Environ. Sci. Technol. 2006, 40, 251-262
Atmospheric Chlorine Chemistry in Southeast Texas: Impacts on Ozone Formation and Control
with OH• radicals (3-5), producing alkyl and alkylperoxy radicals.
SUNGHYE CHANG AND DAVID T. ALLEN* Center for Energy and Environmental Resources (R7100), The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758
Cl2 + hv f Cl• + Cl•
(1)
HOCl + hv f OH• + Cl•
(2)
Cl• + RH f R• + HCl
(3)
R• + O2 f RO2•
(4)
The alkylperoxy radicals then participate in well-known, ozone producing reactions.
Recent evidence has demonstrated that chlorine radical chemistry can enhance tropospheric hydrocarbon oxidation and has the potential to enhance ozone formation in urban atmospheres. To assess these effects quantitatively, an August-September 2000 photochemical episode in southeast Texas was simulated using the comprehensive air quality model, with extensions (CAMx). During this episode, ambient measurements of a unique marker of atmospheric chlorine chemistry, 1-chloro-3-methyl-3butene-2-one (CMBO), were made and model performance was assessed by comparing modeled and observed CMBO mixing ratios. The model predicted ambient CMBO mixing ratios within the uncertainty limits associated with the emissions inventory, so the model was used to assess the impacts of chlorine chemistry on ozone formation. Based on the current emissions inventory, chlorine emissions have the potential to enhance 1-h-averaged ozone mixing ratios by 70 ppb, in very localized areas, during morning hours. Over wider areas, and at times of day when peak ozone concentrations are observed, the impacts of chlorine emissions on ozone concentrations are typically less than 10 ppb. Chlorine emissions also influenced changes in ozone concentrations due to hydrocarbon and NOx emission controls.
Introduction Ozone formation in urban atmospheres typically proceeds through a series of free radical reactions involving volatile organic compounds (VOCs) and oxides of nitrogen (NOx) (1). Other species present in urban atmospheres can affect these photo-oxidation reactions, particularly if they serve as either a source or sink of free radicals. The Houston area, with a population of approximately 4 million people and a large industrial base performing chemical manufacturing and petroleum refining operations, has both typical urban emissions associated with ozone formation and a variety of industrial emissions that are not commonly encountered in urban areas. Among these emissions are releases of molecular chlorine that can influence ozone formation by serving as a source of free radicals. Chang et al. (2) have estimated that atmospheric emissions of molecular chlorine and HOCl total 104 kg/day in southeast Texas. Once emitted, HOCl and Cl2 quickly photodissociate, producing chlorine radicals. Under typical urban conditions, the chlorine radicals abstract hydrogen from hydrocarbons quickly, through reactions similar to those associated * Corresponding author phone: +1 512-475-7842; fax: +1 512471-7060; e-mail:
[email protected]. 10.1021/es050787z CCC: $33.50 Published on Web 12/01/2005
2006 American Chemical Society
RO2• + NO f NO2 + RO•
(5)
RO• + O2 f HO2• + carbonyl
(6)
HO2• + NO f NO2 + OH•
(7)
NO2 + hv f NO + O• (3P)
(8)
O2 + O• (3P) + M f O3 + M
(9)
O3 + NO f NO2 + O2
(10)
Atomic chlorine formed in the troposphere can also serve as an ozone sink:
Net:
Cl• + O3 f ClO• +O2
(11)
ClO• + HO2• f HOCl + O2
(12)
HOCl + hv f OH• + Cl•
(13)
Cl• + O3 + HO2• f Cl• + 2O2 + OH•
(14)
Under typical urban conditions, concentrations of hydrocarbons are sufficiently high so that the rate of reaction of atomic chlorine with hydrocarbons is much faster than the rate of reaction with ozone and the chlorine emissions enhance ozone formation (6, 7). Occurrence of Cl• chemistry in southeast Texas has been confirmed through the detection of a unique marker species, 1-chloro-3-methyl-3butene-2-one (CMBO), which is only produced from the reaction between Cl• and isoprene, with a molar yield of 0.28 (6, 8, 9). Riemer (9) made measurements of CMBO at a monitoring site at La Porte airport, east of Houston in an industrial source region, during August and September 2000. As shown in Figure 1, CMBO concentrations showed sharp maxima in the morning hours with peak concentrations of roughly 100 ppt (parts per trillion). These results will be compared, later in this paper, to predicted values of CMBO concentrations generated by a threedimensional, gridded photochemical model; however, the fact that the marker has been detected at levels significantly above detection limits provides direct evidence that chlorine emissions, and atmospheric chemistry associated with the emissions, do occur in southeast Texas. The goal of this work will be to examine the role that urban chlorine chemistry plays in ozone formation in southeast Texas. Chang et al. (2) performed a preliminary assessment of the role of chlorine emissions on urban ozone formation; however, that assessment was performed for a VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Isoprene and CMBO concentrations measured at the La Porte airport during August and September 2000 (9). 1993 photochemical episode. Since no data on CMBO concentrations were available for the 1993 episode, Chang et al. could only qualitatively evaluate the overall performance of the photochemical model in simulating chlorine chemistry. In this work, photochemical modeling results will be reported for the same episode for which CMBO concentrations are available, allowing quantitative assessment of the model performance. In addition, a series of sensitivity studies will probe the impact of chlorine chemistry on ozone formation and the sensitivity of that impact to NOx and VOC emissions.
Methods Photochemical Modeling. The main focus of this work will be on photochemical modeling of the regional impact of urban atmospheric chlorine chemistry. Regional photochemical models simulate emission, chemical transformation, horizontal advection and diffusion, vertical transport and diffusion, dry deposition, and wet deposition of species in the atmosphere. Although any comparable photochemical grid model could be used, the comprehensive air quality model with extensions (CAMx) (10) was selected for this study because it is currently being used by the State of Texas for attainment demonstrations in areas that have violated the National Ambient Air Quality Standards for ozone. Tanaka et al. (6, 11) modified the chemical mechanism in CAMx version 4.03 so that the model is now capable of assessing the regional impacts of chlorine radical chemistry. Chang et al. (2) used the CAMx model to examine an ozone episode from 1993, which was the most current photochemical modeling episode available for southeast Texas at the time that work was done. Since that time, model input data and monitoring data have been developed for a photochemical episode for the year 2000. Therefore, in this work, the CAMx 4.03 model was used with emission inventories, meteorology, and chemistry updated to the year 2000. The main features of this updated model and the air pollution episode used in the modeling analysis are briefly summarized here; complete details, including model input files, are available from the Texas Commission on Environmental Quality (TCEQ) (12). The modeling domain was a nested regional/urban scale 36 km, 12 km, and 4 km grid, shown in Figure 2. The episode period was August 22 to September 6, 2000. Meteorological inputs required by the model were based on results from the Mesoscale Meteorological Model, version 5 (MM5). The volatile organic compound (VOC) and NOx emission inventories used as input for the modeling episode were prepared by the TCEQ in accordance with U.S. EPA guidance. A MOBILE6-based inventory was developed for on-road mobile source emissions; inventories of emissions for nonroad mobile and area sources were developed using emission 252
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FIGURE 2. Modeling domain used in the study: The regional, East Texas, and Houston-Galveston-Beaumont-Port Arthur nested domains had 36, 12, and 4 km resolution, respectively. factors and the U.S. EPA’s NONROAD model, using local activity data when available. Biogenic emission inventories were estimated using the GLOBEIS emission model with locally developed landcover data (13). Point source emissions data were developed through a special inventory survey and were based on ambient data collected in the source region. Approximately 155 tons per day of VOC emissions at point sources were added to the regular emission inventory. The added VOC emissions were based on reconciliation between ambient measurements and emission rates and are primarily reactive olefins. Details of the VOC and NOx emission inventory development are available from the TCEQ (12). In this work, version IV of the carbon bond mechanism (CB-IV), with updated isoprene chemistry (10), was used to model atmospheric chemistry. The original CB-IV mechanism was modified (6, 11, 14) by adding reactive chlorine chemistry (Cl2, Cl, and ClO reactions) to the mechanism. The base chemical mechanism (the mechanism without chlorine chemistry) uses 96 reactions to describe ozone formation chemistry. Thirteen additional reactions were incorporated into the chemical mechanism, and they describe chlorine chemistry relevant to an urban atmosphere, such as in Houston. Therefore, the chemistry employed during the CAMx simulations in this work included 109 reactions. The 13 reactions added to represent chlorine chemistry and corresponding rate constants are provided in Table 1 (6). To run CAMx simulations with the chlorine chemistry mechanism, it was necessary to develop a chlorine emission inventory. Chang et al. (2, 15) have developed a chlorine emission inventory for a 1993 photochemical episode. In this work, the emission inventory was updated for the 2000 photochemical episode. The methods used in developing the inventory parallel those used by Chang (2) and are presented in the Supporting Information for this paper. Figure 3 shows a summary of the inventory data. The major sources of emissions were volatilization of chlorine from industrial cooling towers (used as a biocide, 6 metric tons/day (mt/d)), emissions of chlorine from swimming pools (5 mt/d), industrial point source emissions (largely from chloro-alkali manufacturing, 1 mt/d), and chlorine produced by the reactions of chloride in sea salt (0.3 mt/d). Other sources
TABLE 1. Chlorine Chemistry Incorporated into CAMx (6) k (cm3 molecule-1 s-1)
reactions (1) Cl2 ) 2Cl (2) HOCl ) OH + Cl (3) Cl + PAR ) HCl + 0.87XO2 + 0.13XO2N + 0.11HO2 + 0.11RCHO + 0.76ROR - 0.11PAR (4) Cl + OLE ) FMCL + RCHO + 2XO2 + HO2-1PAR (5) Cl ) HCl + XO2 + FORM + HO2 (6) Cl + ETH ) FORM + 2XO2 + FMCL + HO2 (7) Cl + ISOP ) 0.15 HCl + XO2 + HO2 0.28ICL1 (8) OH + ICL1 ) ICL2 (9) Cl + BUTA ) XO2 + HO2 + 0.70BCL1 (10) OH + BCL1 ) BCL2 (11) Cl + O3 ) ClO + O2 (12) ClO + NO ) Cl + NO2 (13) ClO + HO2 ) HOCl + O2
a
a a
78*kOH,PAR 20*kOH,OLE 6.6 × 10-12 exp(-1240/T) 12.6*kOH,ETH 4.5*kOH,ISOP 0.19*kOH,ISOP 4.2*kOH,ISOP 0.36*kOH,ISOP 2.9 × 10-11 exp(-260/T)b 6.2 × 10-12 exp(295/T)b 4.6 × 10-13 exp(710/T)b
name
description
name
description
BUTA BCL1 BCL2 ETH Cl Cl2 ClO CO FORM FMCL HCl HOCl
1,3 butadiene 4-chlorocrotonaldehyde (CCA) CCA + OH reaction products ethene chlorine atom molecular chlorine chlorine oxide carbon monoxide formaldehyde formyl chloride hydrochloric acid hypochlorous acid
HO2 ISOP ICL1 ICL2 OH OLE PAR RCHO ROR XO2 XO2N
hydroperoxyl radical isoprene 1-chloro-3-methyl-3-butene-2-one (CMBO) CMBO + OH reaction products hydroxyl radical olefinic bond (carbon double bond) paraffinic carbon higher aldehyde organic nitrate forming peroxy radical universal peroxy radical operator nitrate forming peroxy radical operator
The rate of these photolysis reactions is dependent on calculated sunlight intensity. b Source: ref 5.
were evaluated but found to be negligible. A variety of assumptions were made regarding the temporal and spatial distribution of the emissions (details available in the Supporting Information). Emissions from cooling towers were assumed to be due to shock chlorination done during morning hours and the emissions were assumed to occur at petrochemical facilities; emissions from swimming pools were assumed to occur during the afternoon when pool occupancy and agitation would be greatest and were assumed to occur in high income residential areas; industrial process emissions were assumed to be constant and their location was determined using permit data; reactions of chloride in sea salt were assumed to occur near the coast where oxidant species would be available and the reactions were assumed to occur during daylight hours. Observational Data. To determine if Cl chemistry is occurring in the Houston area, Riemer (9) made measurements of isoprene and CMBO (as well as other species not directly relevant to this work) over the period of August 12 to September 12, 2000, at the La Porte airport, east of Houston near an industrial source region. Ambient air samples were initially drawn from a continuously flushed glass manifold into fused-silica-lined stainless steel traps and sample lines internal to a concentration system. Sample analysis was performed using gas chromatography with mass spectrometric detection. The spectrometer was used in single ion monitoring mode, which allowed very low detection limits. The detection limit for isoprene was
