Environ. Sci. Technol. 1991, 25, 883-890
from as large a number of calibration standards as is appropriate. ( c ) Method Validation. The following paragraphs provide method validation data for AN determination by modified 624. The amount/response calibration curve for the mass spectrometer was found to be linear over the range of 4-1612 pph, with a correlation coefficient of 0.9996 (average of two determinations 6 months apart). The method detection limit (MDL), determined according to the procedure outlined in appendix B to CFR 140 Part 136, was 3 ppb with 95% upper and lower confidence limits of 6 and 2 ppb, respectively. Effluent spiking experiments were carried out on six different effluent samples to determine the effect of the matrix on the accuracy of the method. Table I1 contains the data from these analyses. The average percent recovery is 102.0 with a standard deviation of 7 % . A Student's t test (0.05 level of significance) applied to the mean recovery did not detect a statistically significant difference from 100%. The observed 70 RSD was less than the estimated method precision. Together, these two results show that the matrix has no statistically detectable effect on the method. Four effluent samples were stored a t 5 "C and analyzed at 2-week intervals over 4- and 8-week periods to determine sample stability. The results are summarized in Table 111. AN was not detected in sample 1. Data for sample 2 are given twice, first with and then without a data point presumed to be anomalously low. The third analysis consumed the remaining portion of sample 3. None of the samples showed unusually large percent relative standard deviations or shifts in average over the time period in question. Thus, these samples appeared to be stable over
the test periods when stored a t 5 "C.
Summary Method 603 is sensitive to matrix effects and thus must be selectively applied to waste stream analyses. On the other hand, the modified 624 method for AN analysis is less sensitive to matrix interferences. Furthermore, the variability in the quantitative results was equivalent to published data from method 603. Acknowledgments We thank Dr. D. F. H. Swijter of the Mass Spectrometry Laboratory for his analysis of the AN-spiked effluent. Registry No. Acrylonitrile, 107-13-1; water, 7732-18-5.
Literature Cited (1) Ramstad, T.; Nicholson, L. W. Anal. Chem. 1982,54, 1191. ( 2 ) Guidelines Establishing Test Procedures for the Analysis of Pollutants; Proposed Regulations, 40 CFR Part 136 Fed. Regist. 1979, 44, Part 111. (3) Guidelines Establishing Test Procedures for the Analysis of Pollutants; 40 CFR Part 136 Fed. Regist. 1984,49, Part
VIII. (4) Dreisch, F. A.; Munson, T. 0. J . Chromatogr. Sci. 1983,21, 111. (5) Trussel, A. R.; Moncul, J. G.; Lieu, E'. Y.; Leong, L. Y. C. HRC 8 . 1 CC, J . H i g h Resolut. Chromatog. Chromatogr. C o m m u n . 1981, 4 , 156. ( 6 ) Lucas, S. V.; Cole, T. F.; Riggin, A.; Cooke, W. M. EPA600/4-85-001; U.S.EPA, Cincinnati, OH, December 1984.
Received for review J u n e 25, 1990. Revised manuscript received November 26, 1990. Accepted November 28, 1990.
Electrical Mobility Measurements of Fine-Particle Formation during Chamber Studies of Atmospheric Photochemical Reactions Richard C. Flagan," Shih-Chen Wang, Fangdong Yin, and John H. Seinfeld
California Institute of Technology, Pasadena, California 9 1 125 Georg Reischl, Wolfgang Winklmayr, and Rudolf Karch University of Vienna, Vienna, Austria
w New approaches have been applied to electrical mobility measurement of ultrafine aerosol particles in smog chamber studies of secondary aerosol formation. With several mobility classifiers operating in parallel, rapid new particle formation was followed in the photochemical oxidation of dimethyl disulfide. When foreign particles were present before reaction was initiated, multiple bursts of nucleation and oscillations in the concentrations of 3.4-nm particles were observed. Later experiments used the scanning electrical mobility spectrometer to make high-resolution particle size distribution measurements. With this measurement method, the rapid growth of nuclei from their initial appearance a t 10-nm size was followed in hydrocarbon/NO, and hydrocarbon/NO,/SO2 reactions. Again, multiple bursts of nucleation were observed in some experiments, and insights were gained into particle growth mechanisms. Introduction Homogeneous gas-phase reactions of gaseous pollutants 0013-936X/91/0925-0883$02.50/0
including organics, nitrogen oxides, and sulfur dioxide lead to the formation of condensible vapors and, ultimately, to the production of secondary aerosols. Gas-to-particle conversion is also thought to lead to aerosols in the remote marine environment where dimethyl sulfide (DMS), which is excreted by phytoplankton, reacts to form sulfuric acid and methanesulfonic acid (MSA) (1). Numerous studies have documented the formation of secondary aerosols through homogeneous gas-phase reactions of gaseous pollutants (2-4), but a quantitative understanding of the gas-to-particle conversion has not yet been achieved. While the gas-phase reaction mechanisms leading to O3and other gas-phase products are reasonably well understood in many cases, neither the chemical pathways that ultimately lead to particle formation nor the composition or even the physical properties of the condensing species are wellknown. Part of the reason for this deficiency is that the technology for following the dynamics of rapidly changing aerosols has lagged behind the ability to follow the primary gaseous species. Smog chamber studies provide data on
0 1991 American Chemical Society
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the time variation of major species involved in the atmospheric photochemical reactions. These data have been used to evaluate postulated reaction mechanisms, providing the ability to predict the formation of ozone and other gaseous reaction products. Little is known about the condensible products of these reactions. Chamber studies of aerosol formation require large reactor volumes to minimize the biases introduced by wall losses of the product particles. As a result, studies of aerosol formation in photochemical reactions have been limited to large outdoor reactors. Heisler and Friedlander (3) explored growth laws in the formation of secondary organic aerosols, and McMurry and Friedlander (6) studied sulfate aerosol formation in the oxidation of sulfur dioxide. These experiments utilized untreated air that contained ambient levels of pollutants; hence they were not intended to provide fundamental insight into the gas-phase reaction mechanisms involved in aerosol formation. Yet those early studies provided the first direct measurements of the evolution of particle size distributions in atmospheric photochemical systems. Improvements in gas- and aerosol-phase instrumentation and in experimental control through chemical cleaning of the air with packed-bed scrubbers have made outdoor smog chambers valuable tools in the study of both gasphase chemistry and aerosol formation (7). Leone et al. (8) used such a chamber to probe the mechanisms of photochemical oxidation of toluene. Those gas-phase species that could be measured had behavior consistent with proposed reaction mechanisms, although insufficient information exists to predict the nature or source rates of condensible vapors. Little quantitative understanding of the aerosol products of photochemical reactions has evolved. Stern et al. (9) used the TSI Model 3030 electrical aerosol analyzer (EAA) to follow the dynamics of secondary aerosols in chamber studies of the photochemical oxidation of toluene and other aromatic hydrocarbons. Conclusions from that study relative to the aerosol measuring capability were (i) the EAA does not have the size range or resolution needed for quantitative interpretation and (ii) faster response than the 3-8 min typically required for EAA size distribution measurements is needed to follow the rapid changes due to nucleation in the simulated atmospheric chemistry. A method for making high-resolution particle size distribution measurements has been available for a number of years in the form of the differential mobility analyzer (DMA) ( I O ) , but this instrument has seen relatively little use in the study of time-varying aerosols because the DMA requires sampling times as long as 1 h or more to take advantage of the increased resolution. Sampling times can be reduced by decreasing the number of sizes at which measurements are made, but a t the risk of missing some of the structure of the size distribution. Recent improvements in electrical mobility size distribution analysis make it possible to follow rapidly changing aerosols. This paper will report on outdoor smog chamber studies and use results from these experiments to illustrate some of the complex dynamics of the atmospheric aerosol. Outdoor smog chamber studies have been performed in an effort to understand homogeneous nucleation and particle growth in photochemical oxidation of dimethyl disulfide, or hydrocarbons with nitrogen oxides, both alone and in the presence of sulfur dioxide. Although dimethyl sulfide is the active organosulfur species in the marine atmosphere, dimethyl disulfide (DMDS) was used in the experiments as DMDS has similar photooxidation behavior to DMS but has fewer degradation pathways and, there884
Environ. Sci. Technol., Vol. 25, No. 5, 1991
fore, affords the opportunity to isolate certain aspects of DMS gas-phase chemistry.
Experimental Section The Caltech outdoor smog chamber, which is described in detail by Leone et al. (8), consists of a Teflon balloon located on the roof of the Keck Laboratories and utilizes sunlight to drive the photochemical reactions. The reactor has been operated in two modes: (i) single-bag mode, with a volume of approximately 60 m3 when fully inflated; and (ii) dual bag mode in which the chamber is divided into two roughly equal volumes, - 2 5 m3 each. The former mode offers the largest volume for long-duration experiments or to minimize wall effects. The dual-bag mode allows simultaneous experiments to be performed to observe directly the impact of varying a single parameter such as the initial hydrocarbon concentration or the quantity of particles present in the chamber at the beginning of the experiment. The experimental operation consists of filling the chamber with air that has been cleaned by passing it through a series of packed-bed scrubbers. The filling and subsequent addition of reactants to the chamber was performed with the chamber covered to eliminate photochemical reactions. The reactants consist of selected hydrocarbon or organosulfur species, nitrogen oxides, and, in some experiments, sulfur dioxide or ammonia. An aerosol of fine particles may also be added to the reactant mix to probe the influence of preexisting aerosol on the formation and growth of new particles. For the present experiments, this seed aerosol was produced by atomizing an aqueous solution containing 2 g L-l (NH4)2S0,with a stainless steel, constant-rate atomizer ( 1I ) . The aerosol was passed through a s5Kr decharger before introduction into the chamber. To minimize seed particle deposition, the aerosol was introduced after the gaseous reactants. After the chamber was filled, the reactants were allowed to mix for 30 min to 2 h before beginning the experiment. The photochemical reactions were initiated by uncovering the chamber. The smog chamber was operated as a batch reactor with continuous sample extraction for gas and aerosol measurements. Teflon tubing that extended approximately 30 cm into the chamber through Teflon sampling ports was used for sampling gaseous species. Since Teflon surfaces carry a charge that leads to enhanced losses of the charged fraction of the aerosol (22),particle samples were extracted by using separate copper lines extending -15 cm into the chamber. The gas-phase composition data obtained included concentrations of Os, NO, NO,, SO2,and reactant organic species measured by gas chromatography with photoionization or flame ionization direction. Temperature, relative humidity, and total solar radiation were also measured. The focus of this paper is on the aerosol measurements, so the details of the gas analysis will not be repeated here. The gas-phase measurements are discussed in Yin et al. (13). Aerosol Analysis. Aerosol measurements were made
with TSI differential mobility classifiers (DMA; Model 3071) with TSI clean room condensation nuclei counters (CNC; Model 3760) used as detectors, and with the electrical mobility spectrometer (EMS) developed a t the University of Vienna ( 1 4 ) . Operation of both of these instruments is based upon electrical mobility analysis wherein charged particles are classified according to their mobility in an electric field. Measurements of the number concentration within a narrow increment of mobility are made at a sequence of field strengths to determine the size distribution. Because the smog chamber is a batch reactor,
and the measurements of different sizes are made sequentially, only transients slower than the scan period can be followed with conventional use of these instruments. Hence, we have sought alternate modes of operation that would allow fast transients to be followed. Both the DMA and EMS classify charged particles by migration in an electric field between concentric cylinders. The particles are thus characterized in terms of their electrical mobilities, Z,. A small flow extracted through holes at the downstream end of the central cylinder carries particles with mobilities in a narrow range out of the instrument for analysis, either using an electrometer to measure the current carried by the charged particles (in the EMS) or using a condensation nuclei counter (with the DMA/CNC combination). When measurements are made a t a sequence of field strengths, a differential mobility distribution cW/dZ, = n(2,) (1) can be determined directly. Under typical operating conditions, the range of mobilities extracted a t any time amounts to -10% of the mean value. Determination of the particle size distribution, n(D,), from mobility distribution measurements requires knowledge of the charging and transmission efficiencies of the instrument as a function of particle size. The commercially available EAA utilized a unipolar diffusion charger that generates singly charged particles below 30-nm diameter. Larger particles could have one, two, or more charges; 'hence, a single mobility interval contains particles of a number of sizes. In conventional use, the DMA incorporates a bipolar diffusion charger that shifts the range of multiple charging to larger sizes, greater than 100 nm. Still, multiple charging creates data analysis problems for accumulation-mode aerosols. To determine the particle sizle distribution, it is necessary to invert the data by solving a set of Fredholm integral equations for a best estimate of the actual particle size distribution. The EMS utilizes a new mobility classifier design that has been optimized to minimize diffusional losses and a highly sensitive electrometer with a detection limit of approximately 3 X A to facilitate particle size distribution measurements to sizes as small as 3-nm diameter. With this instrument, 22 point particle size distributions of particles in the 3-150-nm size range can be measured in 4 min. The 'TSI instrument combination can measure particles as small as -10 nm, although the counting efficiency drops dramatically below 20-nm diameter (15). The time required to measure a particle size distribution depends on the resolution sought, and can range from 8 min to more than 1 h in conventional operation. Two EMSs and two DMAs have been employed to follow the rapid dynamics of the aerosols produced in smog chamber experiments. Initially, parallel measurements of the concentrations of particles in a number of size intervals have been used to overcome the time resolution limitations of mobility size distribution analysis. In these experiments with the dimethyl disulfide system, the mobility classifiers were operated in parallel to measure the time variation in the concentrations of particles of several sizes. In this way, the minimum resolvable time was reduced from several minutes to a few seconds. As will be shown below, these measurements revealed dynamics of the aerosols that could not be resolved with conventional mobility analysis of the aerosols, including extremely rapid bursts of homogeneous nucleation and oscillations in the concentrations of fine particles. Motivated by these intriguing dynamics of the fine particles, a new approach to mobility analysis was then
-
-
Table I. Initial Concentrations for Dimethyl Disulfide Photooxidation Experiments concn, ppb expt
NO
NOz
1123 1125
39
2
40
9
(CH312S2
seed, ~ r n - ~ 0 2,000
364 293
developed that allows complete size distributions to be measured in as little as 30 s (15). The long times required to measure a particle size distribution are primarily due to the long flow times in the analyzer column. Between voltage steps, it is necessary to wait long enough for all particles in the column to be flushed out of it before a measurement is made. The critical feature of the mobility analysis of the particle size distribution is that each measurement correspond to particles of a given mobility. It is not necessary that the particles be classified at constant field strength, but rather that each measurement sense only a limited range of mobilities. If the field strength is changed continuously but monotonically, the particles migrate in the analyzer column along characteristic trajectories such that the particles arriving a t the detector at a particular time correspond to a narrow range of mobilities. Wang and Flagan (15)developed a modified mobility analyzer called the scanning electrical mobility spectrometer (SEMS) by a computer control of the commercially available TSI Model 3071 differential mobility analyzer, and using a TSI Model 3760 clean room condensation nuclei counter as a detector. This instrument and its calibration are described in detail elsewhere (15). By use of an exponential ramp on the field strength, 100 point particle size distributions have been acquired in as little as 30 s.
Results Dimethyl Disulfide Photooxidation. Studies of photooxidation of dimethyl disulfide were performed only in the single-chamber reactor mode. Both particle-free and seeded experiments were performed. Mixtures of DMDS and particles were prepared in clean air with the chamber covered. After the reactants were allowed to mix thoroughly, the chamber was uncovered. Both gaseous reaction products and aerosol were followed in all experiments. The experimental conditions are summarized in Table I. Preliminary experiments had indicated that aerosol formation occurred very rapidly, so a series of experiments was undertaken t o resolve the nucleation transients by operating two ultrafine particle mobility analyzers and two conventional mobility analyzers in parallel. In several experiments, the mobility analyzers were operated a t constant classifier field strength to follow a single size of particle with high time resolution, a t least through the initial transient of new particle formation. Figure 1 shows the time variation in the number distributions of several particle sizes in an unseeded DMDS experiment. The EMS measurements in Figure l a show a rapid rise in the number concentration of 11-nm particles -3 min after uncovering the chamber. The peak in the number concentration lasted for only 2 min, after which the number of these fine particles rapidly decreased as they grew to larger sizes. The concentration of 20-nm particles peaks -5 min into the run and then decays, albeit somewhat more slowly than does the concentration of smaller particles. The particles grow to 50-nm size after -10 min. The decay in the number concentration appears to be slower than that of smaller particles. EMS measurements of larger particles, plotted in Figure lb, show
-
Environ. Sci. Technol., Vol. 25,
No. 5, 1991 885
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