Environ. Sci. Technol. 2000, 34, 4656-4661
Investigation of a Long-Term Sampling Period for Monitoring Volatile Organic Compounds in Ambient Air SHIGEHISA UCHIYAMA* AND SHUJI HASEGAWA Chiba City Institute of Health and Environment, 1-3-9, Saiwaicho, Mihama-ku, Chiba City, Chiba, 261-0001 Japan
The potential health effect, induced by exposure to volatile organic compounds (VOCs) in ambient air for long periods, is an important problem. We have investigated a long-term monitoring method for measurement of VOCs in ambient air. Long-term, nonstop sampling periods of 4-weeks, 7-days, and 24-h were evaluated simultaneously, using mass flow controllers set to 0.5, 2.0, and 14 mL/ min, respectively. Ambient air was drawn through a multisorbent sampling tube packed with Carbotrap C, Carbotrap B, and Carboxen 1000. Adsorbates were introduced into a GC/MS using a thermal desorption cold trap injector and determined according to the EPA Method TO14 list. Concentrations of almost all VOCs estimated by the 7-day or 4-week sampling method were approximately equal to the mean value calculated from the 24-h sampling method for each term, except for compounds bearing a vinyl group such as chloroethene, 1,3-butadiene, and styrene. Concentration variations of CFCs, tetrachloromethane, or 1,1,1trichloroethane were small, but the other VOCs exhibited significant day-to-day fluctuation reflecting socioeconomic activities and the direction of wind. As an example, daily concentrations of benzene in February and March 1999 ranged from 2 to 10 µg/m3 and 1.5-8.6 µg/m3, respectively. This wide variation indicates that long-term, nonstop sampling is necessary for measuring accurate average concentrations of VOCs in ambient atmospheres.
Introduction Environmental air contains a large number of volatile organic compounds (VOCs), and these compounds cause an important public health problem throughout the developed and developing world. An increasing number of pollutants in the environment raise the problem of the toxicological risk evaluation of these compounds. Many important questions remain to be clarified in assessing exposure to these compounds. It is important to assess the long-term healthrelated consequences and the toxicological risk evaluation to a general population from long-term, low-level exposures to specific substances in the environment. Many researchers have reported the use of sample collection and measurement apparatus for VOCs (1-3). Conventional methods for measurement of VOCs have been established as TO-1 (4), TO-2 (4), TO-14 (5), and TO-17 (6) * Corresponding author phone: +81-43-238-1900; fax: +81-43238-1901; e-mail:
[email protected]. 4656
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by the U.S. Environmental Protection Agency and are used in many countries. The Environmental Agency of Japan modified these methods to conventional methods (7) and conducts ambient air monitoring once per month throughout a year according to the Air Pollution Control Law. Then, ambient air quality standards of benzene, trichloroethene, and tetrachloroethene were set to 3, 200, and 200 µg/m3 on annual average, respectively (8). However, sampling periods of VOCs in these methods are all specified to be within 24 h. Many pollutants in the general environment are required to be monitored to understand the overall risk to public health. Therefore, monitoring methods need to be designed which accurately estimate the annual average concentration. Instant or short-term sampling can only be used for determination of a momentary value for VOC concentration and cannot be considered representative of the annual or monthly average when actual atmospheric concentrations vary so significantly over time. Atmospheric concentrations of NOx, NO2, SO2, SPM, CO, and NMHC were reported to be lower on holidays (9). Social, economic, or leisure activities are usually dependent on the day of the week. It is therefore necessary to collect ambient air samples over 1 week periods to average out the effects of social, economic, and lifestyle activities plus meteorology at a sampling site. Automated VOC monitoring systems (10-12), which are designed to observe time or day deviation, are able to monitor of VOCs for long periods. However, these methods require expensive equipment and multiple measurements to estimate a yearly averaged concentration. We have developed a long-term, low flow rate monitoring method using a mass flow controller set to a very low flow rate (13). The present investigation was aimed at confirming the validity of continuous weekly or monthly sampling of VOCs in the general environment by using a solid adsorbent.
Experimental Section Sampling Tube. The multisorbent tube used in this experiment consists of three sorbents packed in a stainless steel tube (150 mm × 4 mm i.d.) in the following order: Carbotrap C (20/40 mesh, 250 mg), Carbotrap B (20/40 mesh, 120 mg), and Carboxen 1000 (45/60 mesh, 200 mg) supplied by Supelco, Bellefonte, PA. The above sorbents are listed on the U.S. EPA Method TO-17 (6). Tubes were conditioned for 24 h at 350 °C while passing at least 50 mL/min of pure helium carrier gas through them. Once conditioned, each tube was sealed with Swagelok-type fittings and Vespel ferrules. The sealed tubes were then placed in a Tube Container supplied by GL Sciences Inc., Japan. Drying Tube. The mixture of magnesium perchlorate (24/ 48 mesh, 2 g) as a drying agent and glass beads (20 mesh, 0.5 g) was packed into a glass column (30 mm × 10 mm i.d.) with a silanized quartz wool support. Glass beads were added in order to make way for air flow when magnesium perchlorate was wet by moisture. After preparation, the tubes were cleaned by passing pure air through them. Sampling Procedure. A drying tube was connected to the Carbotrap C end of the sampling tube by a Swageloktype union and Vespel ferrules. The Carboxen 1000 end of the sampling tube was connected to a sampling apparatus by a silicon tube (4 mm i.d.). The sampling apparatus was composed of a mass flow controller (model SEC-400 MARK3; STEC Inc., Kyoto, Japan), an air sampling pump (APN-085V1; Iwaki Co. Ltd., Japan), and a wet gas meter (WS D-1A; Shinagawa Co., Tokyo, Japan) in that order. The mass flow controller was set to 0.5 mL/min for a 4-week sampling, 2.0 10.1021/es990843u CCC: $19.00
2000 American Chemical Society Published on Web 09/29/2000
TABLE 1. Precision and LOD of the Analytical Measurement and Measured Concentrations of Major VOCs Collected with 24-h, 7-Day, and 4-Week Sampling Method in February 1999 at Chiba Citya
CFC11 CFC12 CFC113 CFC114 dichloromethane trichloromethane tetrachloromethane chloroethane 1,2-dichloroethane 1,1,1-trichloroethane chloroethene trichloroethene tetrachloroethene 1,2-dichloropropane 1,3-butadiene acrylonitrile benzene toluene ethylbenzene m,p-xylene o-xylene styrene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene chlorobenzene 1,4-dichlorobenzene a
24-h
7-day
RSD, %
LOD, µg/m3
mean
max.
min.
mean
max.
min.
4-week measd
2.4 5.4 2.2 3.8 2.3 2.5 2.6 7.3 2.4 1.7 7.4 3.3 4.2 1.8 6.6 8.4 2.5 5.2 4.7 3.7 4.6 4.6 5.4 3.5 3.1 8.8
0.01 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.04
1.6 3.1 0.71 0.15 2.5 0.33 0.67 0.10 0.13 0.35 0.07 1.3 0.46 0.08 0.11 0.12 4.6 24 3.9 3.9 2.3 0.36 1.2 3.1 0.03 0.71
2.0 3.3 0.95 0.16 7.4 1.0 0.78 0.40 0.35 0.54 0.62 4.1 1.4 0.19 0.25 0.34 10 49 13 8.7 5.6 0.81 2.4 7.0 0.07 1.5
1.4 2.6 0.63 0.12 0.51 0.08 0.61 0.01 0.05 0.27 0.01 0.30 0.12 0.02 0.02 0.03 2.0 8.2 1.3 1.7 1.0 0.17 0.53 1.4 0.01 0.32
1.6 3.1 0.71 0.15 2.4 0.32 0.67 0.10 0.13 0.34 0.08 1.3 0.46 0.07 0.16 0.09 4.6 25 3.9 4.0 2.4 0.19 1.2 3.1 0.04 0.75
1.7 3.2 0.77 0.16 3.4 0.40 0.70 0.13 0.18 0.37 0.12 1.6 0.54 0.09 0.19 0.18 5.5 26 4.5 4.4 2.7 0.42 1.4 3.7 0.04 0.85
1.5 2.9 0.67 0.14 1.7 0.25 0.66 0.07 0.08 0.31 0.03 0.92 0.29 0.07 0.10 0.05 3.5 21 2.7 3.3 1.7 0.15 0.89 2.4 0.03 0.60
1.5 3.2 0.79 0.15 2.5 0.30 0.68 0.08 0.12 0.32 0.05 1.4 0.46 0.08 0.13 0.10 4.8 24 3.9 4.0 2.5 0.11 1.2 3.2 0.04 0.76
Measured at µg/m3. Each sampling volume is all 20.1 L.
mL/min for a 7-day sampling, or 14 mL/min for a 24-h sampling. The sampling volume was 20.1 L by all 3 methods. Sampling flow rate and actual volume were monitored by using a data logger (UL-100; Unipulse Inc., Saitama, Japan) to record the pulse signal from the wet gas meter. One revolution of a cylinder in the wet gas meter is equal to 1 L gas volume, and one pulse signal is equal to 1 mL (1 p/ml). Analysis. A Chrompack (Middelburg, Netherlands) thermal desorption cold-trap injector (TCT), a Hewlett-Packard (Avondale, PA) 5890 series II gas chromatograph, and a JEOL (Tokyo, Japan) AX-505W mass spectrometer were employed. We connected a condenser (14) between the sampling tube and the cryofocusing trap tube to prevent moisture from freezing in the trap tube. The tube was placed in the TCT for desorption of volatile compounds at 350 °C for 15 min. The condenser was set at 0 °C when the tube was packed with strong adsorbent, such as Carboxen 1000. Compounds were swept into the cold trap at -180 °C via helium carrier gas, and GC injection was performed by flash heating the cold trap to 250 °C for 5 min. Analysis was carried out on a column (50 m × 0.32 mm) of CP-Sil 5CB (5 µm film; Chrompack, Netherlands) with temperature programming from 50 °C (held for 5 min) to 260 °C (held for 6 min) at 15 °C/min. The separated components were identified by ionization current of 70 eV MS, ion source temperature at 280 °C, ionization current at 300 µA, and full scanning between m/z 35-350. Quantitation was based on a single characteristic fragment ion for each compound. The method was calibrated with TO-14 reference gas (43 compounds, 1 ppm, Scott Specialty Gases, Plumsteadville, PA) and the internal standard (4-bromofluorobenzene, 1 ppm Matheson Gas Product, Morrow, GA).
Results and Discussion Precision of Collection and Analysis. Sampling flow rates were monitored during a period of 24 h, 7 days, and 4 weeks
FIGURE 1. Flow rate change with time. The flow rate of 14 mL/min, 2 mL/min, or 0.5 mL/min was monitored and recorded with a data logger at intervals of 1 h, 7 h, or 28 h, respectively. to test the stability of the sampling apparatus (Figure 1). Relative standard deviation (RSD) of flow rate at 14, 2, and 0.5 mL/min was 0.66, 1.5, and 3.0%, respectively. If the flow rate of the sampling apparatus varies widely during a period of collection, the average concentration value will not be measured accurately. The sampling apparatus was found to be suitable for long-term sampling under the 3 sets of conditions tested. Burkart (15) described a general procedure for limit of detection (LOD) calculations in industrial hygiene laboratories, using linear regression theory. The Japan EnvironVOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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mental Agency (7) also described LOD. The calculation of LOD was based on the data points near the detection limit. Five milliliters of the TO-14 reference gas (100 ppb) and 5 mL of the internal standard gas (100 ppb) were introduced into the sampling tubes and then analyzed by TCT-GC/MS. LODs as if they had been introduced in 20.1 L of sampling volume were calculated from the data of seven replicate measurements. The overall precision of VOC measurements was estimated from data for seven sampling tubes into which 5 mL of the TO-14 reference gas standard (1 ppm) and 5 mL of the internal standard (1 ppm) were introduced. LOD and relative standard deviation (RSD) for repeatability are shown in Table 1. RSD for measurements of the most important VOCs ranged from 1.7 to 8.8%, indicating the measurement method had good precision. Comparison of 4-Week, 7-Day, and 24-h Sampling Methods. Ambient air concentrations of VOCs were measured continuously for 5 weeks from November 16 to December 21, 1998 and for 8 weeks from February 1 to March 28, 1999 at an air monitoring station in Chiba City, Japan. Sampling periods were 4 weeks, 7 days, and 24 h. The sampling site was located close to a crossing of main roads in a business district and near to a coastal industrial zone. Table 1 shows concentrations and concentration ranges of major VOCs in February 1999 measured by changing monitoring periods. Twenty-four-h and 7-day data are presented as mean values over 4 weeks and 4-week values are actual data. Mean values calculated from the 24-h and 7-day data agreed well with actual 4-week monitoring data. The concentration variation is larger for 24-h samples than for 7-day samples, but mean values of both methods agree well with each other. These observations remained true for data obtained in March 1999. Figure 2 shows the comparison of 7-day or 4-week monitoring data with mean values of 24-h monitoring data for each sampling period. Data obtained by measurement for 13 weeks are plotted on the figure. Three hundred and seventy four points are scattered on a regression line y ) 1.01x + 0.004, with r2 ) 0.996, indicating that each long-term monitoring value for almost all VOCs was nearly equal to the average value of 24-h monitoring, except for the few compounds mentioned below. Volatile compounds such as CFC-12 (bp -29.8 °C) and relatively high boiling compounds, such as 1,4-dichlorobenzene (bp 174.1 °C), were well recovered by 7-day or 4-week continuous nonstop sampling. Data points for chloroethene, 1,3-butadiene, and styrene are far from the regression line in Figure 2B. A common feature to all of these compounds is a vinyl group. The potential for chemical transformation of olefins during air sampling, due to the presence of ozone, was first reported by Bunch and Pellizzari (16). During sampling, ozone may react with the adsorbed terpenes or terpenoids (16-26). Ye et al. (27) reported that 1,3-butadiene may react with NO2 and decay. A number of materials have been used to remove ozone prior to sampling of terpenes. Presumably, like terpenes, other vinyl compounds may react with ozone. This would account for the losses observed during longer sampling times as an ozone scrubber was not used in this case. Daily and Weekly Variation of VOCs Concentration. Figure 3 shows changes in concentration of major VOCs measured by 24-h and 7-day sampling from February 1 to March 28. Chlorofluorocarbons (CFCs). Mean values of CFC-12, CFC11, CFC-113, and CFC-114 measured by 24-h sampling in February and March of 1999 were 3.1, 1.6, 0.70, and 0.15 µg/m3, respectively. Concentration variations of these compounds were small, and concentrations were approximately constant for 2 months. In January-February 1988, atmospheric concentrations of CFC-12, CFC-11, and CFC-113 in downtown Tokyo were 4.5, 2.9, and 5.0 µg/m3, respectively (28). In November 1996, mean values over 1 week for CFC4658
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FIGURE 2. Relationship between 24-h monitoring data and longterm monitoring data. Plots show 420 data points for 28 compounds listed in Table 1. Data for toluene are divided by five. Data collected using a 24-h monitoring period are presented as mean values for 7 days or 4 weeks and are plotted as ordinate. Seven-day and 4-week monitoring data are plotted as abscissa. The solid regression line is calculated from data points except chloroethene, 1,3butadiene, and styrene. The broken regression line is calculated from data points of chloroethene, 1,3-butadiene, and styrene: (A) all results and (B) results below 1.0 µg/m3. 12, CFC-11, CFC-113, and CFC-114 were 3.0, 0.94, 0.86, and 0.14 µg/m3, respectively (13). The background concentrations of CFC-12, CFC-11, and CFC-113 in the northern hemisphere (Hokkaido, Japan) were 2.3, 0.83, and 0.47 µg/m3, respectively (28). CFCs such as the above compounds damage stratospheric ozone, permitting enhanced levels of ultraviolet B radiation to reach the Earth’s surface. The Montreal Protocol mandated the cessation of production and use of CFCs by 1995 in Japan. As a consequence, concentrations of CFC-12, CFC-11, CFC-113, and CFC-114 have been reduced nearly to a background level and have remained constant thereafter. Chlorocarbons and Chlorohydrocarbons. Concentration variations of tetrachloromethane and 1,1,1-trichloroethane were small, and concentrations were approximately constant for 2 months. However, daily variations in concentrations of the other chlorohydrocarbons were marked: e.g., the concentration range of dichloromethane or trichloroethene was 0.43-7.4 and 0.12-4.1 µg/m3, respectively. Using weekly sampling, their range was reduced to 0.84-3.3 and 0.30-1.6 µg/m3, respectively. Using 4-week sampling, the range was further reduced to 1.6-2.5 and 0.69-1.4 µg/m3, respectively. Trichloroethene, tetrachloroethene, dichloromethane, and trichloromethane are industrial solvents used widely in
FIGURE 3. Concentration variation depending on the day of the week for key VOCs measured in Chiba City. Open circles and horizontal solid bars indicate data obtained using sampling periods of 1 day or 1 week, respectively. A broken line indicates the mean value of a week calculated from 24-h sampling data. degreasing, dry cleaning, and numerous other medical and industrial processes. Therefore, the concentration of these compounds may vary widely, depending on working rates of factories and weather conditions. Aromatic Hydrocarbons. Daily concentrations of aromatic hydrocarbons show large variation, and there are some similarities among the patterns of fluctuation in their concentration. The daily concentration range of benzene, toluene, or ethylbenzene was 1.5-10, 7.8-51, and 1.1-13 µg/m3. Using weekly sampling, their range was reduced to
3.5-5.4, 18-28, and 1.9-4.5 µg/m3, respectively. Using 4-week sampling, the spread was further reduced to 4.6-4.8, 21-24, and 2.8-3.9 µg/m3, respectively. Influence of Socioeconomic Activities and the Direction of Wind. Figure 4 shows the concentration variation of major VOCs with day of the week. The concentration distributions of chlorofluorocarbons and tetrachloromethane (data not shown) display a regular heptagon, but other VOCs display an irregular heptagon with a drop on Saturday and Sunday. This phenomenon was striking for chlorocarbons and VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. The concentration variations of major VOCs depending on day of the week for February and March 1999. The data for each day of the week were obtained from mean values measured using 24-h sampling over 2 months (n)8). Research on Environmental Health from the Ministry of Health and Welfare of the Japanese Government. We thank Elizabeth Woolfenden, MARKES international Limited for her aid in preparation of the manuscript.
Literature Cited
FIGURE 5. The relationship between the concentration of benzene and the direction of the wind for February and March 1999 (µg/m3). Concentrations with each wind direction were mean values obtained using 24-h sampling over 2 months. chlorohydrocarbons such as dichloromethane, trichloromethane, trichloroethene, and tetrachloroethene. Komazaki et al. (29) measured the diurnal variations of formaldehyde and acetaldehyde concentrations in the urban atmosphere by automated continuous measurement system for 7 days and observed lower concentrations on Saturday and Sunday. Socioeconomic activities are dependent on the day of the week, and our observations reflect that automobile traffic and factory operations decrease at the weekend. Figure 5 shows a clear relationship between the concentration of benzene and the wind direction. High concentrations of benzene were observed when the wind blew from the direction between east and south. These compounds are emitted into the atmosphere by the evaporation of fuel or from automobile exhaust (30-32). A main road is located 50 m to the east of the sampling site, and factories manufacturing benzene in a coastal industrial zone are located 2.5 km to the south of the sampling site. The concentration of VOCs will vary widely with industrial work rate, automobile traffic density, and weather conditions around the sampling site. In summary, it is important to assess the risk to the general population from long-term, low-level exposure to specific substances in the environment. The daily change of most of VOCs during the measurement period for 2 months showed very large variation. We have therefore developed a longterm sampling method to more accurately measure average VOC concentrations in the atmospheric environment. Concentrations of almost all VOCs can be monitored by 7-day or 28-day sampling, except vinyl group-bearing compounds such as chloroethene, 1,3-butadiene, and styrene.
Acknowledgments This work was supported by grants for Second Term Comprehensive 10-year Strategy for Cancer Control and 4660
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Received for review July 26, 1999. Revised manuscript received July 7, 2000. Accepted August 10, 2000. ES990843U
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