Environ. Sci. Technol. 1094, 28, 88-98
Modeling the Concentrations of Gas-Phase Toxic Organic Air Pollutants: Direct Emissions and Atmospheric Formation Robert A. Harleyt and Glen R. Cass'
Environmental Engineering Science Department, California Institute of Technology, Pasadena, Californla 9 1125 An Eulerian photochemical air quality model is described for the prediction of the atmospheric transport and chemical reactions of gas-phase toxic organic air pollutants. Model performance was examined in the Los Angeles, CA, area over the period August 27-28, 1987. The organic compounds were drawn from a list of 189 species selected for control as hazardous air pollutants in the Clean Air Act amendments of 1990. The species considered include benzene, various alkylbenzenes, phenol, cresols, 1,3butadiene, acrolein, formaldehyde, acetaldehyde, and perchloroethylene among others. It is found that photochemical generation contributes significantly to formaldehyde, acetaldehyde, acetone, and acrolein concentrations for the 2-day period studied. Phenol concentrations are dominated by direct emissions, despite the existence of a pathway for atmospheric formation from benzene oxidation. The finding that photochemical production can be a major contributor to the total concentrations of some toxic organic species implies that control programs for those species must consider more than just direct emissions. 1. Introduction
Volatile organic compounds (VOCs) are pervasive in indoor and outdoor air (1,2). This is of concern because VOCs promote the photochemical formation of ozone and other oxidants (3) and because some individual VOCs are known or suspected to have adverse effects on human health (4, 5 ) . In the United States, recent amendments to the Clean Air Act (1990) require reductions in the emissions of VOCs that have been identified as toxic air contaminants. The state of California and other jurisdictions also have initiated programs to identify toxic air contaminants and to gather data on the emissions, atmospheric chemistry, ambient concentrations, and health effects of these compounds. Numerous studies have been published that examine individual compounds of concern; for examples, see refs 6-13. In order to verify our understanding of the emissions and atmospheric concentrations of individual toxic air pollutants, it is important to be able to predict the ambient concentrations of toxic pollutants directly from emission data and to compare these results against measured concentration data. However, in past studies, data on ambient concentrations generally have been analyzed separately from pollutant emission data. Some toxic pollutants may be formed in the atmosphere as oxidation products of other VOCs. In all but a few past studies, formation pathways are identified without exploring in detail how important photochemical production might be in determining ambient concentrations. In those cases where photochemical production has been considered, it
has been found to be important. A study of carbonyls (aldehydes and ketones) in Los Angeles air conducted by Grosjean et al. (14) did conclude that photochemical production of carbonyls dominates over direct emissions, based on changes in the measured ratios of carbonyl compound to carbon monoxide concentrations in ambient air. An analysis of carbonyl and non-methane hydrocarbon concentration data acquired during the summer of 1987 in the Los Angeles area indicates low carbonyl to hydrocarbon ratios in early morning samples, with much higher ratios later in the day (15). This result indicates the importance of direct emissions in some early morning samples plus significant photochemical formation of carbonyls later during the daytime. The objectives of the present study are to assess the contributions of various sources to the total direct emissions of individual toxic air contaminants, to model the emissions to air quality relationship for these species, and to investigate the relative importance of direct emissions versus atmospheric photochemical production in determining the atmospheric concentrations of toxic air contaminants. For this purpose, an Eulerian photochemical air quality model is described that tracks the transport and chemical reactions of selected toxic air pollutants. This model is applied to the Los Angeles, CA, area for a summertime period in 1987. 2. Selection of Species Studied
Title I11 of the Clean Air Act Amendments of 1990 specifies a list of 189 inorganic and organic species that are declared to be toxic air contaminants. This list includes the following: various organic solvents; pesticides and herbicides; chemical process feedstocks, intermediates, products and byproducts; some of the hydrocarbons present in gasoline; and combustion-derived species such as formaldehyde and acetaldehyde. In addition to direct emission sources, some of the toxic species such as aldehydes, ketones, phenol, and cresols are formed in the atmosphere as oxidation products of other directly emitted VOCs (16-18). Nineteen species appearing in the Clean Air Act list that are ubiquitous in urban areas are considered in the present study, as shown in Table 1,Many of the species listed in Table 1are emitted directly in motor vehicle exhaust (19-21). 3. Model Description
The CIT airshed model (22-24) is an Eulerian photochemical air quality model that solves the atmospheric diffusion equation
ac --! + V.(iiC,) = V.(KVC;) + Ri ,
at to predict the spatial and temporal distribution of pollutant
+ Present address: Civil Engineering Department, University of California, Berkeley, CA 94720.
concentrations in the atmosphere. The variables referred to in eq 1are defined as follows: Cj is the ensemble mean concentration of species i, ii is the wind velocity vector,
88 Envlron. Sci. Technol., Vol. 26, No. 1, 1994
0013-936X/94/0928-0068$04.50/0
0 1993 American Chemical Society
Table 2. Additional Reactions of Toxic Species Included i n the Model
Table 1. Toxic Organic Species a n d Hydroxyl Radical Reaction Rate Constants
kinetic parametersb
kinetic Darametersb species
koH" a t 300 K
A"
benzene toluene ethylbenzene isopropylbenzene o-xylene m-xylene p-xylene styrene phenol o-cresol p-cresol formaldehyde acetaldehyde 1,3-butadiene acrolein methyl isobutyl ketone ethylene glycol l,l,l-trichloroethane perchloroethylene
1.28 X 10-12 5.91 X 7.10 X 6.50 X 10-12 1.37 X 10-" 2.36 X 10-" 1.43 X 10-l' 5.73 X 10-" 2.63 X 10-l' 4.20 X 10-11 4.70 X 10-l' 9.80 X 1.56 X 10-l1 6.59 X 10-l' 1.99 X 10-11 1.41 X 10-11 7.70 X 10-12 1.23 X 10-14 1.72 X l t 1 3
2.500 X 10-12 1.810 X 7.100 X 6.500 X 1.370 X 2.360 X 10-" 1.430 X 1.070 X 10-" 2.630 X 10-'' 4.200 X 10-" 4.700 X 10-" 1.130 X 10-l' 5.550 X 1.480 X 10-" 1.990 X 10-" 1.410 X 10-" 7.700 X 3.100 X 10-12 9.400 X
Eac ref 0.397 -0.705 0.000 0.000 0.000 0.000 0.000 -1.000 0.000 0.000 0.000 -1.288 -0.618 -0.890 0.000 0.000 MOO 3.299 2.385
25 25 25 25 25 25 25 28 30 30 30 25 25 25 17 17 25 31 31
In cm3 molecule-1 5-1. b The hydroxyl radical reaction rate constant is computed from the kinetic parameters using the expression kOH = A e~p(-EJRT)('l"/300)~ using R = 0.001 987 2 kcal mol-' K-l. B = 0 in all cases except formaldehyde for whichB = 2. In kcal/mol. 0
K is the eddy diffusivity tensor, Ri is the rate of generation of species i by chemical reactions, and t is time. At ground level, the boundary condition is
where K,,is the vertical eddy diffusivity,Ei is the emission flux, and uk is the dry deposition velocity for species i. A no-gradient boundary condition is applied a t the top of the modeling region, and lateral boundary conditions are set using measured pollutant concentrations. Initial conditions also are set using measured data. 3.1. Chemical Mechanism. The gas-phase atmospheric reactions of VOC, NO,, and other pollutants are represented in the model using a chemical mechanism derived from the work of Carter (25). The starting point for this mechanism is the complete set of reactions listed in Table 2 of ref 25. The mechanism is configured with methane, ethane, four lumped higher alkanes, three lumped aromatics, ethene, and three lumped higher olefins. The biogenic emissions of compounds such as isoprene and the terpenes are lumped separately from the other olefins. In this study, reactions are added to the mechanism so that each of the toxic organic species listed in Table 1 is tracked as a separate species. Reaction rate constants of the toxic species with the hydroxyl radical are shown in Table 1. Additional reaction rate constants of toxic species with ozone, the nitrate radical, and monatomic oxygen are listed in Table 2. Those organics not treated explicitly in the modified chemical mechanism remain incorporated into the lumped species categories of ref 25. Oxidation product yields and reaction rate constants for the lumped organic species were calculated using data from ref 25, together with the respeciated VOC emission inventory of Harley et al. (26) and the PREPEMIT computer program described by Carter (27). The extended chemical mechanism includes 76 active chemical species and 226 reactions. All of the extensions to the published mechanism of Carter (25) are listed in supple-
ka at 300 K
reaction
+ +
styrene 0 3 styrene + NO3 styrene 0 phenol + NO3 o-cresol + NO3 p-cresol + No3 formaldehyde HOz formaldehyde + NO3 acetaldehyde + NO3 l,3-butadiene + O3 1,3-butadiene NO3 1,3-butadiene + 0 acrolein + NO3
+
+
1.77 X 1.55 X 1.79 X 3.92 X 1.37 X 1.07 X 7.79 X 6.38 X 2.84 X 7.93 X 1.03 X 2.10 X 1.20 X
3.460 X 6.550 X 1.210 X 10-l2 3.920 X 10-l1 1.370 X 1.070 X lo-" 9.700 X 10-16 2.800 X 1.400 X 10-15 3.300 X l P 1 3 1.480 X 10-l' 2.100 X 1.200 X 10-'5 10-l' lO-l3
Eac
ref
3.144 2.233 -0.235 10-l2 0.000 0.000 lo-" 10-l' 0.000 -1.242 10-12 5.000 10-'2 3.696 4.968 10-11 2.959 10-l' 0.000 10-'6 0.000
28 28 28 30 30 30 25 25 25 25 71 25 17
A" 10-l6
In cm3molecule-' s-l. The rate constant is computed from the kinetic parameters using the expression k = A exp(-EJR!I') using R = 0.001 987 2 kcal mol-' K-l. In kcal/mol.
mentary material. These extensions are documented in the following paragraphs. Reactions of the aromatic hydrocarbons benzene, toluene, ethylbenzene, isopropylbenzene, and each of the three isomers of xylene are taken from Table 7 of ref 25. Styrene is also tracked explicitly in the extended chemical mechanism, using reactions recommended by Carter (28). This mechanism assumes that the initial reactions of styrene are similar to those of the olefins (Le., reactions take place a t the double bond). Formation of ringretaining products in the atmospheric oxidation of benzene and toluene has been studied by Atkinson et al. (16)among others. The yield of phenol from the reaction of benzene with OH was measured to be 0.236 f 0.044. For the reaction of toluene with OH, cresol (methylphenol) yields of 0.204 f 0.027 for o-cresol and 0.048 f 0.009 for the sum of m- and p-cresol were measured. Earlier work by Gery et al. (29) showed similar cresol yields from toluene oxidation and a ratio of p - to m-cresol of 17:2. Therefore, because rn-cresol is a minor product, only o- and p-cresol are represented explicitly in the model, and rn-cresol is lumped together with p-cresol. Subsequent reactions of phenol and cresols have been studied (301, and rate constants are shown in Tables 1 and 2. Direct emissions of phenol are tracked separately from the phenol formed by benzene oxidation, as described in more detail later in the text. Carbonyl species such as formaldehyde, acetaldehyde, propionaldehyde, acetone, and methyl ethyl ketone can be formed photochemically in the atmosphere as oxidation products of other VOCs (17). These aldehydesand ketones are produced in numerous reactions involving many different VOC precursors. Estimates of oxidation product yields for individual VOCs are presented in Tables 6-9 of ref 25. Whereas formaldehyde and acetaldehyde are explicit species in the mechanism, propionaldehyde (RCHO) and methyl ethyl ketone (MEK) are used as surrogate species (Le., the RCHO species in the model represents both true propionaldehyde and higher molecular weight aldehydes; likewise, the MEK species in the model represents both true methyl ketone and higher ketones). Although propionaldehyde and methyl ethyl ketone are identified as toxic air contaminants in the Clean Air Act, concentration estimates produced by the model for these species will not Envlron. Scl. Technol., Vol. 28, NO. 1, 1994 89
...--.-Computational Region Boundary
-.-.-.
0
Special SCAQS air monitoringsite
County Boundary
Flgure 1. Map of southern Callfornla showing the computational region and the locations of special SCAQS air quality monitoring sites.
be examined here because of their use as surrogate species within the chemical mechanism. Acetone is included as an explicit model species, although it is not considered to be a toxic species. Like the other carbonyls mentioned above, acrolein (propenal) can be formed in the atmosphere. However, unlike formaldehyde and acetaldehyde, which have many different VOC precursors, only 1,3-butadiene is expected to be a significant source of secondary acrolein in the atmosphere (10). In the model, one molecule of acrolein is formed for each molecule of 1,3-butadiene that reacts with OH or NO3; a yield of 0.5 molecules of acrolein is used per molecule of l,&butadiene reacting with ozone (25). The reactions of acrolein with OH and NO3 are included in the extended mechanism using rate constants recommended by Atkinson ( 1 7 ) . Acrolein photolysis and the ozone-acrolein reactions are neglected ( 1 7 ) . Several other toxic organic species are tracked explicitly in this study. The reaction of methyl isobutyl ketone with OH is included in the extended chemical mechanism, using a rate constant recommended by Atkinson ( 1 7 ) and the reaction products used by Carter (25)for all Cq and larger ketones. Ethylene glycol also is included in the present study, based on data from Table 8 of ref 25. Reaction rate constants for l,l,l-trichloroethane and perchloroethylene with OH follow the recommendations of Atkinson et al. (311, The reactions of OH with ethene and ethylene glycol form glycolaldehyde, which is represented using acetaldehyde by Carter (25). For the purposes of this study, the glycolaldehyde formed in these reactions is now tracked separately from the true acetaldehyde formed during the atmospheric oxidation of other VOCs. This avoids any miscounting of the glycolaldehyde as if it were acetaldehyde. In cases where photochemical formation may contribute to the ambient concentrations of individual toxic organic air pollutants, the chemical mechanism has been extended so that the contribution of direct emission sources can be resolved separately from the generation of these species by photochemical reactions. This has been accomplished by using separate model species to represent the directly emitted fractions and the photochemically derived fractions for formaldehyde, acetaldehyde, acrolein, acetone, 90
Envlron. Scl. Technol., Vol. 28, No. I , 1994
and phenol. Total predicted concentrations for each species are computed by summing the predicted contributions from direct emissions and from photochemical formation. Predicted formaldehyde, acetaldehyde, and acetone concentrations are further subdivided to track the contribution due to influx of these pollutants across the boundaries of the modeling region and due to initial amounts of these pollutants present at time zero. 3.2. Dry Deposition. Dry deposition velocities are computed in the model by use of a three-resistance scheme (32)that includes turbulent transport through the atmospheric boundary layer (ra),diffusion through a laminar sublayer ( r b ) , and a surface resistance term (r;)to account for differences in pollutant-surface interactions. The deposition velocity u$ for species i is given by u$ = l/(ra
+ rb + ()
(3) The atmospheric resistances (ra and rb) are computed in each model grid square using local surface roughness characteristics and meteorological conditions. Surface resistance values for the hydrocarbons and ketones listed in Table 1are set to 50 s/cm throughout, and therefore dry deposition velocities for these species are limited to a low value of 0.02 cm/s. Surface resistances are lower for species that are highly reactive and more soluble in water (33). Approximate surface resistance values for phenol and the cresols have been set to match the values used for S02. The values used for the SO2 surface resistances are in accordance with those developed for use in the regional acid deposition model (34). For formaldehyde, the surface resistances are set a t half the corresponding values used for SOz and to double the SO2 surface resiatance values for the other aldehydes (35). Atmospheric reaction and advection out of the air basin rather than dry deposition are likely to be the major removal processes for the organic species considered here. 4. Model Application
The model is applied to the Los Angeles, CA, area for the period August 27-28,1987. A regular 5 by 5 km grid system has been superimposed on the modeling region shown in Figure 1,and meteorological and emission data are specified as inputs to the model for each grid square
and at each hour. The modeling region extends to 1100 m above ground level, with five computational layers in the vertical direction. Meteorological fields were developed using the extensive set of measurements available from the Southern California Air Quality Study (SCAQS). Surface-level hourly average wind speed and direction measurements were made at 50 sites. In addition, six upper air soundings per day were made a t each of eight sites to measure the vertical profiles of temperature, humidity, and wind velocity. Hourly measurements of ground-level temperature were made a t 59 sites, and humidity measurements also were reported a t 43 of these sites. Hourly measurements of solar ultraviolet radiation were available a t five sites. The wind, mixing depth, temperature, humidity, and solar radiation fields required by the CIT airshed model were prepared by spatial interpolation of the available observations, as described in more detail elsewhere (24, 36-38). Boundary and initial condition values are set using measured pollutant concentration data. There is no significant input of these toxic organic air pollutants a t the upwind boundary of the modeling region, except for formaldehyde (3 ppb) and acetaldehyde (5 ppb). The model is executed over a 2-day period; the first of these two days (August 27) is used to reduce the effect of uncertainties in specification of the initial conditions. Analysis of model results will focus on the second day (August 28). Emission Inventory. An emission inventory for the Los Angeles area has been prepared by the California Air Resources Board and the South Coast Air Quality Management District (39)and forms the starting point for the emission inventory used in the present study. The inventory specifies hourly emission rates of carbon monoxide, oxides of nitrogen, and volatile organic compounds for over 800 source types. These emission estimates are resolved spatially over a 5 by 5 km square grid system. Mobile source emissions are estimated by use of the EMFAC 7E emission factor model and typical weekday traffic patterns developed by use of a travel demand model (40). Numerous studies have concluded that the official emission inventory prepared by the government for the 1987 period studied here understates the emissions of VOCs and CO (3, 41-43). Emissions of VOCs and CO from on-road vehicles measured in the Van Nuys tunnel during 1987 were found to be up to 3 times higher than the values computed by the EMFAC 7E emissions model (44). Therefore, to compensate a t least in part for understated emissions, the on-road vehicle hot exhaust emissions of VOC and CO were scaled up to 3 times the baseline (EMFAC 7E) values to reflect better the measured emission rates in the Van Nuys tunnel. For the purposes of mapping total VOC emissions to individual organic species, each source is assigned to one of 225 chemical composition profiles. The composition profiles supplied with the official emission inventory have been updated by Harley et al. (26). Results from a study of emissions from 20 old (mostly pre-1975 model year) light-duty noncatalyst vehicles tested burning regular leaded gasoline were used to develop a new description of the chemical composition of VOCs in noncatalyst gasoline engine exhaust (45).VOC exhaust emissions from catalystequipped gasoline engines are speciated on the basis of results from the auto/oil study (46)for 1983-1985 model
Table 3. Emission Composition* of Selected Toxic Species in Direct Source Emissions
species benzene toluene ethylbenzene isopropylbenzene o-xylene m-+ p-xylene styrene phenol formaldehyde acetaldehyde 1,3-butadiene acrolein
gasoline engine unburned gasoline exhaust (summertime) diesel catalyst whole headspace engine noncatalystb equipped' liquidd vapors" exhaust 3.6 5.8 1.2 0.3 1.6 4.2 0.66 0.30 2.30 0.63 1.2 0.47
3.7 8.9 1.3 0.1 1.3 3.5 0.21 0.30 1.20 0.62 0.2 0.14
1.9 10.2 1.9 0.2 3.1 8.3
0.66 0.65 0.03
1.7 1.V 0.1
0.04 0.13
0.6 0.3 0.06 0.14 8.8 3.0 1.6 1.4
0.1
Percent by weight relative to total organic gas emissions. b Exhaust speciation for gasoline-powered light-duty vehicles without
catalytic converters (45). Exhaust speciation for catalyst-equipped gasoline-poweredlight-duty vehicles (46-49). Compositionof whole liquid gasoline (50).e Composition of gasoline vapors in headspace over liquid fuel (50).f Toluene content of diesel engine exhaust may be overstated in this speciation profile. However, contribution of diesel exhaust to overall toluene emissions is still minor when the value stated above is used, as shown in Table 5.
year vehicles tested using industry average gasoline, combined with results from several studies by the U.S. Environmental Protection Agency (47-49). Separate profiles that describe the composition of unburned liquid gasoline and the vapors existing in the headspace over the liquid fuel are derived from measurements made on composite Los Angeles gasoline samples (50), updated to reflect 1987 sales of gasoline by grade (26). Diesel engine exhaust emissions are speciated using the profile supplied with the official state of California emission inventory, which is based on Table 9-07-021of the US. Environmental Protection Agency's species data manual (51). The abundance of the toxic organic species considered in the present study in VOC emissions from these sources is shown in Table 3. A more complete tabulation of the VOC speciation profiles is presented elsewhere (26). The available speciation profiles described above do not include phenol emissions and only include styrene emissions for catalyst-equipped vehicles. Average phenol emission rates of 3.0 mg/km for gasoline-powered vehicles, and 4.5 mg/km for diesel trucks, were measured in the Allegheny tunnel (20). In the same study, styrene emission rates of 6.6 and 1.8 mg/km were measured for gasolinepowered vehicles and diesel trucks, respectively. Using these data and total hydrocarbon emission rates measured in the same tunnel in a separate study (521, phenol and styrene were added to the engine exhaust speciation profiles, as shown in Table 3. Acrolein was missing from the official diesel engine exhaust speciation profile, so a value of 1.4% by weight relative to total organic gas emissions was added based on test results for a heavyduty diesel engine (53).Results of emission tests for two light-duty diesel vehicles (54) were used to establish a content of 1.6 w t % l,&butadiene in diesel engine exhaust. Further study of the emissions from heavy-duty diesel engines is recommended. Total emissions from all sources combined within the region mapped in Figure 1 for each of the toxic organic species tracked in the present study are presented in Table 4. The contribution of individual sources to the total Environ. Scl. Technol., Vol. 28, No. 1, 1994 81
Table 4. Regionwide Emission Totals for Selected Toxic Organic Species. species
regionwide emissions (108 kg/day)
benzene toluene ethylbenzene isopropylbenzene total xylenes styrene phenol formaldehyde
52 144 16 2.3 116 8.5 3.6 29
regipnwide emissions
species
(lo8kg/day)
acetaldehyde 1,3-butadiene acrolein methyl isobutyl ketone ethylene glycol l,l,l-trichloroethane perchloroethylene
9.1 10.1 4.6 14 10
47 28
Emissions inventory region includes most of the mapped area shown in Figure 1. The on-road gasoline-poweredvehicle hot exhaust emissions of all VOCs including toxic species have been scaled up to 3 times the official state of California emission inventory (EMFAC 7E) values, as suggested by on-road vehicle emission rates measured in the Van Nuys tunnel (41). The speciation of VOC emissions is as reported in ref 26, with additional emission data for phenol, styrene, and acrolein as discussed in section 4 of the present study.
Table 5. Source Contributions to Total Emissions of Selected Toxic Organic Species within the Southern California Study Area Shown in Figure 1
source on-road vehicles engine exhaust noncatalysta catalysta diesel evaporative hot soak diurnal running loss other exhaust* solvent emissions house paints water-borne solvent-borne industrial coatings adheskes thinning solvents all other sources total
emissions (109 kg/day) total total 1,3VOC benzene toluene xylenes acrolein butadiene
5. Ambient Concentration Data 590 528 26
21.4 19.6 0.4
34.5 47.0 0.5
33.7 25.4 0.2
98 24 51 74
1.8 0.2 0.3 2.2
10.0 0.2 0.3 3.1
11.2 0.0
3.5
1.1
29.1 3.4 5.6 6.7 144
31.3 0.1 3.2 7.1 116
23 110 198 52 .~ 27 735c 5.9 2536d 52
0.1 2.9
2.8 0.7
6.9
0.4
0.4
0.7
1.0
0.0
0.6 10.1
4.6
1.2
On-road vehicle hot exhaust emissions have been scaled up to 3 times the values found in the official state of California emission inventory in order to reflect better the results of measurements made in the Van Nuys tunnel (41). The speciation of emissions from the sources is as reported in ref 26. Includes off-road mobile sources such as aircraft, boats, trains, and construction equipment. Includes 634 X l o 3 kg/day methane from natural gas leaks and waste decomposition. Total includes 791 X lo3kg/day methane emissions.
*
emissions of benzene, toluene, xylenes, acrolein, and 1,3butadiene are detailed in Table 5. Inspection of Table 5 shows that on-road vehicle exhaust is the biggest contributor to the total emissions of each species listed. Hot soak evaporative emissions contribute significantly to the emissions of the aromatic hydrocarbons. Diurnal and running loss evaporative emissions are enriched in the high-volatility compounds such as butane found in gasoline and, hence, are lower in aromatic content. There are significant contributions to toluene and xylene emissions from industrial surface coating activities (e.g., product finishes). Further analysis of the emission inventory and the technical literature (26,55)indicates that methyl ethyl ketone and methyl isobutyl ketone are used as solvents and that mobile source emissions of these compounds are 92
negligible. Ethylene glycol is used as an organic cosolvent in water-borne paints (55);this compound also is used as a cosolvent in automotive cooling systems, but the emission inventory does not include ethylene glycol emissions from mobile sources. Trichloroethane is used as an industrial degreasing solvent, especially for metal parts prior to painting, and perchloroethylene is used both as a degreasing solvent and as a solvent for dry cleaning of clothes. The other compounds listed in Table 4, including aldehydes, ethylbenzene, isopropylbenzene, styrene, and phenol, are all emitted primarily in the exhaust of on-road vehicle engines. One caveat in the emission estimates for the species considered here is that sources such as wood combustion in fireplaces and stoves and cigarette smoking have not been included in the emission inventory. Phenol and cresols among other species have been measured in emissions associated with residential wood combustion (56). However, in Los Angeles for the hot summertime episode being considered in this study, emissions from fireplaces and woodstoves are of minor importance. Cigarette smoke contributes significantly to the indoor concentrations and human exposure to some toxic organic pollutants (57).
Environ. Sci. Technol., Voi. 28, No. 1, 1994
On selected days during the summer and fall of 1987, speciated hydrocarbon concentration measurements were made a t a network of nine monitoring sites shown in Figure 1, as part of the Southern California Air Quality Study (for an overview,see Lawson (58)).Samples were collected in stainless steel canisters (59) and then analyzed by gas chromatography and gas chromatography-mass spectrometry (60, 61). Aldehyde and ketone sampling was performed over the same network of nine sites, using 2,4dinitrophenylhydrazine (DNPH) impregnated cartridges (62). These data were consolidated into a single data base and further analyzed by Lurmann and Main (15) and provide the principal means for assessing the accuracy of model predictions. These data will be referred to as data set 1. In addition to the formaldehyde concentration measurements available as part of data set 1,concentration data for nitrogen dioxide, nitrous acid, and formaldehyde acquired using differential optical absorption spectroscopy (DOAS) are available a t the Long Beach and Claremont sites (63). An independent set of speciated non-methane hydrocarbon concentration data was acquired by Lonneman et al. (64) at the Long Beach, central Los Angeles, and Claremont sites. These data will be referred to as data set 2. Peroxyacetyl nitrate, trichloroethane, and perchloroethylene concentrations were monitored continuously a t the sites shown in Figure 1 using electron capture gas chromatography (65-68). In summary, ambient concentration data for most of the species listed in Table 1are available for the locations and dates of interest in this study. Unfortunately, however, no measurements of phenol, cresol, ethylene glycol, and acrolein concentrations were made during SCAQS. Styrene concentrations reported in data set 1 may be unreliable (60). An unspecified CS carbonyl routinely reported in data set 1may in fact be methylisobutyl ketone, but this identification could not be confirmed.
Table 6. Statistical Analysis of Model Performance* rangeb of predicted bias0 gross errord concns ppbC % % species (ppb C) ppbC benzene toluene ethylbenzene isopropylbenzene o-xylene m- + p-xylene styrene phenol o-cresolf p-cresolf formaldehyde acetaldehyde 1,3-butadiene acrolein methyl isobutyl ketone ethylene glycol l,l,l-trichloroethane perchloroethylene
14-29 28-12 4-9 0.4-1.2 6-16 14-35 0.8-4.0 0.20-0.98 0.09-0.30 0.02-0.07 8-12 8-15 1.1-3.3 1.1-2.1 0.8-3.2 0.2-2.0 3.1-4.6 0.6-2.3
-0.5 -10.2 -3.4 NDe -1.0 -8.5 -3.2 ND ND ND +0.8 -4.5 +0.4 ND ND ND -4.1 -1.1
+18 +2 -22 ND +9 -13 -37 ND ND ND +24 -1 +64 ND ND ND -41 -46
6.9 21.8 4.3 ND 4.1 11.9 3.6 ND ND ND 3.1 6.9 0.8 ND ND ND 4.8 1.8
41 38 46 ND 41 38 83 ND ND ND 39 37 84 ND ND ND 53 48
Using all valid measured I-h-average concentrations from data set 1 on August 28. Lowest and highest predicted 24-h-average concentration for August 28 over the eight on-land SCAQS intensive monitoring sites shown in Figure 1. Bias is defined as the mean residual (predicted minus observed) concentration. The percent bias values shown are computed by normalizing each residual to the corresponding observed concentration before averaging. d Gross error is defined as the mean absolute value of the residuals. e ND signifies that nomeasured dataare available. f Photochemical production only; there are no direct cresol emissions included in the model.
50
-
40
-
60
Hawthorne
Central LA
50
(I
6. Results
Statistical comparisons between predicted and observed concentrations for individual toxic organic species tracked in the model are presented in Table 6. The range of predicted 24-h-averageconcentrations for each species over the eight mainland sites shown in Figure 1also is indicated in Table 6. The statistical measures of model performance could not be computed for some species where ambient concentration data were not available. Inspection of Table 6 shows that the mean bias in the predicted concentrations of aromatic hydrocarbons is within f 2 5 % with gross error values in the range 38-46% in all cases except styrene. Styrene is underpredicted by a larger amount (-37 5% normalized bias). While ambient concentration data for isopropylbenzene were not available, the predicted concentrations for this species are much lower than predictions for other aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and the xylenes. Predicted 24-h-average concentrations shown in Table 6 for the aromatic hydrocarbons, 1,3-butadiene, and acrolein were all highest a t the central Los Angeles site, where there is a high level of motor vehicle activity. In contrast, predicted aldehyde and ketone concentrations were highest at the inland sites (Azusa, Claremont, Rubidoux). The lowest predicted concentrations for almost all of the toxic organic pollutants were seen at a coastal site (Hawthorne). Two highly reactive pollutants (1,3-butadiene, styrene) were predicted to have low concentrations far inland a t Rubidoux. Times series plots of predicted and observed concentrations are shown for four sites starting a t the coast with Hawthorne and proceeding inland through central Los Angeles to Claremont and Rubidoux. Figure 2 indicates that the highest benzene concentrations occur during the
60
I
Rubdoux
50
oCI
i
c
16 i o
i4 Time (PST)
12
August 27
i
e
12
16
io
J4
August 28
Flgure2. Time series plotsof observedbenzeneconcentrations(plotted as circles for data set 1 and horizontalbars for data set 2) and model predictions (solid line).
morning rush hour period. Concentrations of these species fall to lower values by midday. Similar diurnal patterns are observed for other aromatics. In Figure 3, time series plots are presented for 1,3-butadiene. Again, concentrations are highest during the morning rush hour and fall to low values a t midday. There is one low observed value of 1,3-butadiene reported a t Claremont and another low value a t Long Beach in data set 2 for August 28 that disagree with higher observed values reported in data set 1. Time series plots for perchloroethylene are presented in Figure 4. The diurnal patterns of both predicted and observed concentrations differ from those species shown in earlier figures: perchloroethylene concentrations peak late in the morning a t central Los Angeles. At the downwind sites (Claremont and Rubidoux), peak concentrations are observed later in the afternoon that are higher than predicted values. Additional time series data have been plotted for species that are both formed in the atmosphere and emitted directly. Observed and predicted formaldehyde and acetaldehyde concentrations are shown in Figures 5 and 6, respectively. In these figures, the total predicted concentrations are subdivided to show the contributions from initial and regional background concentrations (area below the dashed line), from direct emission sources (area between the dashed and dot-dashed lines), and from atmospheric photochemical formation (area between the dot-dashed and solid lines). The decay of initial formEnvlron. Scl. Technol., Vol. 28, No. 1, 1994 93
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.
5-
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Figure 3. Time series plots of observed 1,8butadiene concentrations (plotted as circles for data set 1 and horizontal bars for data set 2) and model predictions (solid line).
aldehyde and acetaldehyde concentrations occurs before noon on August 27. Regional background concentrations of these species are noticeable in the model predictions for August 28 at the coastal sites (e.g., Hawthorne); the contribution of regional background concentrations is negligible at the inland sites. Figures 5 and 6 show that photochemical production contributes significantlyto total formaldehyde and acetaldehyde concentrations on the days examined here. The contribution of direct emissions is small during midday hours. In relative terms, direct emissions of acetaldehyde are less important than direct emissions of formaldehyde as a fraction of the total concentrations of the respective aldehyde, although photochemical production dominates the total concentrations for both species during this episode. As shown in Figure 7, direct emissions and photochemical production both make roughly comparable contributions to total predicted acrolein concentrations. For the case of phenol, direct emissions dominate greatly over the fraction of total phenol concentrations contributed by atmospheric benzene oxidation, as shown in Figure 8. Unfortunately, ambient concentration measurements of acrolein and phenol were not made during the SCAQS episodes; therefore, the accuracy of total predicted concentrations for these species is not known. 7. Discussion
Airshed model calculations for formaldehyde, acetaldehyde, acetone, and acrolein indicate significant atmo94
Environ. Sci. Technol., Vol. 28, No. 1, 1994
August 27
August 28
Figure 4. Time series plots of observed perchloroethylene concentrations as measured by Hlsham and Grosjean (67, 64 (plotted as clrcles) and model predictions (solid line).
spheric photochemical formation of these species. This is important because programs designed to reduce ambient concentrations of these species may fail to achieve the desired results if the control programs only target direct emissions. In contrast, photochemical production of phenol is of minor importance when compared to direct emissions. Results reported here are for a sunny summertime episode in Los Angeles, where there is a high level of photochemical activity. Photochemical production of toxic organic species may be less important during the winter months, and high concentrations of directly emitted species could accumulate a t night and during early morning hours under stagnant meteorological conditions. Direct emissions were found to be a smaller contributor to total acetaldehyde concentrations than is the case for direct emissions of formaldehyde. This can be explained by noting that acetaldehyde is less abundant than formaldehyde in vehicle exhaust (see Table 3). Furthermore, a t least for the case of representative alkane mixtures, acetaldehyde is a major oxidation product whereas formaldehyde yields are small (18). As shown in Figure 3 and Table 6, 1,3-butadiene concentrations predicted by the model are somewhat higher than observed values. While butadiene is emitted in engine exhaust, it is not present in unburned fuel (26). Analysis of ambient concentration data and source emission signatures suggests that the atmosphere contains more unburned gasoline (and less of the combustion-derived
25
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August 26
Flgure 5. Time series plots of observed formaldehyde concentratlons (plotted as solid circles (DNPH) and open circles (DOAS)) and model predictions, The total predictedformaldehydeconcentration Is indicated by the solid line. The contributionfrom initialamounts of formaldehyde present at time zero plus regional backgroundformaldehyde Is Indicated by the dashed line; the area between the dashed and dot-dashed llnes representsthe contribution of direct emission sources;the area between the dot-dashed and solld lines represents the contribution due to atmospheric chemical formation.
species, which would include butadiene) than is indicated by the emission inventory (26). Therefore, it is possible that butadiene emissions to the model are overstated. There are also uncertainties in the ambient concentration data for this compound. Given that butadiene concentrations predicted by the model are somewhat higher than observed values, the rate of formation of acrolein as an oxidation product also may be overstated. However, secondary formation of acrolein will likely remain significant even if butadiene levels in the model are reduced to match observed concentrations. If butadiene emissions are controlled because butadiene is itself a toxic pollutant, then it appears that such controls would have additional benefits in the form of reduced secondary formation of acrolein. Turning to other toxic organic species included in the model, it should be noted that, in some cases, model predictions have been reported for species that were not measured during SCAQS. An assessment of model predictions for some of these species still can be performed using measurements reported in other studies, although the comparisons are less definitive. Ambient concentration data for acrolein, summarized by Grosjean (9), ranged
August 27
August 28
Figure 6, Time series plots of observed acetaldehyde concentrations (plotted as circles) and model predictions. The total predicted acetaldehyde conentrationIs Indicated by the solid line. As in Flgure 5, the contributionfrom lnitiaiand background concentrationsis indicated by the dashed line, the contrlbutlon from direct emissions Is indicated by the area between the dashed and dot-dashed lines, and the area between the dot-dashed and solid lines represents the contribution due to atmospheric chemical formation.
up t o 14 parts per billion by volume (ppbv) during the 1960s in the Los Angeles area, while more recent measurements suggest values on the order of l ppbv, in agreement with the predictions of the present study shown in Table 6. Measured concentrations of phenol as high as 420 ppb have been reported near point sources such as phenolic resin factories; values in the range 0.1-1 ppbv are more typical of urban areas ( 2 1 ) . The predicted phenol concentrations of the current study lie in the same range as other measured data for urban areas. Cresols have been measured in the gas phase and in rainwater in Portland, OR (69). Average gas-phase concentrations of 16 parts per trillion by volume (ppt) for o-cresol and 30 ppt for the sum of m- and p-cresol were reported; significant scavenging of these species by rain droplets also was reported. These measured concentrations are lower than the concentrations of cresols predicted in the present study based solely on formation of cresols from toluene oxidation. However, it is not surprising that predicted cresol concentrations in this study are higher than those observed in the study in Portland, given the lower precursor emissions and level of photochemical activity in Portland and the fact that scavenging by rain events occurred in Portland whereas it did not rain on the Environ. Sci. Technol., Vol. 28, No. 1, 1994 96
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Time (PST) August 28
August 27
August 28
Figure 7. Time series plots of predictedacrolein concentrationsshowing the directly emittedcomponent (dot-dashed line)and the total predicted concentrations (solid line). No concentration measurements are available for this species.
Figure8. Time series plots of predicted phenolconcentrationsshowing the directly emittedcomponent (dot-dashed line) and the total predicted concentrations (solid Ilne). No concentratlon measurements are available for this species.
days modeled in Los Angeles. A study of direct phenol and cresol emissions from noncatalyst, diesel, and various types of catalyst-equipped light-duty motor vehicles indicates that phenol emission rates are higher by a factor of 10 or more than the corresponding cresol emission rates (70). While photochemical generation of phenol by benzene oxidation was found to be negligible compared to direct emissions (see Figure €9, the combination of higher cresol production rates from toluene oxidation and lower direct emission rates from motor vehicles implies that photochemical production will be a more significant relative contributor to cresol concentrations than is the case for phenol. However, the contribution of atmospheric formation to total cresol concentrations could still be quite small compared to direct emissions. Since benzene and toluene are listed as toxic air contaminants, emissions of these species may be reduced in response to the Clean Air Act requirements. Results of this study suggest that toluene emissions a t present lead to production of ambient cresol concentrations of about 100parts per trillion by volume. Benzene emission controls are not expected to affect ambient phenol concentrations significantly. In addition to uncertainties in the VOC emission rate estimates, there are uncertainties associated with the chemical mechanism that is used to represent the atmospheric reactions of the organic species. For the primary
(i.e., directly emitted) toxic species, uncertainties in the chemical mechanism probably are not important since all that is required to predict chemical removal rates for these species is knowledge of the initial reaction rate constants. Such rate constants are well-known for many organic species (17). In contrast, the details of how oxidation reactions proceed after the initial step (e.g., hydroxyl radical attack) are not so well established. Therefore, model predictions for the secondary toxic organic species are subject to greater uncertainties than is the case for the primary VOC precursors.
B6 Environ. Scl. Technol., Vol. 28, No. 1, 1994
8. Conclusions
An Eulerian photochemical air quality model has been described that enables prediction of ambient concentrations of individual gas-phase toxic organic air pollutants based on knowledge of direct pollutant emission rates, meteorological conditions, and atmospheric chemistry. The model has been tested against measurements of speciated VOC concentrations for the August 27-28,1987, period in the Los Angeles area. Analysis of the direct emissions to the atmosphere in southern California indicates a significant mobile source contribution to the total direct emissions of many toxic organic species including the aromatic hydrocarbons, the aldehydes, and 1,3-butadiene. Given the presence of large
numbers of motor vehicles in cities, similar findings can be expected in most urban areas. Further analysis indicates that, for the summertime conditions considered here, the total ambient concentrations of certain toxic organics are significantly affected by the contribution from atmospheric photochemical formation that begins with the emissions of other VOC precursors. Photochemical formation is more important than direct emissions for acetaldehyde, formaldehyde, and acetone in Los Angeles on the summertime days studied here; in the winter, or a t night and during the early morning hours, direct emissions can be more important. Direct emissions and photochemical formation contribute in roughly equal amounts to acrolein concentrations. Photochemical formation may contribute significantlyto cresol concentrations. In contrast, despite the existence of a pathway for phenol formation as a product of benzene oxidation in the atmosphere, ambient phenol concentrations are dominated by direct emissions. These results indicate that the control of toxic organic species in the atmosphere in several cases must consider both direct emissions and atmospheric chemical formation.
StationarySourceDivision, CaliforniaAir Resources Board, Sacramento, CA, 1992. (13) Hughes, K. Proposed identification of 1,3-butadiene as a toxic air contaminant. Part B: health assessment. Stationary Source Division, California Air Resources Board, Sacramento, CA, 1992. (14) Grosjean, D.; Swanson, R. D.; Ellis, C. Sei. Total Enuiron.
Acknowledgments
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The authors thank Dr. W. P. L.Carter of the University of California a t Riverside for his assistance. This research was supported by the Electric Power Research Institute under Agreement RP3189-3. Supplementary Material Available
Extensions to the published chemical mechanism of Carter (Carter, W. P. L. Atmos. Environ. 1990,24A,481-518) (9pages) will appear followingthese pages in the microfilm edition of this volume of the journal. Photocopiesof the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. N.W., Washington, DC 20036. Full bibliographic citation (journal, title of article, names of authors, inclusive pagination,volume number, and issuenumber) and prepayment, check or money order for $19.00for photocopy ($21.00foreign) or $10.00 for microfiche ($11.00foreign), are required. Canadian residents should add 7% GST. Literature Cited (1) Shah, J. J.; Singh, H. B. Enuiron. Sei. Technol. 1988,22, 1381-1388. (2) Evans, G. F.; Lumpkin, T. A.; Smith, D. L.; Somerville,M. C. J. Air Waste Manage. Assoc. 1992,42,1319-1323. (3) NationalResearch Council. Rethinking the ozoneproblem in urban and regional air pollution; National Academy Press: Washington, DC, 1991. (4) Goldstein,B. D. J.AirPollut. Control Assoc. 1983,33,454467. (5) Goldstein, B. D. J.Air Pollut. Control Assoc. 1986,36,367370. (6) National Research Council. The alkyl benzenes; National Academy Press: Washington, DC, 1981. (7) National Research Council. Formaldehyde and other aldehydes; National Academy Press: Washington,DC, 1981. (8) Grosjean, D. J.Air Waste Manage. Assoc. 1990,40,13971402. (9) Grosjean, D. J.Air Waste Manage. Assoc. 1990,40,15221531. (10) Grosjean, D. J.Air Waste Manage. Assoc. 1990,40,16641668. (11) Grosjean, D.Sci. Total Enuiron. 1991,100,367-414. (12) Hughes, K. Proposed identification of 1,3-butadiene as a toxic air contaminant. Part A exposure assessment.
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