged PnBN concentrations. There is an overall trend toward higher concentrations during the fall. This trend is s i m i i to that observed for PAN and PPN during the same period and at the same location (IO). The thermal decomposition of peroxyacyl nitrates increases rapidly with temperature (15),and conversely, their stability in ambient air increases at lower temperature during the fall. Diurnal Variations. Ambient concentrations of PnBN exhibited strong diurnal variations and included late afternoon maxima (Figure 4). These maxima coincided with those observed for other photochemical oxidants including ozone, PAN and PPN ( 1 0 , I I ) . Diurnal variations of ambient levels of PnBN are consistent with transport of photochemically polluted air from the Los Angeles urban area to the mountain forest location. These diurnal variations have been discussed in detail for PAN and PPN ( 1 0 , I I ) and reflect “horizontal”transport downwind of the urban area as well as “vertical” transport up the mountain slopes. The possible contribution of biogenic hydrocarbons cannot be tested at the present time since, to our knowledge, no studies have shown PnBN to be a product of biogenic hydrocarbon oxidation. PnBN/PAN Concentration Ratio. Since the phytotoxicity of PnBN may be different from that of PAN (8), it is of interest to examine the PnBN/PAN ambient concentration ratios. In addition, diurnal variations in this ratio may reflect differences and similarities in formation and removal processes for the two peroxyacyl nitrates. A frequency distribution plot of the PnBN/PAN concentration ratios is shown in Figure 5. If one excludes
Figure 3. Tlme serles plot of highest).( and 24-h-averaged (0)PnBN concentrations, Tanbark Fiat, CA, Aug 8-0ct 15, 1989. 7 ,
6
5 n n n
c‘
4
I-
5
CI
c
3
v 0
0
2
1
0 0
4
8
12
16
20
24
Time, PDT
Flgure 4. Dlurnal varlations of ambient PnBN (A)and PAN (0)concentratlons, Tanbark Fiat, CA, Sept 27, 1989.
those ratios corresponding to levels of PnBN that were below detection, the data shown in Figure 5 suggest that ambient levels of the two compounds exhibited similar variations. Indeed, least squares regression analysis of the PnBN/PAN ratios for PnBN > 0.18 ppb (Le., 3 times the detection limit) yielded a near-zero intercept (0.08 h 0.01 ppb), a slope of 0.077 f 0.002, and a correlation coefficient of 0.72. This observation suggests a common origin for both compounds, i.e., photochemical oxidation of hydrocarbons during air mass transport. To further investigate the variations of the PnBN/PAN concentration ratio, we have constructed composite diurnal profiles, i.e., diurnal variations that are averaged over the entire study period. These composite profiles are shown in Figure 6 for ambient concentrations of PAN and PnBN Environ. Sci. Technol., Vol. 27, No. 2, 1993 329
0.30 - 0.90
0.27
-
il
O.1°
-7
0.30
0.08-
0.24 - 0.27
0.21
I-..
- 0.24 0.06 -
I
z
m
d-. 5 R
0.02
200
0
600
400
800
1000
..
.
0.04 -
0.03- 0.06
0.00- 0.03
. . . ' .
. .. . .
i I
i
1200
Number of Observations
Flgure 5. Frequency distribution of PnBN/PAN concentration ratios.
0
4
8
12
16
20
24
Time, PST
Figure 7. Composite diurnal profile of the PnBN/PAN concentratlon ratio, Tanbark Flat, CA, Aug 8-0ct 16, 1989. i
I
. .
8
s
0
-1. .. 0
0 4 " 0
"
4
"
"
8
'
"
1.2
16
20
1
1
24
Time, PST
Flgure 6. Composite diurnal profiles for PnBN (X10, 0)and PAN (m), Tanbark Flat, CA, Aug 8-Oct 16, 1989.
and in Figure 7 for the PnBN/PAN concentration ratio. As discussed before, diurnal variations of PnBN and PAN were similar and exhibited late afternoon maxima that coincided with those of ozone. The PnBN/PAN concentration ratio also exhibited an afternoon maximum which coincided with the maximum PnBN, PAN, and ozone concentrations. 330
Environ. Scl. Technoi., Voi. 27, No. 2, 1993
Factors that may affect the PnBN/PAN ratio include diurnal changes in the relative abundance of hydrocarbons that are precursors to PnBN and PAN, differences in thermal stability, differences in dry deposition rates, and differences in their rate of removal by chemical reactions. Substantial diurnal variations in the relative abundance of hydrocarbon precursors cannot be ruled out but are unlikely. PnBN and PAN presumably have similar rates of thermal decomposition and similar dry deposition velocities. Unlike PAN, PnBN contains several secondary C-H bonds and therefore is expected to react faster with OH than PAN does, thus resulting in a more rapid removal during long-range transport. However, removal of PnBN by reaction with OH is probably not important (relative to removal of PAN) in view of the short transport time, a few hours, from the urban area to the mountain forest location. In fact, contrary to observations, removal of PnBN by reaction with OH would result in a decrease in the PnBN/PAN ratio at the time of maximum ozone, PAN, and PnBN concentrations. Thus, the observed increase in PnBN/PAN ratio is consistent with a higher in-situ production rate for PnBN relative to that for PAN during episodes of higher photochemical activity. This trend would indeed be expected if hydrocarbons that are precursors to PnBN are more reactive than those that are precursors to PAN. Because of the short air mass transport time involved, this higher production rate for PnBN would not be offset by removal of PnBN by reaction with OH. Hydrocarbons that are precursors to PnBN and PPN are discussed in more detail below. Precursors of PnBN and PAN. Formation rates for PnBN and PAN are proportional to the emission rates and chemical reactivity of their precursor hydrocarbons. Even if reliable hydrocarbon emission inventories were available (18,19),precise calculations of PnBN and PAN formation rates would require, for each hydrocarbon, a detailed knowledge of photooxidation pathways, of the relative
Table 11. Hydrocarbon Precursors of PnBN precursor 1-pentene n-propylbenzene di-n-propylbenzene (isomers) n-butanal n-pentane 2-methylpentane n-hexane 3-methylhexane n-heptane 2-methylhexane
A = emissn rate, metric tons/day 31.1
