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Washington, DC 20460, Nov 1980. “Standard Methods for the Examination of Water and Wastewater”, 14th ed.; American Public Health Association: Washington, DC, 1975. Price, K. S.; Waggy, G. T.; Conway, R. A. J. Water Pollut. Control Fed. 1974, 46,63-77. Committee on Methods for Toxicity Tests with Aquatic Organisms, “Methods for Acute Toxicity Tests with Fish, Macroinvertebrates and Amphibians”; EPA-660/3-75-009, Apr 1975. Alsop, G. M.; Waggy,G. T.; Conway, R. A. J. Water Pollut. Control Fed. 1980,52, 2452-2456. Liss, P. S.; Slater, P. G. Nature (London) 1974, 247, 181-184. Mackay, D.; Leionen, P. J. Environ. Sci. Technol. 1975,9, 1180. Neely, W. B. Proceedings of the 1976 National Conference of Hazardous Material Spills, 1976, pp 197-200. Dilling, W. L. Enuiron. Sci. Technol. 1977, 11, 405-409. Sweeris, S. Prog. Water Technol. 1979,11, 37-47. Smith, J. H.; Bomberger, D. C. “Hydrocarbons and Halogenated Hydrocarbons in the Aquatic Environment“; Afghan, B. K., MacKay, D., Ed.; Plenum Press: New York, 1980, pp 445-451. Smith, J. H.; e t al. Enuiron. Sci. Technol. 1980, 14, 1332-1337. Smith, J. H.; et al. Chemosphere 1981, 10, 281-289. Cawse, J. N.; et al. In “Kirk-Othmer Encyclopedia of Chemical Technology”, 3rd ed.; Wiley: New York, 1980; Vol. 9, pp 432-471.
(18) MacCormack, K. E.; J. H. B. Chenier Ind Eng Chem. 1955, 47, 1454-1458. (19) Reid, R. C.; e t al. ”The Properties of Gases and Liquids”, 3rd ed.; McGraw-Hill: New York, 1977; p 567. (20) Rathburn, R. E.; Tai, D. Y. Water Res. 1981,15,243-250. (21) Mabey, W.; Mill, T. J. Phys. Chem., Ref. Data 1978, 7, 383-415. (22) Mill, T.; Mabey, W. R.; Hendry, D. G. “The Fate of Selected Pollutants in Freshwater Aquatic Systems. Protocol 1: Hydrolysis”; SRI International: Menlo Park, CA 94025, Contract 68-03-2227, 1978. (23) Bronsted, J. N.; Kilpatrick, M.; Kilpatrick, M. J. Am. Chem. SOC.1929,51, 428. (24) Bridie, A. L.; Wolff, C. J. M.; Winter, M. Water Res. 1979, 13,623-626. (25) Verschueren, K. “Handbook of Environmental Data on Organic Chemicals”; Van Nostrand Reinhold: New York, 1977. (26) Miller, I. M. “Investigation of Selected Potential Environmental Contaminants: Ethylene Glycol, Propylene Glycols, and Butylene Glycols”;Franklin Research Center: Philadelphia, PA, 1979 (prepared for U.S. EPA, EPA 560/ 11-79-006). (27) Neely, W. B. ”Chemicals in the Environmenti Distribution, Transport, Fate, Analysis”; Marcel Dekker: New York, 1980. (28) Conway, R. A. “Environmental Risk Analysis for Chemicals”; Van Nostrand Reinhold: New York, 1982.
Received for review April 9, 1982. Accepted October 4, 1982.
Smog Chamber Studies of NO, Transformation Rate and Nitrate/Precursor Relationships Chester W. Spicer Battelle Columbus Laboratories, Columbus, Ohio 43201 An environmental chamber study of nitrogen oxide reactions is described. The aim of the investigation was to determine (1)the rate of NO, transformation to nitrate products, (2) the environmental factors that influence the transformation rate, (3) the relationship between nitrate products and their hydrocarbon and NO, precursors, and (4) relationships among NO,, 03,and nitrates. The experiments made use of a 17-component hydrocarbon mixture designed to represent the major organic constituents of urban air. The results of the chamber experiments demonstrate that the transformation of NO2 to nitrate products follows pseudo-first-order kinetics, and the transformation rate constant is proportional to the initial NMHC/NO, ratio. Nitrogen balances of 290% were obtained when “OB deposition to the chamber surface was taken into account. The fractional conversion of NO, to products was shown to depend on initial NMHC/NO,. For this particular hydrocarbon mixture, the PAN/HN03 ratio was directly proportional to initial NMHC/NO, for NHMC/NO, 120. For a typical urban NMHC/NO, ratio of 8, about 3 times as much HNO, as PAN is produced in the chamber. The implications and limitations of the experimental results are discussed. Introduction The conversion of nitrogen oxides (NO, = NO + N0.J to gaseous and particulate nitrates in the atmosphere is an important but poorly understood process. The rate of the conversion affects ozone formation and the ultimate fate of the nitrogen oxides. The nitrate products that are formed may play a role in the acidification of precipitation 112
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and have been reported to have a deleterious effect on respiratory function and on human health in general. In view of the vital role of these species in atmospheric chemistry, much more needs to be learned about the mechanisms and the dynamics of the processes that convert NO, to nitrates. The conversion of NO, to products has been the subject of several previous laboratory studies (1-8). However, little has been published on the rate of transformation of NO, to nitrates and the factors that affect this rate. There is also limited information available concerning the relationships between the nitrate reaction products and the hydrocarbon and nitrogen oxide precursors. Since the environmental effects of the major reaction products nitric acid, peroxyacetyl nitrate (PAN), and particulate nitrate are different, it is important to understand how the distribution of these products varies with changing precursor concentrations and environmental conditions. It is also necessary to understand these relationships so that we may predict the consequent changes in NO, reaction product distribution of (1) proposed pollutant control strategies, (2) changes in hydrocarbon or NO, emissions due to future energy or environmental constraints, (3) changes in hydrocarbon composition due to changing energy sources or transportation modes, or (4) other future changes in environmental conditions. This study was undertaken to provide a better understanding of atmospheric NO, transformation rate, the factors that affect the rate in urban air, and the relationship between nitrate products and their hydrocarbon and NO, precursors. Variations in the initial hydrocarbon and NO, concentrations and ratios were employed to derive
00 13-936X/83/0917-0112$01.50/0
0 1983 American Chemical Society
Table I. Composition of Synthetic Urban Hydrocarbon Mixture re1 re1 molar molar compound copcn compound concn acetylene 0.136 propylene 0.035 ethane 0.100 trans-2-butene 0.043 propane 0.040 2-methylbutene-2 0.013 2-methylpropane 0.023 benzene 0.029 n-butane 0.099 toluene 0.061 0.0 7 0 rn-xylene 0.069 2-methylbutane 0.025 n-pentane 0.037 p-ethyltoluene 2-methylpentane 0.044 1,2,4-trimethyl0.013 ethylene 0.162 benzene
information on the kinetics and mechanisms of NO, reactions in simulated urban air.
Experimental Section All smog chamber experiments were conducted in a 17.3-m3chamber, with fluorescent lamps (black and sun) for irradiation. The design of the chamber and its operating characteristics have been described previously (8,9). The chamber was lined with 5-mil Teflon (FEP) film for this set of experiments to minimize nitrogen compound losses. The lining only slightly reduces the actinic light intensity. No contamination from the Teflon liner has been detected by gas chromatography (detection limit for C2-C12compounds 1 ppbC). For this series of experiments the chamber dilution rate, which includes air withdrawn for sampling requirements, was 0.059 f 0.002 h-l as measured with an inert tracer. The ozone decay rate in the unlighted chamber was 0.036 h-l, corrected for dilution. With the lights on, the dilution-corrected decay rate was 0.069 h-l. Light intensity (Itl) as determined by NO2 photolysis in clean air was 0.22 min-l (IO). Experiments were conducted with a 17-component hydrocarbon mixture designed to simulate a typical urban hydrocarbon distribution. The distribution of hydrocarbons in this synthetic urban mixture is shown in Table I. Hydrocarbon distribution data from several urban areas were used to formulate this mixture. In general, each species in the surrogate mixture represents several closely related hydrocarbons observed in urban air. For each experiment the chamber was loaded to the design hydrocarbon concentration (1.5, 4.5, or 9.0 ppmC NMHC); a Beckman 6800 chromatograph was used for total NMHC measurements. The hydrocarbon concentrations used for
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data interpretation are based on detailed C2-C10 gas chromatographic analyses conducted hourly from the start of the experiment. The initial NMHC and NO, concentrations were varied from run to run. The initial distribution of NO, was set at N02/N0, = 0.5. All experiments were conducted at 33 f 1"C at an initial relative humidity of approximately 55%, except where noted. The analytical methods employed during the study are listed in Table 11. Detailed C2-Clo hydrocarbon measurements were made each hour, and CHzO and total aldehydes were measured twice during most experiments. PAN measurements were made every half hour, and the remainder of the species were monitored continuously. Because of the critical importance of the oxidized nitrogen compound measurements, redundant systems were usually employed for NO, NO2, HN03, PAN, and 03.It should be noted that the analytical methods employed for the oxidized nitrogen species were selected for specificity as well as sensitivity, and the measurements reported here are believed to be free from interferences. A more detailed description of the analytical methods and calibration procedures has been reported separately (2, 8, 11).
Results and Discussion Eighteen smog chamber experiments were performed with synthetic urban air to investigate the rate of NO, conversion and the relationship between nitrate products and their hydrocarbon and NO, precursors. The irradiations were generally continued until O3and PAN reached maximum values. Length of irradiation varied during the study from 3 to 21 h, but most of the experiments lasted 6-7 h. The initial conditions for the 18 experiments are shown in Table 111. All experiments were run at 55% relative humidity, except KN-23 which had a relative humidity of 15%. NO, Transformation Rate. The profiles of selected reactants and products from experiment KN-28 are shown in Figure 1. The NO, transformation rates for the smog chamber experiments are derived from the maximum slope of semilog plots of NO, concentration vs. reaction time. A representative plot for experiment KN-28 is shown in Figure 2. During the initial phase of the experiments, NO is converted to NOz, with little net loss of NO,, so the plots show an initial curvature. This phenomenon is most obvious for the slower experiments. Once NO has been largely converted to NO2, the curve becomes reasonably linear, demonstrating pseudo-first-order behavior, and the
Table 11. Analytical Methods Used during Smog Chamber Experiments determination 0 3
NO NO, HNO, N, (total oxidized N) PAN THC, CH,, CO, C,H,, C,H, C,-C, hydrocarbons C,-C,, hydrocarbons total aldehydes CH,O NO;, 5 0 ; temp re1 humidity light-scattering aerosol
method chemiluminescence; UV photometry chemiluminescence colorimetric (automated Saltzman) colorimetric (automated Saltzman) pulsed fluorescence chemiluminescence chemiluminescence chemiluminescence electron-capture GC electron-capture GC FID GC FID GC FID GC bisulfite collection/titration colorimetric (Chromotropic acid) quartz filter collection/ion chromatography thermocouple condensation/transmission integrating nephelometry
instrument(s) REM Model 612-B; Dasibi Model 1 0 0 3 AAS Bendix 8101B and CSI 1600 Beckman Acralyzer Beckman Acralyzer Aerospace Corp. (prototype) TECO-14D modified (11 ); Monitor Labs 8440 HP modified (11) TECO-14D Varian 3700 with 63Nidetector; Varian 1200 with 3H detector Beckman 6800 chromatograph Varian 2800 Varian 2800 (with cryogenic preconcentration) NA Beckman DB-G D-ion-X 10 NA EG&G 800 MRI Environ. Sci. Technot., Vol. 17,No. 2, 1983
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Table 111. Initial Conditions, NO, Transformation Rates, and Peak 0, Concentrations for Chamber Experiments
1000
[NMHCI, [NO], [NO,], NMHC/ [O,lnl,, expt ppmC ppm ppm NO, Ru pprn KN-17 15.2 0.52 0.403 4.52 0.160 0.138 KN-18 4.32 0.236 0.220 9.5 0.35 0.468 KN-19 4.01 28.0 0.68 0.300 0.077 0.066 KN-20 4.29 0.151 0.135 15.0 0.58 0.362 KN-21 4.52 0.330 0.280 7.4 0.27 0.385 KN-22 7.64 16.9 0.69 0.470 0.227 0.224 KN-23 7.17 15.8 0.72 0.494 0.225 0.228 KN-24 1.29 0.082 0.082 7.9 0.23 0.229 KN-25 1.60 0.159 0.152 5.1 0.19 0.264 KN-26 9.31 0.072 0.076 62.9 2.39 0.276 KN-27 9.92 0.144 0.165 32.1 0.81 0.441 KN-28 9.30 0.301 0.320 15.0 0.61 0.533 KN-29 1.67 0.225 0.234 3.6 0.11 0.242 KN-30 4.73 0.218 0.215 10.9 0.52 0.421 KN-31 4.52 0.082 0.083 27.4 1.09 0.295 KN-32 8.59 0.094 0.098 44.7 1.95 0.334 KN-33 4.55 0.210 0.227 10.4 0.82 0.480 KN-34 4.59 0.049 0.053 45.0 1.42 0.233 R = NO, transformation rate (in h-l) after NO is largely converted t o NO,. 0.6 0
1
2
3
4
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Reaction Time, hr.
Flgure 2. Concentration of NO chamber experiment KN-28.
0.5
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Flgure 1. Profiles of selected reactants and products for smog chamber run KN-28.
slope of the linear portion of the curve is used to derive the transformation rate. The decrease in NO, concentration in the chamber is due both to chemical conversion to nitrate products and to dilution. The curve shown in Figure 2 represents the combined effects of both processes. The dilution rates were measured during the chamber experiments, and they have been subtracted from the total removal rates in deriving the NO, transformation rates reported in Table 111. The maximum ozone concentrations observed during the experiments are also listed in the table. Factors Affecting NO, Transformation Rate. Two factors that have been shown to affect NO, transformation rate are relative humidity and NMHC/NO, ratio (8). All but one of the experiments in this series was conducted at relative humidity -55%. Experiment KN-23 was initiated with a relative humidity of 15%. Thus KN-23 can be compared with a similar experiment at higher relative humidity, namely KN-22. For both these experiments, O3 114
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+ NO2 vs. reaction time for smog
peaked at 3 h, at 0.470 ppm for KN-22 and 0.494 ppm for KN-23. As noted in Table 111, the NO, transformation rate was also slightly greater for KN-23. These differences are within the run-to-run variation of the chamber experiments, so that no definite effect of relative humidity can be cited. This is consistent with the earlier finding (8)that the influence of water vapor is greatest at low reaction rates, corresponding to low NMHC/NO, or low reactivity hydrocarbons, and is minimal for rapid photooxidation, These observations are also in agreement with the recent work of Sakamaki et al. (12),who have investigated the effect of water vapor on the C,H,-NO, system and report a significant increase in the rate of NO oxidation but a small effect on [O,],, with increasing humidity. A quantitative relationship between NO, transformation rate and NMHC/NO, ratio has not been reported previously. In this study, the NMHC/NO, ratio was varied over the range 3.6-62.9, and the transformation rate covered the range 0.11-2.39 h-l. A plot of the NO, transformation rate vs. initial NMHC/NO, is shown in Figure 3. There is clearly a linear relationship between these two variables. The correlation coefficient is 0.94, and the slope of the least-squares line through the origin is 0.036 h-l. This relationship between NO, transformation rate and NMHC/NO, ratio is valid for the surrogate urban hydrocarbon mixture used in this study. Different hydrocarbon compositions are expected to yield different slopes, depending on the reactivity of the mixture. It should be possible to draw a family of curves that represent the general types of hydrocarbon compositions present in the atmosphere, Of special interest will be the relationship for reacted urban air, which will be important during multiday transport, and nonurban compositions, which become important when NO, from an industrial source such as a power plant mixes with nonurban air. The relationship between transformation rate and NMHC/NO, ratio for such mixtures is currently under study. On the basis of the linear relationship between NO, transformation rate and NMHC/NO, ratio, the range of
2.40
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'7
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0 0
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20 30 NMHC/NO,
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Figure 4. Calculated lifetime of NO, (principally NO,) vs. NMHCINO,. NMHC/NO,
Figure 3. Pseudo-first-orderrate constant for NO, transformation vs. initial NMHCINO, .
typical transformation rates for urban atmospheres can be estimated. Chan et al. (13) have reported average 0600-0900 EDT NMHC/NO, values of 4.6, 6.4, and 8.7 for three sites in Philadelphia during the summer of 1979. Figure 3 indicates that such ratios should yield NO, transformation rates of 0.17-0.31 h-l. During the same period, we investigated NO, chemistry and transformation rate in the Philadelphia plume at a site nearly 80 km downwind of the city in the New Jersey pine barrens (14). For 2 days when the site was under the influence of the Philadelphia plume, lower limit transformation rates of 0.12 and 0.19 h-l were observed. Allowing for the fact that these rates are lower limits and that the relative hydrocarbon composition of Philadelphia may be somewhat different from the hydrocarbon distribution used in the chamber experiments, the values are reasonably consistent. The range of NO, transformation rates reported above also can be compared with rates recently measured in the urban plume from Boston, MA (15). Under clear skies on 4 summer days, transformation rates in the Boston plume ranged from 0.14 to 0.24 h-l. These rates are in reasonable agreement with the range of rates estimated from the chamber results for typical urban NMHC/NO,. It should be noted that extrapolations of chamber results to the ambient atmosphere must be used with caution, due to the potential effects of the chamber walls, for example, as a source of extraneous radicals. Such effects should be minimized in these experiments by the use of a relatively reactive hydrocarbon mixture, for which the homogeneous source of radicals should dominate any contribution due to the walls. Implications for NO, Lifetime in Urban Air. The conversion lifetime of a species in the atmosphere can be defined as the time required for the concentration to decrease to l / e of its original value. Therefore, in the first-order rate expression In ([NO,],/[NO,],) = k t , when [NO,],-J[NO,], = e, t becomes the lifetime 7,where 7 = l/k. The relationship between 7 and NMHC/NO, is shown in Figure 4,with use of the slope from Figure 3. It is clear from the figure that in the range of typical urban NMHC/NO, values, the lifetime increases rapidly as NMHC/NO, decreases. As a consequence, urban plumes of lower initial NMHC/NO, will be diluted more, more widely dispersed, and transported over greater distances before complete NO, conversion. The lifetime represented
in Figure 4 is principally the lifetime of NO2,since the NO, transformation rates were computed after most of the NO had been converted to NOz. Thus the total lifetime of NO,, after emission as NO, typically will be 1-3 h longer than indicated in Figure 4, depending on the rate of NO photooxidation. Nevertheless, it is apparent that the lifetime of NO, in urban air during warm sunny days will generally be less than 1solar day. By comparison, SO2 transformation rates are generally less than 0.05 h-l (16), indicating a conversion lifetime of several days. The consequences of this difference in NO, and SO2 conversion rates are significant when considering the impact of the nitrate and sulfate reaction products on downwind receptor areas. For example, if one considers only homogeneous transformations, it is apparent that NO, will be converted to products more rapidly than SO2,so that the effects of the secondary products on wet and dry deposition will be more localized for NO, than for SO2. The relative rates also suggest that under similar emission conditions, a greater fraction of NO, will be converted to products (e.g., "OB, PAN, NO3-) than SOz, since a larger proportion of SO2 will be deposited before chemical conversion. It is important in considering the effects of NO, on precipitation chemistry to know the distribution of the NO, reaction products and the environmental conditions that affect the distribution. These topics are considered below. Nitrogen Balance. Before the relationships between nitrate reaction products and their hydrocarbon and NO, precursors are discussed, it must be demonstrated that all significant NO, reaction products are accounted for. The task of performing a nitrogen balance in the chamber is complicated by the fact that the chamber is continuously diluted with clean air at a rate of 0.059 f 0.002 h-' to allow for instrument sampling requirements and leakage around the Teflon windows of the chamber. Further complications are introduced by the fact that HNO, and N20, are deposited on the chamber surfaces during the experiments (8). Both the dilution rate and the wall loss rate must be taken into account in order to reach an acceptable nitrogen mass balance. A dynamic nitrogen balance diagram for experiment KN-33 is shown in Figure 5. The various oxidized nitrogen species are plotted in a cumulative manner. Curve C represents the sum of all the measured gaseous nitrogen compounds and includes NO, NO2,PAN, and HNO,. Curve A represents the exponential dilution of the original 0.437 ppm NO, in the chamber by the measured dilution rate. After the NO, maximum at 1-1.5 h, the sum of the measured nitrogen compounds falls short of the expected oxidized nitrogen concentration by inEnviron. Sci. Technol.. Vol. 17, No. 2, 1983
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0 5 Total Oxidized Nitrogen as Measured by Chemiluminescence
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Sum of Measured Nitrogen Compounds
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g 0.2
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Flgure 5. Cumulative plot of oxidized nitrogen compounds for KN-33.
creasingly greater amounts. The explanation for much of the nitrogen deficit is the loss of HNO, and Nz05to the chamber surfaces. This loss rate has been measured in a separate series of experiments conducted under conditions similar to the runs reported here, except that only the species of interest was added to the chamber air. The rate of HNO, and N206wall loss averages 1.07 X min-’ over a relative humidity range of 13-65%. This compares to an 0, loss rate of 6.00 X lo4 rnin-l and implies a nitric acid lifetime of -1.5 h in the chamber. Surface loss of nitric acid is suggested by the HNO, curve in Figure 1. After rising during the first 3 h of the experiment, the concentration of HNO, in the gas phase decreased, even though NO2 was still present and O3and PAN continued to increase. It is clear that the surface loss rate plus the dilution rate exceeded the HNO, production rate after 3 h of irradiation, so the gas-phase concentration decreased. The surface loss rates for HNO,, N205,and 0, have been incorporated into a 58-step kinetic mechanism used to simulate smog chamber and ambient air photochemistry. The model uses a three-class parametrization for hydrocarbon species. When used to simulate the conditions of experiment KN-33, the model reveals a considerable quantity of HNO, lost to the chamber surface. This adsorbed HNO, is represented by the shaded area between curves C and B in Figure 5. Addition of the concentration of adsorbed HNO, to the concentrations of the other measured nitrogen species (curve B) reveals that 90% of the oxidized nitrogen is accounted for at the end of the experiment. The greatest deficit occurs at 2.5 h, when 18% of the nitrogen is missing. The deficit at this time is at least partly explained by the fact that the model is initially slower to produce HNO, than the smog chamber experiment, so the amount of HNO:, lost to the chamber surfaces is underestimated until late in the experiment. The model results indicate that only 0.001 ppm N205is present in the gas phase and 0.003 ppm N205has been lost to the surface by the end of the irradiation. It might be speculated that a part of the remaining nitrogen deficit could be due to unmeasured reaction products. However, the amount of particulate nitrate formed in these experiments is usually
