A Technical Overview on Beta-Attenuation Method for the 1 Monitoring of Particulate Matter in Ambient Air 2

Beta-attenuation technique is one of the widely used real-time technique for ambient particulate 13 matter (PM) measurements since it allows continuous measurement while requiring minimal 14 operator attention. Like any other technique, this method has several limitations that have been 15 recorded in many studies. Beta-attenuation technique is dependent on the meteorological 16 conditions as well as operational factors, which lead to over- or underestimation in the mass 17 measurements in comparison to the reference method. However, the factors that affect its 18 measurement and the variations in its performance under different conditions are not listed or 19 reviewed in a comprehensive manner in a single document. Also, the systematic advancement 20 of its development and implementation in ambient air measurements have not been 21 documented in literature used by the air quality community. It is important for the user of this 22 technique to have a detailed understanding of its principle and operation. Consequently, this 23 article discusses the research, development, technology, and measurement of beta-attenuation 24 method in depth. Although this review emphasizes primarily PM 10 measurement results but 25 some PM 2.5 studies are also included. Our review reveals that federal equivalent method (FEM) 26 designated beta gauge monitors in various studies performed better (with slope < 1.5 and 27 intercept < 2 µg m -3 ) during high RH ambient conditions against reference or federal reference 28 method (FRM). Studies related to PM 10 showed that cut-off size, high mass loading and high 29 ambient RH (>80%) have impact on beta gauge measurements. Therefore, it is recommended 30 to clean inlet once a week and use smart heater to control RH at or below 35%. PM 2.5 studies 31 also confirm the effect of relative humidity on beta gauge measurements. the thermodynamic and evaporation model and the result shows that PM 10 concentration readings simulated by water evaporation loss model are in good accordance with true readings in comparison to thermodynamic ISORROPIA model. The result indicates that the ISORROPIA model can predict water content of particles only when they are in air, whereas evaporation model gives better prediction of beta-gauge readings. This study can be helpful to to set RH for heater activation.

event data with limited operator involvement, its operating principle is reported to be dependent 79 on meteorological conditions which differ widely in many countries. Therefore, we provide a 80 single document detailing the history of its development and implementation in ambient air 81 measurements with studied factors influencing the measurement technique. For several years, beta gauges working on beta-ray attenuation has have been used in 88 applications requiring constant, nondestructive monitoring of thin films. A beta particle is a 89 high-energy, high-speed electron or positron, which is emitted by the radioactive decay of an 90 atomic nucleus during the process of beta decay. Beta rays can either be absorbed, reflected, or system's radioisotopic generators in a fixed geometry have been tested individually as 3 H, 14 C, 102 and 63 Ni pure beta emitters. (Fig. 2).  The gravimetric method is used as primary technique to estimate the concentration of 126 particulate matter (PM) by measuring samples deposited on filters using high-or low-volume 127 samplers over a 24-hour span (Aggarwal et al., 2013). When employing the gravimetric method, 128 a collection period of a few hours to 24 hours or even days is expected. (US EPA, 2016). As a 129 result, the gravimetric approach is ineffective for monitoring peak concentration hours or 130 observing short-term concentration changes. Furthermore, this technique necessitates weighing 131 each filter separately prior to and after sampling, preventing sampling and measurement 132 recording from being automated. Therefore, despite the availability of primary gravimetric 133 method, there is still a need for a real-time automated technique that produce data which (i) 134 directly correlate with mass; (ii) enable data from various cities and seasons to be combined 135 for analysis; (iii) minimise handling of the fragile filter; (iv) allows short-term (1 to 8-hour 136 intervals) ambient particulate concentration measurements in the same units (µg m -3 ) like long-137 term (24-hour interval) samples. The short-term measurements should aid in assessing short 138 events so that better policies can be formulated.

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Many authors have used the beta-ray attenuation method to measure particulate matter 140 obtained from the ambient sampling (Nader and Allen, 1960;Salkowski, 1964;Dresia and 141 Spohr, 1971;Hussar, 1974). A schematic of the beta gauge monitor is shown in Fig. 3. It 142 consists of a beta particle source, a detector, and filter holder for collecting sample deposits on 143 filter tape. The attenuated beta particles reach the detector after passing through the particulate 144 deposited filter tape. The amount of radiation reaching the detector is decreased as the mass of 145 the particulate spot increases. This method has the advantages of non-destructive physical 146 measurement and fast automated operation, both of which are desirable when dealing with 147 large amounts of data requirement.

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The beta gauge method is based on the attenuation of beta particles passing through a fine 149 layer of particles. The relationship between the decrease in counts and particulate mass was 150 computed initially in Jaklevic et al. (1981). where, Io is the incident unattenuated beta counts, I is the attenuated beta counts through a 155 substrate of mass density x (mg cm -2 ) and µ is mass absorption or attenuation coefficient (cm 2 156 mg -1 ). Measurement of I can be directly attributed to the mass of a sample deposit if µ and Io The attenuation coefficient, μ is a constant specific to the absorbed material on filter tape. After 164 knowing I and Io, the mass density of the material can be determined. For a given time t, 165 atmospheric air is sampled at a fixed flow rate Q and passed by a filter of surface area A. After where A is in cm 2 , µ is in cm 2 mg -1 , Q is in L min -1 and time in minutes. The choice of a radioactive source for a specific application, depends on the beta particle 184 spectrum of the source. Fig. 4 shows a beta-particle spectrum characterised by a continuous 185 distribution of energy with a maximum energy Emax at end, which is specific to the isotope used.

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The shaded portion depicts the measured intensity I, while Edisc represents the discriminator 187 level below which the device is not responsive or capable enough to measure it precisely. The Edisc and Emin is the threshold limit below which attenuation cannot be used for the measurement 194 of thickness precisely and accurately.

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Matching the effective range (Emin to Emax) of the beta spectrum to the range of 196 sample thickness to be assessed is important when choosing a radioactive beta-particle source.

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If the overall range of the particles energy in the distribution is less than the thickness to be 198 determined, only a few electrons can reach the sample, and then Eq. (1) is no longer true.

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Likewise, if the particles are highly energetic in comparison to any reductions in a thin sample, 200 there would be a little or no impact on the spectrum, making it impossible to make a sensitive 201 mass measurement. As a result, the source is selected so that beta particle production is the

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To enable measurements with the required resolution, the detector must be receptive to beta 217 particles (i.e., electrons) in the relevant energy range and able to count discrete events quickly.

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Since 1976, different types of detectors have been used in beta gauge systems. These detectors element that detects radiation and processing electronics constitutes a Geiger counter (Fig. 5).

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An inert gas, such as helium, neon, or argon, is injected into the Geiger-Müller tube at a low 228

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11 pressure while being exposed to a high voltage. When a particle or photon of incident radiation 229 ionises the gas, the tube conducts electrical charge. Another type of detector which is widely 230 used is photomultiplier tube with scintillation device. It has a sensitive photodetector 231 (photomultiplier tube) that transforms light into an electrical signal after being exposed to 232 radiation, electronics to process the signal, and a scintillator that produces photons in reaction 233 to incident radiation. In 1982, Jaklevic et al. devised a precise beta gauge using a detector 234 consisting of a plastic scintillator and a photomultiplier tube (Fig. 6). This detector shows 235 enhanced counting rate capability allowing precision of ± 2 µg cm -2 . Current beta gauge 236 monitors are usually equipped with photomultiplier tube with scintillation device. Beta gauge technique for particle measurement is used to calculate the mass accumulated 262 on a filter by evaluating the relative shift in the intensity of the beta particle traveling through  In this section, different types of developments made in the beta gauge system since 1976 are 271 discussed, and summarised in Table 2. efficiency. The instrument's precision was less than ± 5 µg cm -2 for a measuring interval of one 308 minute per sample.  Device (CFD). The beta particle source used was 100 µCi (3.7 × 10 6 Bq) 14 C with solid-state 337 semiconductor detectors. The CFD regulates the sampling rate to 18.9 lpm ± 5% and has a 50% 338

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15 cutoff size of 10 μm. Instrument compared with 24-hour average samples obtained with 339 Wedding reference method sampler, with the slope of 1.2 and intercept below 1.5 µg m -3 with 340 R 2 of 0.99 or higher, and a resolution of less than 3 µg m -3 resolution. Wedding beta gauge 341 sampler has been used in many studies and discussed in the later section (Tsai, 1995  particle sizing followed by scattering based detection. Also, particle density is needed to be 387 considered while calculating particle mass from its size (volume).

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The Met One BAM-1020 particulate monitor is an US EPA designated FEM for PM10, PM2.5 389 and PM10-2.5. This instrument is currently used at most of the air quality monitoring stations  period from a few hours to 24-hour or few days. Therefore, the gravimetric approach is not 429 optimal for tracking high concentration events or detecting short-term concentration shifts. On found to be less than 10% when the particle collection surface (impactor plate) was cleaned 472 periodically reducing loading effect. Wedding beta gauge samplers running at flow rate 18.9 473 lpm measured regular PM10 concentrations within ± 10% of those of Wedding high-volume 474 samplers. In both gravimetric and beta gauge samplers, loading effect was found, and to prevent 475 it, it was recommended that the inlet be cleaned once a week in contaminated areas to keep the 476 error to less than 10%. indicates its D50 cutoff to be only 3.5 µm which is too small for it to be referred as PM10 sampler.

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Therefore, daily average PM10 concentrations measured by Kimoto 180 were much lower than 491 the measurements of Wedding beta gauge and SA 1200 high-volume sampler because the 492 cutoff size of its inlet was much smaller than 10 µm. The PM10 concentration measured by 493 Wedding beta gauge sampler was identified to be more precise due to a cutoff size close to 10 494 µm. Within experimental errors, water vapour content has no effect on the Wedding beta gauge 495 readings, but the fractional difference (difference between daily average PM10 concentrations 496 concentrations were far higher than real readings from the beta gauge monitor. This was due 513 to unbound water evaporation from the aerosols accumulated on the beta gauge monitor's filter 514 tape. In order to link theoretical hourly "wet" PM10 concentration with real measurements of 515 beta gauge monitor, a correction factor was applied to compensate for the evaporative losses.

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The average temperature and relative humidity of the filter weighing room should be held 551 between 20-23 °C ± 2 °C and 30-40% ± 5%, respectively, according to the US EPA's standard 552 operating procedure (2016 The effects of relative humidity on beta attenuation technique used for atmospheric 560 particulate monitoring have been discussed in many literatures. The reported effect is such that 561 wet droplets form when dry inorganic particles absorb water when relative humidity rises over 562 a set point, and they continue to absorb as relative humidity increases leading to high 563 concentration values. To resolve this problem, beta gauge monitors were upgraded with a 564 heater system with a controller that heated the air stream to a predetermined relative humidity 565 level. Since the RH of the beta gauge monitor was set below a particular point, the inlet heater 566 was supposed to remove the hygroscopic and condensation effect of particles on the filter 567 influencing the mass concentration readings. In some areas, the ambient RH is quite high, and 568 at high ambient RH, the inlet heater's increased temperature could not be enough to lower the 569 water content of particles in beta gauge. Therefore, it is critical to assess the beta gauge's 570 performance in these circumstances using a range of RH set points. This section includes and Tokiwa, 1981; McMurry, 1987, 1992). However, for the first time in this paper, 581 evaporation of particle-bound water during the sampling process was addressed and measured, 582 focusing on the fact that when RH is higher than deliquescence RH, water may be one of the 583 most abundant species in particles, and particle-bound water can evaporate during sampling 584 primarily due to pressure drop across the collection media. The simulated results of this study 585 show that when the relative humidity in Taiwan is less than 85%, all of the absorbed water 586 evaporates completely, but when the relative humidity is higher than this, complete evaporation 587 is not possible, resulting in higher beta gauge monitor readings. with water vapour so that it has higher RH than monitor 2. The results showed that PM10 594 concentration in both the monitors were nearly the same as long as RH remains below 80-85%.

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However, when RH surpasses this threshold, monitor 1's PM10 values are higher than monitor 596 2's, and this discrepancy grows as RH increases. Beta-gauge readings were simulated using where, p is the nearest day preceding day i for which a PM10 gravimetric value is available.

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f is nearest day following day i for which a PM10 gravimetric value is available.

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These correction procedures show good agreement at all the sites except at low particle 699 pollution leading to greater uncertainty for single daily values.  The calibration of beta gauge monitor is performed by using a film of known mass density 764 to determine beta-rays absorption efficiency but this attenuation coefficient is dependent on to compare attenuated and un-attenuated beta-rays count but the stability associated with long-810 term calibration cannot be assured. As a result, each sequence of measurements should include 811 specific calibration readings. To assess if any systematic calibration changes have occurred, 812 standard samples should be measured repeatedly. Additionally, it's crucial to adjust the inlet 813 heater system to the precise ambient RH ranges of the operational areas. As a result, for better 814 and comparable air quality data generation, the beta gauge system's instrumentation must be 815 modified along with the region-specific calibration method.