Outdoor Smog Chamber Studies: Light Effects Relative to Indoor Chambers Harvey Jeffries’, Donald Fox, and Richard Kamens Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, Chapel Hill, N.C. 27514
An outdoor smog chamber is described which uses natural conditions of light, temperature, and humidity to more closely simulate conditions in urban atmospheres. This 312 m3, dual chamber is used to perform a number of studies of hydrocarbonloxides of nitrogen/ozone systems. The performance of the outdoor chamber is discussed. Mathematical modeling is used to compare outdoor and indoor chamber experiments. The outdoor chamber appears to exhibit very low heterogeneous character. The effects of natural light intensity variations on urban smog systems are discussed. Diurnal light intensity effects may be significant for oxidant control strategies derived from smog chamber studies.
A single smog chamber cannot be used to provide all the information necessary to understand atmospheric photochemistry. Indoor smog chambers have been used to demonstrate that certain atmospheric chemical processes occur (1, 2 ) . Later studies were designed to elucidate some of the mechanisms involved in smog reactions (3-5). More recently, smog chambers have been used to investigate control strategies (6-8). These chambers were operated with constant temperature, light intensity, and relative humidity because the chemical aspects of the systems were of prime concern. Extrapolation of the results of smog chamber experiments to the urban atmosphere has been reasonably successful for the qualitative behavior but may be questionable with respect to defining the detailed behavior (9).Thus, the utility of such data in establishing air quality standards was quite limited, and aerometric data served as the primary source for deriving numerical values. Because a large number of factors are operating in urban atmospheres, it is usually not possible to fully interpret aerometric data, thus limiting its usefulness in air pollution decision making. The authors, therefore, undertook the design and construction of a chamber facility which exhibits characteristics that are more representative of the open atmosphere than are most indoor chambers. This large chamber (312 m3) is located outdoors and constructed of a transparent material. I t is subject to natural sunlight, temperature, pressure, and humidity. Since these factors exhibit large variations, they must be carefully considered in any experimental design. To control for these variations, the chamber has been divided in half so that two simultaneous experiments, differing in one variable such as hydrocarbon concentration, can be performed under the same physical conditions. Effects due to day-to-day variations in meteorological parameters are determined by applying the same methods used to interpret ambient air data and by performing photochemical modeling studies.
Outdoor Chamber Location. A site with relatively clean air for filling the chamber was located in Chatham County, N.C. This location, like most of North Carolina, is rural and heavily wooded. The background concentrations of NO, and nonmethane hydrocarbons are less than 0.015 ppm and less than 0.20 ppmC. More importantly, the background air exhibits very low reactivity in the chamber. 1006
Environmental Science & Technology
Materials. The most important factors that influenced the choice of materials were that the chamber surfaces had to be chemically inert, durable, and have a very high transmission for.the entire solar spectrum. Fluorinated ethylene propylene (FEP) Teflon film was chosen because its transmission in the UV-VIS region of the solar spectrum is excellent (6, l o ) ,and it exhibits only a few absorption bands in the IR, a necessary property to reduce the “greenhouse effect”. The film has a very low permeability for most chemical species (6, l l ) ,and it is possible to make very durable heat seals for the construction of large panels. Its worst property, for this application, is its ability to hold a static charge for very long periods. Description. The frame is A-shaped, 9.14 m wide, 12.19 m long, and 6.10 m high a t peak on a plywood platform 1.22 m above the ground. Because several fittings such as inlet and outlet doors, mixing fans, and manifolds must enter the film in an airtight manner, these fittings are inserted through a solid floor, leaving the entire sides free for light entry. The floor of the chamber is elevated to allow for these accesses. One-piece, heat-sealed Teflon film panels are used for the floors and triangular end-panels. Twelve individual, 1.0-m wide, 15.2-m long Teflon panels are overlapped and secured to the exterior frame with aluminum u-channels to form the chamber sides. A single unsupported heat-sealed Teflon panel similar to the end panels is used to separate the chamber into two halves of equal volume (156 m3). Injection System. Pollutants are injected into the chamber halves via the return side of 3.75-cm diameter glass sampling manifolds with >60 lpm flow rates. The manifolds then enter the chamber halves under the mixing fans. The injection process involves the use of gas cylinders of pollutants at high concentrations (1000-10 OOO ppm range), two-stage, stainless steel regulators, on-off solenoid valves, and capillary flow restrictors or precision needle valves with vernier handles. Conditions may be varied sufficiently to have the injection time range from a few minutes for each component to >12 h for a programmed injection simulating the buildup of pollutants in urban areas. Instrumentation a n d Calibration Systems. Instrumentation is located in the temperature-controlled laboratory adjacent to the two chambers. Gas instrumentation presently includes equipment for measuring total hydrocarbons, methane, carbon monoxide, nitric oxide, nitrogen dioxide, and ozone. Standard meteorological instruments are used to measure solar radiation, ultraviolet radiation, air temperature, and dew point. An automatic isothermal, three-column, flame ionization gas chromatograph (GC) is used for detailed c 1 - C ~ hydrocarbon analysis. The instruments are time-shared between the two chamber halves by means of three-way Teflon solenoid valves connecting instrument intakes to the two sampling manifolds. The sampling requirements of the instruments are less than 1%of the chamber volume in 12 h, but chamber leaks cause 7-12% dilution over the same period; the data are not corrected for this. The techniques used to calibrate the continuous monitoring instruments closely follow the procedures set forth by the Environmental Protection Agency (12). Gas-phase titration of NO by O3 is used to verify the NO, and O3 meter calibrations. Commercially prepared butane, hexane, and toluene cylinders are compared with a volumetrically calibrated
propane permeation tube for environmental and gas chromatograph calibration. Data Acquisition System. A computer-based data acquisition and control system (DAS) is used to acquire, process, and record data for the chamber-instrument system. The DAS consists of a P D P 11/40 computer with 32-kilobytes of core memory; high-speed papertape reader/punch; 2.4 million byte cartridge disk system; 30-character per second terminal; time-of-day clock; 5lh-digit, digital voltmeter; 200-channel, 3-wire crossbar scanner; and solid-state, isolated AC switches and reed relays. A 12-h run is divided into 4-min intervals. A digital timer operates the Teflon solenoid valves behind each instrument to connect the instruments to the proper manifold for each 4-min period. The DAS acquires the signal outputs every minute and converts the voltage readings to physical units using calibration factors stored in memory. These values are printed out for the operator. At the end of each 4-min period, the valves are switched and the physical data are written on the disk systems for subsequent data processing. All instruments and sensors used in this system have a response time of less than 2 min except the gas and environmental chromatographs; the environmental chromatograph produces a single set of readings within each 4-min period, and the gas chromatograph requires 15 min. The computer automatically starts the run in the early morning before sunrise and also controls injections and chamber flushing operations.
Discussion of Performance Mixing. Actual instrument readings show almost no fluctuations in readings 3 min after injections. When the inlets and outlets of the sampling manifolds were reversed, there was no discernible difference in the instrument readings even though the sample was drawn from a corner. In addition, there are no discernible changes in readings if the mixing fans are turned off-on-off on lh-1-h periods. Temperature and Light Conditions. The desired criterion was to have solar radiation inside the chamber a t or near the levels occurring in outdoor conditions. The chamber air temperature is very close to ambient with chamber heating of less than 4.4 "C above ambient throughout the course of a day. One measure of photochemical inducing radiation is the specific photolysis rate, aka, for the photodissociation of NO2 (13).The results of a series of measurements of a k a inside the chamber for three different solar intensities are tabulated in Table 1. These values indicate that the radiation flux inside the chamber is actually greater than outside the chamber because of the reflective surface beneath the Teflon floor. This more than compensates for any losses due to absorption of the radiation on passage through the film. Studies (14) have shown that the maximum a k a in the outdoor chamber often exceeds the constant values used in indoor chambers. Instrumentation. The performance of the chemiluminescent ozone meter used in this study has been reported elsewhere ( 1 5 ) . Interferences with both the NO and NO, modes of the NO, analyzer were observed (16) and were similar to those described by Winer et al. ( 1 7 ) and Miller (18). Winer presented data to show essentially a 100% response to PAN and methyl nitrate, but found an erratic response to HON02. Miller presented data which indicated a 100% response to HON02, but found an erratic response to PAN. A propylene/NO, experiment was conducted with two Bendix chemiluminescent NO, analyzers. The project analyzer was operated in its normal manner with a Teflon in-line sample filter; the second NO, analyzer had a nylon fiber HOND2 scrubber (19) installed in front of the Teflon in-line filter so that it would not respond to HON02. Manual Saltzman bubbler data were also taken. The results of this test are
shown in Figure 1. The two chemiluminescent instruments agreed almost perfectly the entire run, indicating that the project analyzer was not measuring HON02. Subsequent measurements of HONO2 with the Battelle subtractive method (19)have shown that HON02 was present in the sampling manifold a t the expected concentrations during the propylene/NO, experiments. We have concluded that the particular sampling arrangement probably influences whether the Bendix NO, analyzer can measure HON02, and that in our arrangement, the analyzer does not detect HON02, but does respond to PAN, probably a t 100% efficiency. Therefore, NO2 data from the project NO, analyzer after the NO2 peak is probably NO2 PAN. On the other hand, urban hydrocarbon mix/NO, smog systems do not require a significant correction to NO2 chemiluminescent measurements even after the NO2 maximum, because these systems do not generate large PAN concentrations. The NO2 data presented here are not corrected for PAN response. The environmental chromatograph (EC) is not subject to interference, per se, but can exhibit different responses to different hydrocarbon compounds. A recent study by Scott Research (20),however, did show that a THC response of 4 . 5 ppmC was not uncommon when samples of very carefully prepared and GC analyzed HC-free air were being measured by those types of instruments. The Federal Register method (12)states that both the CH4 and THC modes of the EC should be calibrated with CH4 gas mixtures. This calibration method will work only when the detector response to CH4 and to higher hydrocarbon compounds is the same; that is, each carbon giving the same increment in signal per carbon atom regardless of hydrocarbon identity. This was not true for this environmental chromatograph. The instrument used in this study had an average NMHC response of 64% when both THC and CH4 modes were calibrated with methane. Propane, butane, hexane, and toluene in the ppm range were used routinely to determine this average response factor.
+
Table 1. Specific Photolysis Rate for Nitrogen Dioxide in Outdoor Smog Chamber During September 1973a lncldent total solar rad, aka lnslde chamber 2 ft aka outside chamber 2 ft cal cm-2 mln-1 above floor, min-1 above grass, min-1
0.22 0.28 0.37
0.63 0.78 1.09
0.13 0.17 0.27
These experiments were performed in 100-1. Teflon bags
,
0.so
,
0.45 0.40
,,,,
,
c
0.40 0.35 0.30
0.25
0.25
P 0.20
0.20
8
0.15 0.10 0,05
0 00
k
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L
5
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'
6
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8
9
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* a
I I 10 11 12 HOURS, EDT
I
I
13
I
l
14
,
I
15
/
1
16
/
17
0 0 0
Figure 1. Comparison of three different NO2 measurement methods
in propylene/NO, system Curve 1, Bendix NO, analyzer with Teflon filter: curve 2, Bendix NO, analyzer with nylon " 0 3 scrubber: A, Saltzman bubbler measurements; NMHC concentration 4.06 ppmC
Volume 10, Number 10, October 1976 1007
Two other instruments belonging to other research groups (one produced by a different manufacturer) were tested. The chromatograph by the other manufacturer had a 74% response to NMHC when both the THC and CH4 modes were calibrated with CH4. The second instrument produced by the same manufacturer as the one used in this study also had a 6044% response to NMHC. Recently, EPA (21)examined the response of several different environmental chromatographs and found a wide range of responses with discrepancies exceeding 40%. These findings were consistent with data in the Scott Laboratories report which showed an average response of only 70% to nonmethane hydrocarbons in the 16 environmental chromatographs that they tested. Therefore, the actual response efficiency of each environmental chromatograph should be determined for the compounds of interest, rather than assuming that the response is the same as CH4. NMHC data in this paper were corrected for response efficiency of the EC used. Reactivity of Background Air. Background air reactivity has been tested in a number of all day runs. The rates of NO and 0 3 decay in background air were determined for each chamber half in separate experiments in which the chamber was charged with either 0.45 ppm NO plus 0.05 ppm NO2 or 0.87 ppm O3 and background air. An average daytime and nighttime rate of destruction of O3in rural North Carolina air of 0.11 ppb/min (22) is nearly equal to the nighttime rate observed in the chamber, 0.13 ppb/min (03= 0.6 ppm). The daytime rate observed in the chamber was approximately three times higher than that in the natural rural air, was a function of light intensity, and was approximately equal to the theoretical loss of O3 due to photolysis in a humid atmosphere. The O3 half-life in the outdoor chamber would be in excess of 20 h in the light and 45 h in the dark. Other chamber 0 3 half-lives reported range from 1.5 to 20 h ( 2 3 , 2 4 ) . As in other chambers, the rate of NO disappearance in the dark in this study was negligible. During the daytime, the average rate was 0.20-0.30 ppb/min using starting concentrations of 0.45-0.65 ppm NO, which was 1090 NO2. The rate was a function of light intensity and approximately six times the thermal rate of NO oxidation at midday. Blank runs in the chamber produced 0.05-0.08 pprn of 03.This compared well with 0.08 ppm O3from the Bureau of Mines study (6, 7) and 0.05 ppm from SRI (23). Comparison of Photochemical Systems in Outdoor and Indoor Chambers. One of the differences between experiments performed in the outdoor chamber and those performed in indoor chambers is the light intensity profile. The natural diurnal variation in solar intensity, the occurrence of clouds with associated sudden decreases in intensity, overcast days, and “choppiness” in solar radiation can have a major effect on the outcome of any experiment and thus can make it more difficult to perform a set of experiments under similar conditions. Air parcels in the urban environment, however, are subject to just these phenomena, and it is important to understand the effects of such variations on photochemical systems to better extrapolate theoretical and laboratory results to the real world. Photochemical modeling studies of the outdoor chamber have been used to investigate the behavior of propylene/NO, systems in the outdoor chamber and to compare the outdoor chamber performance with indoor chambers. A specific photochemical model for propylene (C3H6) was developed (25) based on the work of Demerjian et al. (26). A computer program supplied by EPA (27) was modified to allow variable light intensity in the model. The model was validated at several initial conditions for the outdoor chamber and for several runs in indoor smog chambers. In the indoor validations, only the light intensity and the rate constant for the combined homogeneous and heterogeneous reaction 1008
Environmental Science & Technology
NO
+ NO2 + H2O
-
2HN02
(1)
were changed from those values used in the outdoor simulations. The values of rate constants in the outdoor simulations were those recommended by the National Bureau of Standards (28) or, in the absence of recommended values, those of Demerjian et al. (26) or Hecht and Dodge (29). Figures 2 and 3 compare model and experimental results for propylene/NO, runs in the outdoor chamber (May 25, 1974) with a diurnal light intensity and in an indoor chamber (EPA-325) with a constant light intensity (aka = 0.40 min-l for NO*). The simulations are adequate except for the behavior from NO-NO2 crossover to NO2 maximum where the model exhibits accelerated NO conversion. The outdoor chamber run is slow (3.25 h to NO-NO2 crossover) relative to the indoor chamber run (0.5 h to NO-NO2 crossover) even though the HC/NO, were nearly identical: 3.14 ppmC/ppm outdoors and 3.36 ppmC/ppm indoors. The effect of using constant light intensity (equal to that in the indoor model of Figure 3) in the outdoor model of Figure 2 is shown in Figure 4; the time to reach NO-NO2 crossover is reduced to 1.4 h. Other than initial conditions, the only factor that is different between the indoor chamber simulation and the simulation shown in Figure 4 is the rate constant for Reaction 1. The model runs for Figures 2 and 4 used a pseudo-second-order rate constant value of 3.5 X ppm-l min-’, whereas the model run for Figure 3 used a value of 1.5
1.00 0.90 0.80
I
0.10
0.60 0” z 0.50
4
0.40
0” 0.30
0.20 0.10
0.00 HOURS. EOT
Figure 2. Comparison of propylene/NO, run in outdoor smog chamber (dashed lines) and kinetics model profiles (solid lines)
SR is solar radiation plotted at 2.00 cal cm2 min-’ full scale. kl is specific photolysis rate for NO2 used in model. Fluctuations in SR at 1400 not duplicated in kl profile. Initial conditions: NOx, 0.351 pprn; N02, 0.044 ppm; NMHC, 1.10 ppmC propylene
0.90
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0.80
-
0.70
-
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-
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,,,
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6
7
8
9
10
11 12 HOURS
13
14
15
16
17
Figure 3. Comparison of propylene/NO, run in indoor smog chamber (symbols) and kinetics model profiles (solid lines) Initial conditions: NO, 0.35 ppm; NOz, 0.04 ppm; NMHC, 1.35 ppmC propylene.
+, NO; A ?NOz; 0 , C3HB; 0.0 3
X loA2ppm-l min-l, the same value used by Hecht et al. ( 5 ) in their simulation of EPA-325. The first value is close to the upper limit of the homogeneous rate constant suggested by the National Bureau of Standards (28).Heterogeneous effects apparently increased the rate constant in the EPA-325 run. If the value 1.5 X ppm-l min-l and a constant light intensity are used in the model that produced Figure 2, the profiles shown in Figure 5 result. These appear quite similar to those in the EPA-325 run (Figure 3). Similar results (25) were found in comparisons of simulations of outdoor chamber runs with indoor chamber runs from the University of California at Riverside (30) and Lockheed ( 3 1 )chambers which used a rate constant for Reaction 1 of 8.6 X and 8 X ppm-1 min-1.
0.80
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Thus, although the diurnal behavior of light intensity has a strong effect on the concentration-time profiles, the surface chemistry of the chamber can exert a stronger effect. The rate is important because "02 photolyzes of formation of "02 to give NO and a hydroxyl radical (OH). The OH radical can rapidly react with hydrocarbon (in these cases, C3H6), thereby initiating the chain reaction that oxidizes NO to NO2 and consumes the propylene. In the absence of a source of "02, the initiation must depend upon atomic oxygen (0)or 0 3 attack on C3H6, or on other photoacceptors such as aldehydes. The concentrations of 0 and O3 are kept quite low early in a run because of the high NO to NO2 ratio and, in the variable light simulation, also by the small rate of NO2 photolysis a t the beginning of the run. Aldehydes were not present initially in the runs discussed above. The surface-to-volume ratio for the outdoor chamber is 1.31 m2/m3 and for the EPA chamber is 3.28 m2/m3, but this difference does not seem sufficient to account for the differences noted above. There were more than 20 000 ppm of water available in the outdoor chamber. In the Lockheed studies o f , factors affecting smog chamber performance (31),Teflon was the slowest material studied. The outdoor chamber is more than 99% Teflon, and this seems to be the only factor that can in account for the apparently slow rate of formation of "02 the outdoor chamber. The large Teflon aerosol chamber (32) adjacent to this outdoor chamber has several square meters of aluminum surface, and the rate constant of Reaction 1 needed to model C3H6/NOXruns in that chamber is larger than that used in the outdoor simulations above. The importance of "02 formation in ambient air is not known, but in the presence of aldehydes from auto exhaust, its formation rate is not expected to govern the initiation time
0.10
1
0
2
3
4
7
5 6 HOURS
9 1 0
8
Figure 4. Constant light intensity model (kl= 0.38min-I). Other conditions identical to Figure 2
1.00
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0.80 0.70
--
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,
,
,
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(26).
Other Light Intensity Effects. Table I1 compares two dual runs a t two different initial conditions of C3H6/NOZ and different solar radiation conditions (Figure 6). These data suggest that fluctuations in solar radiation, provided they are small, tend to accelerate a photochemical system giving higher rates of NO2 and O3 formation and larger maximum values. For changes approximately 20-30 min in duration, it appears that the system does not slow down as much for a decrease in solar radiation as it speeds up for an increase in solar radiation. For solar radiation decreases that last one or more hours and occur before the 03 maximum, the 0 3 formation rate and maximum value are reduced (the October 16,2.0 ppmC run in Table 11). Side-to-side, same day runs with matched initial concentrations have NO,, HC, and 0 3 profiles that are almost identical; maximum O3concentrations agree to within 5% (25). In a detailed study of effects of hydrocarbon reduction on NO2 formation (139 runs) with a simulated urban hydrocarbon mix (33),a t constant initial conditions the highest NO2 maximum concentration never occurred under clear skies. I t
1,oo
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FRST HNOa RERCTION MOOEL
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Flgure 5. Constant light intensity (k,= 0.38min-') and fast HNOPreaction (1.5X lo-' ppm-l min-') model. Other conditions identical to Figure 2
Table II. Comparison of Two Dual Runs with Different Solar Radiation Conditionsa inltlal concentrationsb
Maximum rated
NMnC, ppmC
T h e X,= rnln
0.447 0.058
1.98
184
B
0.469 0.059
2.04
136
Oct. 15
B
0.448 0.064
3.88
128
Oct. 16
R
0.468 0.059
3.88
112
Date
Side
Oct. 15
R
Oct. 16
NO,
NOZ,
ppm
ppm
*
NO,
ppblmin
3.01 (368)e 3.40 (356) 4.35 (300) 5.47 (320)
Integrated solar rad
Max concn
NOz, ppb/mln
039 ppblrnln
NOz, ppm
03,
@(390),'
@(720),e
ppm
caicm-2
caicm-2
2.02 (360) 2.96 (356) 3.78 (300) 4.88 (320)
4.56 (464) 4.45 (468) 11.16 (372) 15.44 (368)
0.365 (424) 0.417 (404) 0.400 (340) 0.428 (344)
0.804 (664) 0.700 (636) 0.754 (660) 0.941 (448)
126.3
369.8
124.1
333.3
126.3
369.8
124.1
333.3
a Solar radiation profiles given in Figure 6. Conditions established before sunrise. Time from sunrise until [NO] = [NOz]. fit to 8-min data points. e Numbers in parentheses are minutes after 0500 h EDT at which event occurred.
Computed by 5-point numerical
Volume 10,Number 10,October 1976
1009
also never occurred under ’partly cloudy conditions which resulted in low yalues of average solar radiation. The highest values of NO2 maximum for a given HC to NO, ratio always occurred under partly cloudy conditions which gave “choppy” solar radiation curves such as the first half of the October 16 solar radiation profile. Preliminary modeling studies using “choppy” radiation profiles suggest that the hydroxyl and hydroperoxy radical concentrations fluctuate strongly with changes in light intensity and “overshoot” the values in smooth light intensity runs. Presumably, such overshoots can move the system forward in “spurts” more than it falls behind in the “dips” in light intensity. A similar effect has been seen in condensation nuclei counts in the outdoor chamber photochemical oxidation of so2 (34). Another finding apparently related to light intensity profiles is shown in Figure 7 . The dashed lines are based on the findings of Dimitriades (7) and are often cited in discussions of oxidant control strategies (35,36).The solid lines are based on examination of some clear all-day urban hydrocarbon mix/NO, runs in the outdoor chamber. These data suggest that at high HC concentrations, more O3 is formed, and at lower HC concentrations, less O3 is formed in the outdoor chamber than in the,Bureau of Mines (BOM) indoor chamber. The BOM study used dilute auto exhaust irradiated at constant light intensity for 6 h, and the outdoor chamber study used an urban mix and natural sunlight from sunrise to 1700 h. By use of data on the relationship between a k a for NO2 and total solar radiation obtained in another study ( 2 4 ) , it was concluded that in the summer time the maximum NO2 photolysis rate (light intensity) in the outdoor chamber exceeds that in the BOM chamber by as much as 0.1 min-l. Since the 2.0
2.0
1.8
1.8
1.6
1.6
‘Ec
1.4
?E
1.2
1.4 1.2
OCT. 15
Y
i m
3
Conclusions The outdoor chamber has proved to be a valuable addition to the techniques used to study photochemical smog systems. It would appear that in large Teflon chambers, heterogeneous reactions are less significant than in smaller metal and glass chambers. Diurnal light intensity effects may be significant for oxidant control strategies derived from smog chamber studies. Acknowledgment The following contributed substantially to this work: F. Malcolm, W. Pendergraft, and J. Brown, construction; L. Alexander and R. Baker, data reduction; G. Feigley, D. Stotts, and G. Siple, chamber operation. Discussions with Basil Dimitriades were most appreciated.
1.0
1.0
0.8
0.8
0.6
0.6
Literature Cited
0.4
0.4
0.2
0.2
(1) Haagen-Smit, A. J., Bradley, C. E., Fox, M. M., Znd. Eng. Chem., 45 (91,2080-89 (1953). (2) Neligan, R. E., Arch. Enuiron. Health, 5,581-91 (1962). (3) Altshuller, A. P., Kopczynski, S. L., Lonneman, W. A., Becker, T. L., Slater, R., Enuiron. Sci. Technol., 1,899 (1967). (4) Niki, H., Daby, E. E., Weinstock, B., “Mechanisms of Smog Reactions”, Advances in Chemistry Series, No. 113, “Photochemical Smog and Ozone Reactions”, American Chemical Societv. “ , Washingtin, D.C., 1972. (5) Hecht. T. A., Seinfeld, J. H., Dodge, - M. C., Enuiron. Sci. Technol., 8,327-39 (1974). (6) Dimitriades, B., J . Air. Pollut. Control Assoc., 17,460-66 (1967). (7) Dimitriades, B., Enuiron. Sci. Technol., 6, 253-60 (1972). (8) Wilson, W. E., Miller, D. F., Levy, A., Stone, R. K., J . Air Pollut. Control Assoc., 23,949-56 (1973). (9) “Air Quality Criteria for Photochemical Oxidants”, National Air Pollution Control Administration Publ. No. AP-63 2-14, HEW, GPO, Washington, D.C., 1970. (10) Dupont Technical Information Bulletin T-5 “Optical Properties”. (11) DuDont Technical Information Bulletin T-3D. “Chemical PropLrties”. (12) Fed. Regist., 36,8186-201 (1971). (131 Holmes. J. R.. O’Brien. J. 0.. Crabtree. J. H.. Hecht. T. A.. Seinfeld, J: H., ELuiron. Sci. Tecknol., 7, 519-23 i1973). ’ (14) Sickles, J., Jeffries, H. E., “Development and Operation of a Device for the Continuous Measurement of Photolysis Rate of Nitrogen Dioxide”, Univ. of North Carolina, Dept. of Environ. Sci. and Eng., Publ. No. 396,1975. (15) Feigley, C. E., “Some Statistical Aspects of Air Monitoring Instrument Calibration”, 67th APCA Annual Meeting, Denver, Colo., June 9-13.1974.
0
a
O3 maximum concentration is proportional to the light intensity under which it forms, outdoor runs in which the HC concentration was high could reach the 0 3 forming state when the light intensity was high. Since the intensity could be greater than that in the BOM chamber, those runs could produce more O3 than in the BOM runs. On the other hand, runs at lower HC concentrations require a longer time to reach the O3 forming state. In the BOM runs the light intensity did not change with time, and high intensity light was available continuously. Under diurnal intensity variation outdoors, such low HC conditions resulted in long times (afternoon or late afternoon) to 0 3 maximum and therefore continuously decreasing light intensity conditions. The speed of a photochemical system will largely be governed by its rate of initiation as discussed above, and an increased rate of initiation would tend to decrease the slope of the outdoor 03-HC curves of Figure 7 and bring the two sets of data in closer agreement. At lower HC concentrations, however, the decreasing light intensity would still result in lower 0 3 . It thus appears that oxidant control strategies based on constant light intensity studies would result in a conservative strategy at low HC and, unless maximum outdoor light intensities are used, an optimistic strategy at high HC. Of course, the low HC situation is the more critical region.
0
0.0
5
7
6
8
9
10 1 1 12 HOURS, EDT
13
14
15
16
17
0.0
Figure 6. Comparison of total solar radiation for typically clear day (Oct. 15) and partly cloudy day (Oct. 16) 1.00 0.90
t
I
1
I
’
1
1
’
’
4 1.00 0.90 0.80
0.70 0.60 0.50 0.40
0.30
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0.20 0.10 0.00
0.10 0.00
I
0
I
1
1
1
I
I
2 3 NONMETHANE HYDROCRRBON , ppmC
I 4
, 5
Figure 7. Comparison of Bureau of Mines and UNC results for oxidant as function of initial hydrocarbon at constant initial NO, _ _ _ BOM indoor chamber - UNC outdoor chamber 1010
Environmental Science & Technology
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(16) Jeffries, H. E., Fox, D., Kamens, R., “Outdoor Smog Chambers: Performance with Respect to Natural Sunlight and Temperature Variations”, EPA Smog Chamber Conference, Research Triangle Park, N.C., Oct. 24-25,1974. (17) Winer, A. W., et al., Enuiron. Sci. Technol., 8, 1116-21 (1974). (18) Miller, D., “Measurement of NOz”, EPA Smog Chamber Conference, Research Triangle Park, N.C., Oct. 24-25, 1974. (19) Spicer, C. W., Miller, D. F., J. Air Pollut. Control Assoc., 26 (l), 45-50 (Jan. 1976). (20) Reckner, L. R., “Survey of Users of the EPA-Reference Method for Measurement of Non-Methane Hydrocarbons in Ambient Air”, EPA-650/4-75-008,December 1974. (21) McElroy, F. F., Thomson, V. L., “Hydrocarbon Measurement Discrepancies Among Various Analyzers Using Flame-Ionization Detectors”, EPA-600/4-75-010,September 1975. (22) Jeffries, H. E., PhD thesis, University of North Carolina, Chapel Hill, N.C., 1971. (23) Doyle, G. J., Enuiron. Sci. Technol., 4,907-16 (1970). (24) Levy, A., Miller, D. F., “The Role of Solvents in Photochemical Smog”, APCA Meeting, St. Paul, Minn., June 1968. (25) Jeffries, H., Fox, D., Kamens, R., “Outdoor Smog Chamber Studies: Effect of Hydrocarbon Reduction on Nitrogen Dioxide”, Final Report, EPA-650/3-75-011,June 1975. (26) Demerjian, K., Kerr, J. A., Calvert, J., (1974), in “Advances in Environmental Science and Technology”,Vol4, J. N. Pitts and R. L. Metcalf, Eds., Wiley, New York, N.Y. (27) Dodge, M. C., Overton, J., Environmental Protection Agency, Research Triangle Park, N.C., personal communication, 1974. (28) Garvin, D., Hampson, R. F., “Chemical Kinetics Data Survey. VII. Tables of Rate and Photochemical Data for Modeling the Stratosphere”, rev., NBS Report, NBSIR 74-430, 1974.
(29) Hecht, T. A,, Dodge, M. C., “Rate Constant Measurements Needed to Improve a General Kinetic Mechanism for Photochemical Smog”, in Procgedings of Symposium #1, 1975, Int. J . Chem. Kinet., Wiley, pp 155-64. (30) Hecht, T. A., Lui, M., Whitney,D. C., “Mathematical Simulation of Smog Chamber Photochemical Experiments”, Final Report, Contract 68-02-0580,EPA-650/4-74-040,November 1974. (31) Jaffe, R. J., “Study of Factors Affecting Reactions in Environmental Chambers”. Final ReDort, Coordinating Research Council Contract No. CAP1 1-69(1-?2, 1:73), EPA Contract Nos. 68-020287,68-02-1270,June 30,1975. (32) Fox. D. L.. Sickles. J. E.. Kuhlman. M. R.. Reist. P. C.. Wilson. W. E.,’J. Ai; Pollut.’ Conirol Assoc.,’ 25 (101, 1049-54 (October 1975). (33) Jeffries, H., Fox, D. L., Kamens, R., ‘‘Photochemical Conversion of NO to NO2 by Hydrocarbonsin an’0utdoor Chamber”,presented at APCA Meeting, Boston, 1975; J. Air Pollut. Control Assoc., in press (1976). (34) Fox, D. L., Kuhlman, M. R., Reist, P. C., “Sulfate Aerosol Formation under Conditions of Variable Light Intensity,” presented at International Conference on Colloids and Surface, San Juan, P.R., June 21-25,1976. (35) “EPA Scientific Seminar on Automotive Pollution”, EPA600/9-75-O03,’Washington, D.C., February 10-12, 1975. (36) “A Critique of the 1975-1976 Federal Automobile Emissions Standards for Hydrocarbons and Oxides of Nitrogen”, pp 11-12, Report of Panel 1 on Standards to Committee on Motor Vehicles Emissions, National Academy of Sciences, Washington, D.C., 1973. Receiued for review January 16,1975.Accepted April 5,1976. Project supported by Enuironmental Protection Agency Grant 800926.
Study of Fly Ash Emission During Combustion of Coal Chantal Block and Richard Dams* Institute for Nuclear Sciences, Rijksuniversiteit Gent, Proeftuinstraat 86, E-9000 Gent, Belgium
Fly ash sampling performed in a large combustion facility and in the stack of a private house indicates that the composition of fly ash is significantly different from the original coal composition. Based on their mass-size distributions, the elements can be divided into three groups. A first group of elements (Mg, Al, Ca, Sc, Ti, Cr, Fe, Co, Rb, Ba, Hf, the rare earths, and Th belonging to the rock fraction of coal) are associated with large particles and are depleted relative to the original coal composition. A second group of elements (Cl, Cu, Zn, As, Se, Br, In, Sb, I, and Hg belonging to the organic fraction of coal) have a maximum concentration of small particles and are enriched in fly ash. For a third group of elements (Na, K, V, Mn, Ni, Ga, Mo, Ag, Cd, Cs, W, and Au), the mass-size functions show an intermediate behavior. Their concentration ratios in fly ash and coal are near unity. This division into enriched and depleted elements appears to be roughly independent of the operating conditions of the combustion facilities. There is some indication that the specific mass-size distribution in fly ash emitted by small home heating facilities is different from those emitted by large facilities.
In spite of the rising importance of fuel oil for combustion purposes, coal production has only slightly decreased. In 1973 fuel oil production exceeded coal production slightly ( I ) , but since the ash content of coal is approximately two orders of magnitude higher than that of fuel oil, it is obvious that coal combustion is responsible for the emission of larger amounts of particulates into the atmosphere. In fact, from a worldwide
inventory for emission of particulates drawn u p by Robinson and Robbins ( 2 ) , coal combustion appears to be the major source. Also in Belgium, coal is responsible for a large fraction of particulate emissions. A recently calculated emission inventory for Belgium for the year 1972 ( 3 ) indicates that approximately 33% of the particulates smaller than 10 pm emitted to the atmosphere are combustion derived, coal combustion being the major source. A study dealing with the inorganic composition of Belgian coals and their ashes was reported recently ( 4 ) . However, the elements are not emitted during combustion in concentration ratios identical to those in the original coal or coal ash. Selective volatilization for some elements results in an enrichment in the fly ash, while other elements are depleted ( 5 ) .Consequently, knowledge of emission factors of individual elements is required. These emission factors are determined not only by the chemical properties of the elements but also by different operating parameters such as the temperature of the effluent gas stream, the combustion efficiency of the boiler, and the use of a control device. Therefore, elemental emission efficiencies were assessed by chemical analysis of coal, coal ash, and fly ash emitted during combustion of the same coal. Extensive data on the composition of fly ash have only recently become available (6-8). The size of the emitted fly ash particles is of particular interest. In fact, meteorological effects, chemical interactions in the atmosphere, and especially deposition efficiency in the innermost regions of the human respiratory tract strongly depend on the size of the particles (9, 10). Several relatively toxic elements such as As, Sb, Se, Zn, Pb, and Cd, which are generally present on small particles in urban aerosols, have enrichment factors as high as 100 to 10 000 when calculated Volume 10, Number 10, October 1976
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