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.
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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
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 21, 2000
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 June 14, 2000. Accepted August 3, 2000.
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