Atmospheric Particulate Mass Measurement with Beta Attenuation Mass Monitor Edward S. Macias" Department of Chemistry, Washington University, St. Louis, Mo. 63130
Rudolf B. Husar Air Pollution Research Laboratory, Department of Mechanical Engineering, Washington University, St. Louis, Mo. 63130
A two-stage on-line mass monitor with aerosol size separator (TWOMASS) employing the beta attenuation technique has been developed for monitoring of atmospheric aerosols with high time resolution. This instrument independently analyzes the mass concentration of two particle size fractions. Coarse particles are inertially separated by impaction on filter paper. The fine particle fraction is collected on a high-efficiency glass-fiber filter. Carbon-14 is used as a source of beta particles which are detected by a silicon surface-barrier detector coupled to fast, low-noise nuclear electronics. A programmable calculator controls TWOMASS, calculates the mass concentration of each size fraction, plots, prints, and stores the data on tape. The instrument has been gravimetrically calibrated using laboratory and atmospheric aerosols. For 10-min sampling intervals, the precision of the instrument is 4 pglm3, and the accuracy is 11%.It has subsequently been field tested by sampling atmospheric aerosol over an extended period which indicates that it is capable of ambient air monitoring with a time response on the order of 10 min.
The present air quality standard for particulate matter is expressed in terms of the total aerosol mass concentration in pg/m3 as measured by the high-volume air filter sampler. Recently, there has been concern among researchers, as well as within control agencies, that total particulate mass or any other single parameter is an inadequate and, to some extent, misleading measure of adverse effects of atmospheric particles. There is an increasing body of evidence which suggests that atmospheric particles of diameters less than 5 pm contribute significantly to the adverse effects of air pollution and in fact constitute a large segment of the total air pollution problem ( I ) . Accordingly, the current thinking on a first step toward a proper characterization of atmospheric particles is to divide the total aerosol population into two size classes, i.e., f i n e and coarse particles ( 2 ) .A study of the characteristics of the Los Angeles smog aerosol by Whitby et al. (3) and subsequent studies of other locations by Durham et al. ( 4 ) showed that most of the atmospheric aerosol volume is distributed bimodally, as shown in Figure 1. The lower mass mode is in the size range 0.1-1.0 pm, while the upper mode is over 5 pm. The minimum between the two mass (or volume) modes is between 0.8 and 3.0 pm. The bimodal aerosol mass spectrum is significant because the two modes have distinctly different elemental composition, as shown by Dzubay and Stevens ( 5 ) and physical characteristics (6) (particle shape, volatility). Furthermore, the two modes are produced by different sources ( 7 , 8 ) , and they are associated with different effects. These findings provide a scientific rationale for the separate consideration of fine and coarse particles. It is anticipated that within the next few years, these findings may be recognized by establishing new standards requiring the determination of aerosol mass and composition as a function of particle size. Independent determination of the mass concentration of only two size fractions will probably be adequate for monitoring purposes ( 2 ) .The division of lower 904
Environmental Science & Technology
and upper mass modes will depend on two major criteria. First, the separation should reflect the bimodality of the atmospheric aerosol size spectrum. Secondly, the separation should take into account the ability of different size particles to affect human health, visibility, weather and climate, etc. (Figure 1). The attenuation of beta particles as they pass through an absorber can be used as a thickness gauge or mass monitor. In earlier studies, various source and detector combinations were used (9-13). An application of the beta absorption technique for determining the mass concentration of atmospheric particles was reported by Nader and Allen in 1960 ( 1 4 ) . That work demonstrated the feasibility of this technique for use in automated tape samples. More recently, many researchers have reported the use of beta attenuation mass monitors using a variety of experimental configurations (15-25). In a recent comprehensive study of potential techniques for measurements of emissions from fossil fuel combustion sources, Sem et al. (26) identified beta attenuation as a promising method for sensing particle mass concentration. A detailed review of the application of this method for mass measurement is given elsewhere (27). This paper is concerned with application of the beta attenuation technique for automated atmospheric particle mass measurements. A two stage on-line mass monitor with aerosol size separator (TWOMASS) which monitors atmospheric aerosols in two size ranges is described. The operating characteristics, calibration of the instrument, and field tests are discussed. Physics of Beta Absorption
In the process of p- decay, an electron is emitted from the atomic nucleus, and the nuclear charge changes from 2 to 2 t 1 in units of the electronic charge. This process transmutes the beta-active element into the next heavier element. Beta particles are emitted with a continuous energy spectrum rather than a single discrete energy equal to the transition energy. For I 4 C , the beta source used in this work, the decay can be depicted as follows: 14C
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The average beta particle energy is approximately one-third of the maximum energy for most beta emitters. Beta particles interact with matter through elastic and inelastic scattering with atomic electrons and through elastic scattering with nuclei. For low-energy electrons ( E D< 0.5 MeV), inelastic scattering with atomic electrons (ionization) is the predominant mode of energy loss (28).The number of beta particles passing through an absorber decreases exponentially with absorber thickness, to a good approximation, as given by the following (29)
I = I,e-rmx (2) where I , is the beta intensity without an absorber, I is the intensity observed through an absorber of thickness x , and pm is the mass absorption coefficient. The exponential form of the curve is fortuitous, since it also includes the effects of
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the continuous energy distribution of the beta particles and the scattering of the particles by the absorber. The range, R , is the distance traversed by the most energetic particles emitted and corresponds to the energy at the endpoint of the continuous spectrum. The simple exponential absorption of beta particles holds for absorber thicknesses which are small compared to the beta particle range (29). For low-energy beta emitters, the mass absorption coefficient is nearly independent of the chemical composition of the absorber. The absorption of high-energy radiation (E8 > 1 MeV) includes a bremsstrahlung radiation component (inelastic scattering with the nuclear coulomb field). This effect causes large variations in the mass absorption coefficient with increasing atomic number (30) and is largest for heavy element absorbers. I n s t r u m e n t Design
The TWOMASS instrument was designed to separate particles into two size classes and to measure aerosol mass concentration with high time resolution. Many of the details of optimal design criteria are given in a recent review (27).
Aerosol Collection System. The flow system separates particles into two size fractions. Coarse particles are impacted on a glass fiber filter; the remaining particles are collected on an identical high-efficiency glass fiber filter. This system is shown schematically in Figure 2. The single-stage impactor head has a 4.5-mm diam inlet aperture with a 4.5-mm jetto-plate distance. The impactor was designed to have 50% efficiency for particles of 3.5 wm diam. This cutoff size was chosen as a compromise between the proper separation of the two modes of the atmospheric aerosol and the simulation of the aerosol removal characteristics of the human upper respiratory system. Various impaction materials were tested, and a Pallflex (Pallflex Products Corp., Putnam, Conn.) glass fiber filter with thin cellulose backing was the most satisfactory because of its low mass density, efficient impaction properties, and uniform thickness. Lilienfeld and Dulchinos (19)found that this filter has high particle collection efficiency a t high face velocities, low flow resistance, low tare weight, adequate mechanical strength, and adequate filter capacity before clogging. The mass density of this filter is 3.2 mg/cm2, and the efficiency is over 99% for both cigarette smoke and 0.3 km diam. polystyrene latex spheres (19). This filter was used for both stages of the sampler. The flow rate through the TWOMASS was set at 400 cm3/s (24 l./min). The face velocity under operating conditions was 13 m/s. Source and Detection System. Both the impaction and filtration heads of TWOMASS have independent sourcedetector systems. The beta source chosen was 3 mCi of 14C deposited as sodium acetate in a 6-mm diameter copper disk covered with 800 kg/cm2 of mylar film. This source is a pure low-energy beta emitter with 5.7 X lo3 year half life, E,,, = 156 keV, and range R = 28.3 mg/cm2 (31).The total density of all absorbers (air, detector window, filters, deposited mass, etc.) in the system is about 6 mg/cm2 or about 21% of the range of I4C beta particles, which is optimal for measurement sensitivity (11). The cross-sectional areas of the collection spots in the filtration and impaction states are 32 and 16 mm2, respectively. The beta detector (ORTEC, Inc., Oak Ridge, Tenn.) is a silicon surface-barrier detector with a 50-mm2 surface area, a 40+g aluminum window, and a noise width less than 11keV. This detector is light-tight and rugged, and hence suited for field operation. It was chosen because of its high count-rate capability, low-noise characteristics, low cost, and simplicity of operation. The amplified output of each detector is acquired Volume 10, Number 9, September 1976 905
with two high-speed counters and further processed by a Tektronix programmable calculator with an x-y point plotter, printer, and magnetic tape cassette. The average aerosol mass concentration, Ah4 in wg/cm2, is given by
where A is the cross-sectional area of the collector, At is the counting and collection interval in seconds, Q is the flow rate in cm3/s, pm is the mass attenuation coefficient in units of cm2/pg, Z is the count rate of the current interval, and Z, is the count rate through the filter before the addition of AM. On-Line Operation. The TWOMASS instrument is operated as a spot sampler with aerosol deposited continuously on both stages. The filter tapes are advanced simultaneously every 2 h to avoid clogging. In operation with atmospheric aerosol, the change in the flow rate is less than 5% during a 2-h interval. The beta particles which penetrate the filter paper are counted as aerosol is being deposited. The values of Z and I,, appearing in Equation 3, are the number of counts accumulated in the current and previous time intervals, respectively. The mass determination is made for each 10-min counting interval. In this way, 11 mass concentrations are acquired during the accumulation of each 2-h filter spot. (No mass measurement can be made during the first time interval.) Cali brat i o n
The instrument was calibrated gravimetrically with laboratory and St. Louis atmospheric aerosols. The results of these measurements yield an exponential relationship between beta attenuation (Z/Zo) and gravimetric mass, both for laboratory aerosols in the range of 1-7 mg/cm2 and for atmospheric aerosols in the range of 0.2-1 mg/cm2, as shown in Figure 3. The calibration was carried out over a much wider mass range than required for TWOMASS when used for ambient monitoring. In a typical 3-h period of aerosol collection in St. Louis, the deposited particulate mass on either stage is no more than 1 mg/cm2.
The mass attenuation coefficient, wm, depends on the chemical form of the aerosol as shown in Figure 3. These data indicate that I,,, varies from 0.20 for glycine to 0.26 for atmospheric aerosol. Thus, we suggest that the determination of pm is necessary when using TWOMASS for studying a specific aerosol. However, our data indicate that, a t least for St. Louis, the atmospheric aerosol has a value of wm = 0.26 f 0.03 cm2/mg. This value, perhaps fortuitously, is identical with the standard literature value of 0.26 cm2/mg (311. Precision. The total precision of the instrument is a function of the stability of the system and the counting statistics. Typical counting rates are 10 000 cps with the unloaded filter tape in place. The count rate is reduced by 0.4% due to an aerosol mass collected in a 10-min interval. This high counting rate was possible with minimal dead time (
