Environ. Sci. Technol. 2000, 34, 4470-4473
Anthropogenic Sources of Chlorine and Ozone Formation in Urban Atmospheres PAUL L. TANAKA,† SARAH OLDFIELD,† JAMES D. NEECE,‡ CHARLES B. MULLINS,† AND D A V I D T . A L L E N * ,† Department of Chemical Engineering and Center for Energy and Environmental Resources, M/C R7100, University of Texas at Austin, 10100 Burnet Road, Building 133, Austin, Texas 78758-4497, and Texas Natural Resource Conservation Commission, P.O. Box 13087, Austin, Texas 78711-3087
In this paper, we present ambient monitoring data from Houston, TX along with results from environmental chamber studies to suggest that molecular chlorine (Cl2), a photolytic source of chlorine atoms (Cl•), may contribute significantly to ozone (O3) formation in some urban environments. The ambient data were collected during an ozone episode in August 1993 that involved an alkane-rich hydrocarbon plume passing over anthropogenic sources of Cl2. Two unusual observations were made about the plume a few hours after it had passed over the Cl2 sources: (1) a rapid loss of alkanes and (2) a large increase in ozone concentration. Neither of these observations could be explained with models employing hydroxyl radical (OH•) chemistry (OH• are generally accepted to control oxidative chemistry in the daytime troposphere). Environmental chamber experiments were performed to determine whether the addition of Cl2 to a mixture of air, hydrocarbons, and nitrogen oxides (NOx) representative of conditions in the Houston area would yield similar results. The results of these chamber experiments indicated that Cl2 enhances O3 production when alkanes dominate the hydrocarbon mixture, with a possible enhancement in ozone production of between 5 and 10 mol of ozone produced per mol of Cl2.
Introduction It has been suggested that Cl• and other reactive halogen species can contribute significantly to or even locally dominate tropospheric oxidative chemistry in marine environments (1-4). Recently, halogen atoms, primarily bromine and to a lesser extent chlorine, have been identified as the species responsible for observations documenting the consumption of ground-level O3 in the Arctic (4-7). It has also been suggested that Cl• may promote the formation of O3 in the presence of VOCs and NOx (2, 3). However, there has been little attention recently directed at characterizing the ability for Cl• to promote O3 formation in urban environments, where NOx and volatile organic compounds (VOCs) are ubiquitous and many anthropogenic sources of Cl2 exist. * Corresponding author phone: (512)475-7842; fax: (512)471-1720; e-mail:
[email protected]. † University of Texas at Austin. ‡ Texas Natural Resource Conservation Commission. 4470
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Hov8 reported the results of a modeling study in 1985 in which the role of industrial chlorine emissions on photochemical oxidant formation was investigated downwind from a Norwegian industrial center. The industrial center contained numerous emission sources of NOx, hydrocarbons, and Cl2, among other species. By using emissions data for southern Telemark, Norway from 1980 to 1981, Hov was able to show that chlorine causes large increases in photochemical activity that translated into rapid promotion of ozone and PAN formation downwind of the industrial center. In addition, the fractional decomposition of C1-4 alkanes, C2,3 alkenes, and m-xylene were calculated for a 1 h period in the presence of OH• only and OH• present with Cl• from industrial Cl2 emissions. The calculation indicated that the fraction of m-xylene and propene that decomposed after 1 h increased by a factor of 2 with the addition of Cl2 emissions, whereas the fractional decomposition of C1-4 alkanes increased by up to 56 times with the addition of Cl2 emissions. For ethane and n-butane, the fractional decomposition increased from 0.5 and 4% to 29 and 26%, respectively, with addition of a Cl2 source (8). Although the Hov study focused on chlorine released from sources in southern Telemark, Norway, large sources of Cl2 exist in the United States. These large anthropogenic sources of Cl2 emissions include chemical production facilities, water treatment plants, smelters, and paper production operations. Figure 1 contains a map showing the locations of industrial air emission sources of Cl2 that emitted greater than 50,000 pounds of Cl2 to the atmosphere in 1996. The region in and around Houston, TX provides an opportunity to study the chemistry of anthropogenic emissions of Cl2 within an urban environment. As shown in Figure 1, several large sources of Cl2 exist in the Houston area. According to the Toxics Release Inventory (9) (TRI), approximately 95,000 kilograms of Cl2 were emitted by industrial sources in Houston and the remainder of Harris County in 1993. This estimate is likely a lower bound on Cl2 emissions because the inventory is limited to operations with emissions that exceed a threshold quantity. Nevertheless, the data from the TRI provide a reasonable starting point for assessing the potential role of Cl2 in the chemistry of urban atmospheres. To determine the potential role of this anthropogenic Cl2 in Houston oxidative chemistry, a comparison must be made between the availability of other oxidative species (such as OH•) and that of Cl•. Because the TRI only reports total mass emitted on an annual basis, it is not possible to directly quantify the role of Cl2 in controlling oxidation chemistry within a volume of influence. However, since the primary mechanism for Cl2 destruction in the daytime troposphere is photolysis, an estimate can be made of the number concentration of Cl• emitted in the volume of influence (VOI) by assuming that the Cl2 emissions are constant and well mixed within the VOI. The locations of the Cl2 emission sources are concentrated near the Houston Ship Channel within an approximate area of 20 kilometers (km) by 10 km. Given this 200 km2 area, a mixing layer of approximately 500 m, and assuming that the 95,000 kg of Cl2 (1.3 × 106 mol Cl2) is emitted at a constant rate, the rate of emission is equivalent to 1.8 × 109 Cl• hour-1 cm-3. It should be noted that the rate of Cl2 emission in Harris County (in 1996) is on the same order of magnitude as that of Telemark, Norway in 1985. The 1985 modeling study by Hov showed that Cl• from industrial sources could rapidly increase ozone and PAN formation rates while causing rapid depletion of alkanes. 10.1021/es991380v CCC: $19.00
2000 American Chemical Society Published on Web 09/15/2000
FIGURE 1. Industrial sources emitting greater than 50,000 pounds of Cl2 in 1996 (9).
FIGURE 2. Possible effect of Harris County chlorine sources on ozone formation, August 19, 1993. Notes: (1) VOC (including alkanes) and chlorine emissions mixed with the air mass as it approached the Clinton monitor but did not have time to react. The Clinton monitor reported high alkane concentrations but only 115 ppb O3 at 1400 h. (2) An hour later, the HRM3 monitor (downwind) reported 170 ppb O3 for the same air mass. (3) Another hour later, the air mass reached the Aldine monitor, which reported greatly reduced alkane concentrations and a large increase in O3 concentration (231 ppb). (4) Since the peak O3 concentrations detected at surrounding monitors peaked at approximately 160 ppb O3, it is suspected that chlorine emissions caused an increase of 50-80 ppb O3. The clearest evidence in ambient monitoring data for the role of anthropogenic Cl2 emissions in O3 formation in the Houston area is provided by an unusual episode that occurred on August 19, 1993, during the last major air quality field study conducted in Southeast Texas (10). During this episode, data collected by monitoring stations located near Clinton Drive in Houston (upwind) and Aldine (downwind) (see Figure 2) documented a very large increase in O3 concentration, loss of alkenes and substituted aromatics, and a dramatic loss of alkanes. This air mass contained an atypically high concentration of substituted alkanes prior to passing over known sources of Cl2 (11). According to trajectory calculations, the hydrocarbon plume traveled from the Clinton monitoring station to the Aldine station (Figure 2) in approximately 2-3 h. Upon arrival of this air mass over Aldine, a sharp rise in O3 concentration was observed. The peak concentration of O3 measured by the Aldine monitoring
station was 50 to 80 parts per billion volume (ppbv) higher than the level recorded at any other monitoring station in the Houston area. The observed loss of alkenes and substituted aromatics from the plume can be explained by the fast rates of reaction between OH• and substituted aromatics or olefins. However, the relatively slow rates of reaction between OH• and alkanes (12-17) cannot account for the observed loss of alkanes during the 2-3 h that elapsed between the time the air mass was sampled at the upwind (Clinton) and downwind (Aldine) monitors. In addition, reaction of longer-chain alkanes (C6+) with OH• has been shown to decrease ozone production in model urban atmospheres (18). The rapid loss of alkanes and very large increase in O3 concentrations suggest that an oxidizing species other than OH• was responsible for initiating the August 1993 episode. Because the plume exhibited a rapid loss of alkanes after passing over known sources of Cl2, VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Rate Constants for Selected Hydrocarbons compound
kOH‚ (10-14 cm3 molecule-1 s-1)
kCl‚ (10-14 cm3 molecule-1 s-1)
Alkanes methane (12) ethane (12) propane (12) n-butane n-pentane (13) n-hexane (13)
0.64 25 110 244 (13) 400 545
ethene propene
900 (12) 3000 (12)
benzene toluene
111 (14) 5960 (14)
10 5900 14,000 22,000 (12) 28,000 34,000
Alkenes 10,700 (13) 28,000 (13)
Aromatics 0.9 (15) 5890 (16)
chlorine oxidation chemistry was suspected for the observed chemistry. Chlorine atoms, formed by the rapid photolysis of Cl2, react with alkanes up to 2 orders of magnitude faster than do hydroxyl radicals (12-17) and can promote the formation of O3 in the presence of VOCs and NOx (2, 3). For example, assuming a concentration of OH radicals of 1 × 107 molecules per cubic centimeter, the lifetime of hexane is approximately 5 h. For the same concentration of Cl radicals, however, the lifetime of hexane is approximately 5 min. Rate coefficients are provided in Table 1 for the reaction of Cl and OH radicals with selected hydrocarbons. To determine whether the anthropogenic chlorine could explain the loss of alkanes and large increase in observed O3 concentration, a series of experiments was performed in outdoor Teflon environmental chambers at our laboratory. The results of these experiments provided data to quantitatively characterize the role chlorine plays in O3 formation in model urban atmospheres.
Experimental Section The environmental chambers used in this study were approximately 2 cubic meters (m3) in volume with internal volume-to-surface ratios of approximately 0.13 m when fully inflated. The chambers were conditioned (19, 20) and subsequently prepared by flushing with clean, dry air overnight. A commercially available mixture of 56 hydrocarbons (Matheson “Enviro-Mat” Ozone Precursor) as well as individual hydrocarbon reactants were used in the experiments. NOx (Praxair- NO/NO2 at a ratio of 200:1) was also injected into the chamber, while the chamber was covered with an opaque tarp. After these reactants were allowed sufficient time to mix, Cl2 (Air Products and Chemicals) was injected, the tarp was removed, and gas sampling was begun. Gas withdrawn from the chamber was delivered to O3 (Dasibi 1008AH or 1003PC) and NOx (Monitor Laboratories 9841 or Columbia Scientific Industries 1600) analyzers. These continuous measurements were collected as 5-min averages by Climatronics Corporation IMP 850 microloggers. Air samples for hydrocarbon analysis were collected in 6-liter stainless steel Summa canisters and analyzed by a HP 5890A gas chromatograph (GC) equipped with a flame ionization detector (FID) and/or a HP 6890 GC with a HP 5972 mass selective detector and Entech 7000 preconcentrator/cryofocuser.
Results and Discussion A first set of chamber experiments was directed at showing whether the addition of Cl2 to a mixture of VOCs and NOx (to model conditions found in Houston) would promote the formation of O3. A summary of the experimental results is provided in Table 2. Initial reactants included a mixture of 56 hydrocarbons with a total hydrocarbon mixing ratio of 4472
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approximately 1 part per million carbon (ppmc). Sufficient NOx was injected to yield an initial VOC/NOx ratio of 10 ppbc/ ppbv. Initial Cl2 concentrations were between 0 and 47 ppbv. Each run was conducted under conditions of similar solar flux and temperature. The data summarized in Table 2 show that the peak O3 concentration ([O3]peak) increases by up to a factor of 6 with the addition of Cl2, and the time required to reach 0.63*[O3]peak was reduced by up to a factor of 3.5. In addition, experiments that included injections of Cl2 showed significant losses of alkanes, specifically substituted alkanes. The loss of alkanes increased from less than 10% in experiments without added Cl2 to greater than 60% for hexane, 2,3-dimethylpentane, and octane in experiments with added Cl2. Losses of alkenes and aromatic compounds were similar between runs with and without added Cl2 (Table 3). These two phenomenasa dramatic increase in the production of O3 and loss of alkanesswere also observed during the August 19, 1993 episode described above. The environmental chamber data can also be used in a more quantitative explanation of the August 19, 1993 episode. We note that the estimated rates of Cl2 release from sources near the Clinton monitoring station were 100 mol per hour (based on data from the 1993 Toxics Release Inventory) and that approximately five mol of O3 are produced per mol of molecular chlorine injected in chamber experiments (Table 2). This suggests that the chlorine injection would have resulted in the formation of an additional 500 mol of O3 per hour. This quantity of O3 corresponds to an additional 50 ppb of O3 distributed evenly throughout a volume of approximately 0.26 cubic kilometer. Given a mixing height of 0.5 kilometer, this corresponds to a 0.7-kilometer by 0.7kilometer area of dramatically elevated O3 concentrations per hour of emissions. These estimates of O3 production associated with anthropogenic chlorine emissions are based exclusively on emissions reported through the Toxics Release Inventory and therefore represent a lower bound on the influence of Cl2 on an urban center such as Houston, where other sources of Cl2 emissions exist. All of these estimates of O3 production are based on the O3 yield for chlorine injected into a mixture of 56 hydrocarbons, representative of urban air mixtures. Further, all of the initial experiments were performed at a VOC/NOx ratio of approximately 10 ppbc/ppbv. A second set of chamber experiments were run to identify whether variations in the hydrocarbon precursor composition and the VOC/NOx ratio would affect O3 production associated with the addition of Cl2. These experiments used n-pentane (EM Omnisolv), propene (Matheson-C.P. grade), and benzene (EM-reagent grade) to represent major classes (alkanes, alkenes, and aromatics) of hydrocarbons found in the urban troposphere. In runs with a high VOC/NOx ratio (20-30) and approximately equal concentrations (in ppbc) of benzene, pentane, and propene, no significant changes in peak O3 concentration or rate of O3 formation were observed with the addition of Cl2. However, for runs with a low VOC/NOx ratio (5) containing pentane but no benzene or propene, injection of Cl2 at the start of experiment caused a significant increase in peak O3 concentration (Table 2). This increase was calculated to be equivalent to the formation of approximately 10 additional mol of ozone per mol of Cl2 injected. These preliminary experiments indicate that the O3 formation associated with chlorine releases will depend on hydrocarbon composition and the availability of NOx. At one extreme set of conditions (high VOC/NOx ratios with hydrocarbons that are reactive with OH•), chlorine injections produced no additional O3. At another extreme set of conditions (low VOC/NOx ratios with hydrocarbons that are relatively unreactive with OH•), chlorine injections produced approximately twice the amount of O3 per mol of Cl2 injected
TABLE 2. Environmental Chamber Data: Mol of Additional Ozone Generated Per Mol of Chlorine Injected, Relative to a Base Case Chamber Experimenta HC/NOx (ppbc/ppbv)
[Cl2]o (ppbv)
[O3]Peak (ppbv)
time to reach 0.63* [O3]Peak (min)
(∆O3-NO)(ppbv at max)/ Cl2(ppbv injected)
mixture of 56 hydrocarbons mixture of 56 hydrocarbons mixture of 56 hydrocarbons mixture of 56 hydrocarbons
10 10 10 10
0 14 20 47
37 85 120 262
pentane, benzene and propene pentane, benzene and propene pentane pentane
20 30 5 5
0 5 0 5
312 305 90 138
95 41 52 27 av: 165 137 227 197
b 4.5 5.0 5.1 4.9 b c b 9.6
hydrocarbon (HC) mixture
a Data are reported for a variety of hydrocarbon precursor mixtures. bNot applicable. c No increase in peak ozone concentration was observed relative to the base case.
TABLE 3. Fractional Losses of Selected VOCs in Environmental Chamber Experiments Performed with a 56-Compound Mixture as the Hydrocarbon Precursor fraction lost [Cl2]o ) compound
0 ppbv
14 ppbv
20 ppbv
47 ppbv
0.30 0.39 0.32 0.52
0.62 0.37 0.55 0.43
0.63 0.60 0.76 0.43
0.24 1.00
0.32 1.00
0.32 1.00
0.01 0.10
0.01 0.01
0.11 0.21
Alkanes 2,3-dimethylpentane n-hexane n-octane n-nonane
0.13 0.02 0.06 0.01
Alkenes 1-pentene 2-methyl-1,3-butadiene
0.31 0.88
Aromatics benzene toluene
0.17 0.18
than in the base set of experiments (56 compound hydrocarbon mixture and VOC/NOx ratio of 10 ppbc/ppbv). Based on the results of these experiments, it can be concluded that hydrocarbon mixtures containing a high concentration of species that react quickly with OH• to yield O3 (such as olefins) are not affected by addition of Cl2. However, in mixtures containing species that do not rapidly react with OH• to initiate O3 formation (such as paraffins), the addition of Cl2 is very important. Because the added Cl2 photolyzes rapidly and the Cl• reacts very quickly with alkanes, paraffinic ozone precursor species that otherwise would be inert to OH• attack can be rapidly activated by Cl• and contribute to ozone formation. Because Cl• reacts with alkanes via hydrogen abstraction to form photolytically inactive hydrogen chloride (HCl), Cl• availability decreases over time unless a continual source is present. In the experiments described here, Cl2 was injected only at the beginning of each run. Therefore, in the chamber experiments where alkanes dominated the hydrocarbon mixture, chlorine activated paraffins early in the run to form alkyl radicals and photolytically inactive HCl through hydrogen abstraction. Although Cl• is consumed, the resulting alkyl radical reacts rapidly with oxygen to form an alkylperoxy radical (RO2). The RO2 can then react with NO to form an alkoxy radical (RO) and NO2. Finally, the RO radical can react rapidly with O2 to form HO2 and a set of carbonyls that can continue to react. The reaction of Cl• with paraffins rapidly promotes ozone formation by initiating a set of reactions leading to conversion of NO to NO2 and formation of HOx. Our initial findings are presented as evidence to suggest that Cl2 can promote O3 formation in some urban areas. By adding Cl2 to a mixture of VOCs and NOx in air typical of urban atmospheres, we show that the rate of formation and peak concentrations of O3 significantly increase relative to
control runs without added molecular chlorine. In addition to an increase in O3 formation, Cl•-dominated chemistry appears evident given the significant loss of alkanes over the time scale described by the August 1993 episode. Continuing environmental chamber and modeling studies should probe the O3 formation potential of chlorine under a variety of conditions and the role of possible natural sources of molecular chlorine, such as chlorine liberation from sea salt aerosols in urban environments.
Acknowledgments The authors thank Peter Breitenbach of the Texas Natural Resource Conservation Commission for initially organizing and advocating this project and Dr. Gary Vliet and Ian Bird of the Solar Energy Laboratory at the University of Texas at Austin for providing solar flux data. C.B.M. acknowledges the generous support of the Welch Foundation. This work was supported by the Texas Natural Resource Conservation Commission (Contract 98-80076000).
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Received for review December 14, 1999. Revised manuscript received June 14, 2000. Accepted August 3, 2000. ES991380V
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