On the techniques and standards of particulate matter sampling

ABSTRACT

Air pollution and its limits are regulated by the environmental protection agency of an individual country according to their National Ambient Air Quality Standards (NAAQS). Particulate matter (e.g., TSP, PM₁₀, and PM_2.5) is one of the important criteria pollutants of NAAQS. Their measurement methods are specified in NAAQS, and detailed technical descriptions are given in standards. This review focuses on the sampling and analysis techniques and methods in the context of PM samplers’ design mentioned in countries specific PM measuring standards (e.g., EPA Part 50, CEN 12341, IS 5182(23), etc.) and their comparison wherever is necessary. It discusses, different designs of PM samplers mentioned in standards and its important components, e.g., size fractionators cutoff efficiency, PM sampler head design, flow measurement, and calibration, and also addresses the important issues that are the limitation of present standards. Our review reveals that most of the country-specific standards show common practice in measuring PM_2.5 using WINS impactor and VSCC cyclone as mentioned in EPA Part 50, except European Union (EU) standards, which has different design and parameters. For PM₁₀ measurement, sampler design is different in EU and Indian standards than that of U.S. EPA and other countries’ standards, which is discussed in length here. All standards lack in pointing some inherent problems like change in D₅₀ cutoff of size fractionator of sampler under a high particle mass loading condition, which is common in countries like China and India. Other important issues where most of the standards lack include PM head design and specification, a key component of PM sampler on which the mass measurement results are largely dependent.

Implications: The review paper discusses the air quality standards compliances of different countries and their comparisons. It focuses on the sampling and analysis techniques in context of PM samplers’ design mentioned in countries specific PM measuring standards, and also addresses the important issues that are not mentioned in standards. Therefore, the discussions and findings of the review may be very useful while revising the existing air quality standards of different countries and to fill the research gap in this domain. Further, we have discussed several technical issues described in standards related to PM sampling which may be very helpful for PM sampler designing or modification in current designs as per the prevailing ambient conditions of a country.

Introduction

Globally, the severity of particulate matter (PM) pollution is well known, and its control is a complicated task. Its sources are many (e.g., household, vehicular, industrial, biomass burning, etc.) and not limited to local but regional or intercontinental. Many countries such as the United States of America (USA), European Union (EU), Japan, China, India categorized it as criteria pollutants under their National Ambient Air Quality Standards (NAAQS) (Aggarwal et al. 2013; Chow et al. 2007; Gilliam and Hall 2016; Green et al. 2002). The categorization of PM mostly depends on the aerodynamic diameter size of particles (dp). It is divided into three or four subclasses, such as total suspended particulate matter (TSPM/TSP or SPM), respirable suspended particulate matter (RSPM), PM₁₀ (dp ≤ 10 µm) and PM_2.5 (dp ≤ 2.5 µm) in most of the NAAQS (Hinds 1999). This classification is mainly based on one or more reasons: (i) Direct impact on health, i.e., according to the sizes, particles penetrate in the human respiratory system and in a more general way, the smaller the size of PM, the deeper its penetration in the lungs. Thus, a higher fraction of PM_2.5 penetrates deep into the lungs than that of the PM₁₀ fraction (Heydar et al. 1986; Soderholm 1989). (ii) Ambient particle size distribution, such as PM_2.5, lies between accumulation and coarse mode (Seinfeld and Pandis 2016). (iii) Differences in the origination of PM, such as PM_2.5 emits mainly from burning of biomass, tailpipe, volcanic eruption, firecrackers, chemical industries (Huang et al. 2014; Liu et al. 2019; Wang et al. 2015; Zheng et al. 2005), while PM₁₀ emits from mechanical practices including resuspension of existing settled roadside dust (Amato et al. 2009; Jancsek-Turoczi et al. 2013; Sanchez-Soberon et al. 2015). Therefore, size segregated measurement of PM is an important task to understand their fundamentals (Poschl 2005), impacts (Ebi and McGregor 2008), sources (Bi et al. 2007; Chow et al. 1992), and also to control the PM pollution by implementing various action plans (Amann et al. 2020; Li, Jin, and Kan 2019). It is also helpful for weather forecasting (Fernando et al. 2012; Stadlober, Hörmann, and Pfeiler 2008) and in mapping and validating satellite data to the reference ground-based measurement of PM data (Hong et al. 2020; Lin et al. 2015).

The ground-level measurements of PM were carried out in one or two ways, i.e., nearly real-time (continuous) and offline (manual) methods. Several manufacturers exploit the principle of optical/laser scattering (Liu, Slaughter, and Larson 2002; Zhang, Marto, and Schwab 2018) in commercializing instruments for real-time measurements, such as optical particle counter and sizer (OPCS) (Bächler et al. 2020; Braniš et al. 2005; Peters, Ott, and O’shaughnessy 2006), condensation particle counter (Hermann et al. 2007), low-cost PM sensor (Kelly et al. 2017). The other types of PM monitors which require a physical size segregator (impactor or cyclone) at the inlet to classify PM class are Beta attenuation monitor (BAM) (Gobeli, Schloesser, and Pottberg 2008; Macias and Husar 1976) and frequency-based devices like tapered element oscillating monitors (TEOM) (Patashnick and Rupprecht 1991). Offline or manual measurement primarily consists of mechanical impactors and cyclones that first segregate the particle on the basis of aerodynamic diameter and later the mass of PM determined by weighing the filter before and after the sampling. For size segregation sampling, commonly used devices are Andersen cascade impactor sampler (Andersen 1958), PM_2.5 Very Sharp Cut Cyclone (VSCC) (Kenny et al. 2004), PM_2.5 Well Impactor Ninety-Six (WINS) (Peters et al. 2001a), etc. In contrast to real-time, the offline methods are more reliable in measurement as they are directly traceable to SI units of related observation, such as particle cutoff, mass, and airflow (Aggarwal et al. 2013). The measurement methods based on Optical/laser scattering, radiation absorption, change in frequency can be affected by particle physical property (Rosenberg et al. 2012) and meteorological parameters (Shukla and Aggarwal 2021). Therefore, most of the nation adopted the manual method as a primary or reference method to measure the particulate matter concentration and perform audit checks of real-time methods (also called secondary or equivalent methods). Further, this filter-based measurement also helps in carrying out chemical analysis for source apportionment study and many other fundamental studies.

Impactors and cyclones play a significant role in carrying out the size-segregated measurements of PM. These devices work on the principles of inertia and centrifugal force (Hinds 1999). For ambient PM monitoring, countries like the US and European Nations (EN)/Union (EU) successfully adopt cyclones- and impactors-based samplers (Cao et al. 2013). Many countries have adopted these methods as a reference at their NAAQS. In fact, the US and EU have even notified the reference designs of PM samplers in their country-specific standards such as 40 CFR part 50 of US Environment Protection Agency (EPA 1998) and EN 12341 (2014) of the EU, and in that they have mentioned the PM₁₀ and PM_2.5 designs and measurement procedures. It is common in fast-growing nations like China and India that their NAAQS and standards follow the same principle techniques for measuring PM. However, the problems are still with notified methods, which may be inherent in PM measuring techniques or vary from country to country as per prevailing ambient conditions. Since weather conditions vary from region to region, its effect can reduce measurement quality. Moreover, most countries have accepted either U.S. EPA or EN standards, which may or may not be applicable or suitable to their regions, where weather conditions are different. Therefore, this review discusses the major points affecting PM measurement, especially in terms of

1. Different setups to measure PMs, focused on size segregator (impactor and cyclone) designs and parameters to evaluate these designs (Figure 1),

   Figure 1. Particulate matter sampling, criteria parameter and measurement techniques.

   

2. Flow measurement and calibrations techniques,

3. Chemical parameters (dependent on PM measurement) notified in NAAQS.

The review starts with standards followed in the US, EU, Japan, and highly populated countries in the world, i.e. China and India. After that, a detailed review of abovesaid factors affecting PMs measurement is done and discussed here.

NAAQS adoption in the world

The US was the first who introduce the standards for TSP in 1971 in its NAAQS regulations (36 FR 8186). The measurement instruments mainly were high-volume samplers (HVS) in these regulations. The sampler consisted of a large filter of 8-inch x 10-inch size, a flow meter, and a vacuum pump that sucks particle-laden air through the filter at the flow rate between 40 and 60 cfm. The mass measurement of collected TSP is being carried out by the gravimetric method. The TSP sampler is defined as the ability to draw particles inside the sampler of size up to 100 µm and requires no particle segregation device (EPA 1971). After that, in place of TSP, the PM₁₀ was introduced in 1987 in US NAAQS (52 FR 24634). The basic setup is the same as earlier in TSP HVS, except it has a particle segregation device at the top, followed by the filter and pump unit. This particle segregation device allows only PM₁₀ to be collected on the filter (McFarland and Ortiz 1982). The U.S. EPA has defined the PM₁₀ sampler as a respirable particulate matter sampler, which should have a D₅₀ cutoff of 10 µm and particle sampling effectiveness curve according to U.S. EPA PM₁₀ ideal samplers (Figure 2). They followed the design based on the criteria of the US PM₁₀ convention (EPA 1999a) that correlates the health implication of size-selective PM to ambient aerosol sampling devices. Table 1 summarizes the vital parameter required to assess the size segregating instrument and final sampler.

Table 1. Important parameters and terminology associated with PM Samplers.

S. No.	Prameters	Relevant equation	Explanation of terms used	Discussion	References
1	Effectiveness curve	$\mathrm{E}\left(\mathrm{\%}\right)=\left\{\frac{Mas{s}_{afterimpactor}\left(D\right)}{Mas{s}_{beforeimpactor}\left(D\right)}\right\}\times 100$	Percentage of total airborne particles (plot between particle concentration and aerodynamic diameter).	Curve defined based on sampling convention for various particle size fractions used for assessing human health and its adoption to monitor the PM in ambient air.	ISO 7708 1995; EN 12341 2014
2	Cutoff size	$D_{50}={\left(\frac{{\displaystyle 9n\pi \mu w^{3}St{k}_{50}}}{{\displaystyle 4{\delta }_p{C}_{c}Q}}\right)}^{1/2}$	Aerodynamic diameter (D) size at a given percent (%) sampler effectiveness curve. Example: for PM2.5 WINS impactor, 2.5 µm sized aerodynamic diameter at 50% sampler effectiveness curve.	Theoretically calculated while designing the size-selective inlets based on stokes number and also experimentally validated by measuring particle size and mass (or number count), upstream and downstream to the size segregator.	Hinds 1999; Marple and Willeke 1976
3	GSD	$GSD=\sqrt{\frac{D_{15.9}}{D_{84.1}}}$	Dispersion or variability of particle size distribution data from the median (D50) of sampler effectiveness curve.	It is unitless, which measures the cutoff sharpness of the size segregating device. Usually, GSD < 1.2 is considered sharp.	Kulkarni, Baron, and Willeke 2011; Peters, Vanderpool, and Wiener 2001a
4	Field co-comparisons		To perform equivalency test of candidate PM sampler to reference sampler.	Statistical analysis for linearity (correlation coefficient, slope and intercept)	EPA, 1998; Noble et al. 2001

Later on, in 1997, the US also included the PM_2.5 in its NAAQS (62 FR 38652). In this standard, they have given reference design only for PM 2.5 sampling and don’t lay any basic criteria like their earlier standard for TSP and PM₁₀ sampler. This design is also commonly known as PM_2.5 Federal Reference Method (FRM) (EPA, 1999a). In this, the setup is based on low-volume sampling (LVS), i.e. 1 m³ h⁻¹ as compared to that of HVS (>1 m³ min⁻¹). The critical designs of the PM₁₀ inlet, PM_2.5 WINS impactor, and filter holder are specified. The setup requires a mandatory PM₁₀ segregation inlet followed by a PM_2.5 WINS impactor. The low-volume PM₁₀ inlet follows the sampler effectiveness curve of the ideal sampler (Figure 2), while PM_2.5 WINS impactor D₅₀ cutoff is set to be 2.5 ± 0.2 µm (Kenny et al. 2004). Along with this, the U.S. EPA has defined the calibration of the cutoff size of particle separators and a scheme for the designation of PM₁₀ and PM_2.5 samplers by conducting collocation field sampling at designated sites (EPA 2020; Noble et al. 2001).

On the other hand, the European Union issued its first directive in 1980 as “Air quality limit values and guide values for sulphur dioxide and suspended particulates” (Directive 1980). Subsequently, PM₁₀ and PM_2.5 as pollutants and their limits were released in the form of directives in 1996 (Directive 1996) and 2008 (Union 2008), respectively. Current standards for PM₁₀ and PM_2.5 have the flexibility to use both low- to high-volume samplers. The procedure for measurement, uncertainty calculation, and intercomparison of field samplers is defined well in (EN 12341 2014). Moreover, EU standards also provide a reference design for air inlet and particle separators for PM₁₀ and PM_2.5 at a flow rate of 2.3 m³ min⁻¹ (EN 12341 2014). The design of reference samplers is different from that of U.S. EPA. However, the D₅₀ cutoff size definition is the same. In comparison to U.S. EPA, the EU PM₁₀ does not follow the effectiveness curve of PM₁₀ ideal samplers of U.S. EPA.

In Japan, the Ministry of Environment (MoE) issues the environment quality standards. Their regulations came into effect in 1973 for SPM (Kawamoto et al. 2011) and for PM_2.5 in 2009 (Akimoto 2017). In contrast to other nations’ NAAQS, Japan is the only nation where PM₁₀ is not included. Japanese standards give the flexibility to use both high- to low-volume samplers for SPM sampling. It has defined basic criteria that SPM samplers must follow, i.e., the penetration efficiency curve for SPM should have a D100 cutoff point at 10 µm (in terms of D₅₀ cutoff equivalent to 7 µm). For PM_2.5 sampling, Japan has released its standards (JIS, 2008), which have the same reference design and setup as provided by U.S. EPA. However, it has also defined the Geometric Standards Deviation (GSD) or slope calculation for the penetration efficiency curve of size fractionator in PM_2.5 sampler as the square root of the ratio of D₈₀ to D₂₀ particle diameters. The GSD here is used to evaluate the particle size distribution of aerosol. In general, for the sampler size fractionator evaluation, the ratio of D_84.1 to D_15.9 (± 1σ size deviation from median D₅₀ cutoff size) particle diameters is in practice. The field co-comparison (i.e., field comparison including reference sampler and other samplers) of sampler for equivalency test is conducted periodically (JIS Z 2008).

In China, the first NAAQS norms (GB 3095–82) was issued in 1982, which included the TSP and PM₁₀ as particulate pollutants (Siddiq et al. 1984; Zhao et al. 2016). In 2012, PM_2.5 was included for the first time in the revised NAAQS (GB 3095-2012), which came into effect from 2016 onwards (Jin, Andersson, and Zhang 2016). The current standards that measure the ambient PM consist of two measuring methods, i.e., BAM and TEOM, which follow HJ 618 2011 as a reference, which further describes a gravimetric method for the determination of PM₁₀ and PM_2.5 mass concentration in ambient air. The standards are comprehensive but different from other nation PM measurement standards. For example, D₅₀ cutoff and GSD are defined for both PM₁₀ and PM_2.5 separators in HJ 618 2011. The limit for GSD is ~1.5 and <1.2 for PM₁₀ and PM_2.5 separators, respectively, whereas in U.S. EPA and EU standards for these PM classes, the GSD term is not given. It may be because the GSD parameters are dependent on the type of aerosol used. The monodisperse aerosol used for calibration could give lower GSD than polydisperse aerosol for the same PM separator (Du et al. 2020; Patel et al. 2021). Moreover, in ambient air, the aerosols are polydisperse, and normally, ambient air PM particle size distribution suggests GSD typically lies in between 1.3 and 1.5. With this type of polydispersity, it is challenging to achieve a GSD of <1.2 for the D₅₀ cutoff size of 2.5 µm. The PM₁₀ criteria, like in the U.S. EPA PM₁₀ ideal sampler, is not defined in NAAQS of China and neither mentioned to follow ISO 7708 (1995), which defines sampling based on penetration of particles and different particles size fractions inside the respiratory tract of humans (Figure 2).

In India, The NAAQS first came into effect in 1982, and the SPM was introduced (CPCB 2016). Then, it was revised in 1994, in which PM₁₀ was included, and a regulatory limit was defined for all criteria pollutants (CPCB 2012a). In 2009, PM_2.5 was incorporated as a criteria pollutant, whereas TSP was withdrawn from the NAAQS (CPCB 2009). PM measurement in India also follows some standards and technical documents developed by the Bureau of Indian Standards (BIS) and the Central Pollution Control Board (CPCB). CPCB (2009) gives the flexibility to use all three commonly used methods for PM monitoring purposes. CPCB also published the technical documents and practices for measuring the PM (CPCB 2012b). The measurement standards for PM₁₀ in the form of Indian standards (IS) came in 2006 (IS 2006), while for PM_2.5 in 2019 (IS 2019). In PM₁₀ standards, the reference design which is commonly followed in India is given. The design is discussed in the latter part of this review. The standard suggests using high-volume for PM₁₀ gravimetric monitoring. At the same time, for PM_2.5, it is mentioned to use the same reference design as given in U.S. EPA standard with PM₁₀ inlet and WINS impactor at a flow rate of 1 m³ min⁻¹. The basic setup is also the same compared to other nations’ measurement standards. However, these standards are not thorough and lack some basic information. For example, according to U.S. EPA in the US, D₅₀ cutoff and sampler designing parameters with minimum criteria are needed with QA/QC requirements, and thus given in standards, whereas in Indian standards, i.e. IS 5182 Part 24 (2019) (IS 2019) and CPCB guidelines lack this information. Regarding calibration of PM samplers, the size separators cutoff point calibration is missed and only flow calibration is mentioned. Other parameters such as aspiration efficiency/transport efficiency of PM inlet is also missing.

It is essential to understand the basic structure/setup laid by different nations’ standards and their technical aspects and limitations under certain conditions. Figure 1 describes the basic format of PM measurement and sampling techniques as per NAAQS that is commonly followed in different nations. First, the basic setup is selected based on sampling flow rate, then tagged as LVS or HVS. Most of the PM_2.5 sampling is carried out at a low flow rate, while for TSP, SPM, and PM₁₀ sampling flow, much flexibility is given, where LVS, HVS, or MVS (mid-volume sampler) can be used. The basic arrangement is as follows, first the aerosol inlet (PM head) is positioned from where the aerosol is drawn inside the sampler. After that, the inline particle separator segregates the PM into class, i.e. SPM, PM₁₀, and PM_2.5. These particle separators are generally called impactors or cyclone separators. These separators allow only the lower fraction of the PM class to move to the filter where it gets deposited or collected. These filters are commonly made up of glass fiber, polytetrafluoroethylene (PTFE), quartz fiber, cellulose ester, etc. The type of filters used varies based on the purpose of sampling.

Figure 2. ISO 7708 and U.S. EPA PM₁₀ ideal effectiveness curve.

PM sampler components

PM head

Sampled particle-laden air enters the sampler through the PM head. Hence, the design of the PM head should be fabricated in such a way as to fulfill the objective that the drawn air inside the sampler is representative of particle concentration (number and size) present in ambient air. Usually, it is a circular annulus in shape to sample particles from all directions uniformly. In most countries, louvered PM inlet design given by U.S. EPA is in practice for low-volume samplers, Figure 3 (Kenny et al. 2005). Another notable PM head is the EU reference design, Figure 4 (EN 12341 2014), which is simple in design and smaller than the U.S. EPA reference at low flow (1 m³ h⁻¹). The circumferential inlet diameter of the common high-volume sampler is large such as EPA designated PM₁₀ sampler (EPA 2017) (diameter = 700 mm), Figure 5. Contrary to this design, the Indian reference design as mentioned in BIS (IS 2006) and CPCB technical manual is comparatively very small (CPCB 2012b) (diameter = 185 mm). And, even smaller than the low-volume U.S. EPA reference louvered inlet (diameter = 246.4 mm), Figure 3.

Figure 3. U.S. EPA Louvered TSPM inlet.

Figure 4. PM₁₀ or PM_2.5 low-volume sampler (European Union).

Figure 5. PM₁₀ high-volume sampler (U.S. EPA and European Union).

Another notable PM inlet, different from all above, is TSPM/SPM (HVS) inlet. Its role as a sampler is currently limited because PM air quality standards shift from TSPM to PM₁₀. However, it is still in use for sampling airborne particles for chemical constituent determination (in particle phase), such as Pb (lead) in air, as mentioned in EPA (2008). Figure 6 shows a typical louvered PM inlet used in TSPM/TSP/SPM or high-volume sampler. The gabled roof or triangular prism-shaped design of the inlet serves two purposes, (i) act as shelter, to protect the filter from direct exposure to external environmental factors such as wind, sunlight, rain, etc., and (ii) to collect the particle uniformly on the filter. However, some issue always exists with designs. In this case, there is a lack of exact design dimension for the gabled roof in air quality standards, which is an issue and vague definition of TSPM (HVS) inlet as pointed out in EPA (1998) and Krug et al. (2017). It says as “(i) The sampler flow rate and the geometry of the shelter favor the collection of particles up to 25–50 µm (aerodynamic diameter), depending on wind speed and direction. (ii) The sampler cover or roof shall overhang the sampler housing somewhat and shall be mounted so as to form an air inlet gap between the cover and the sampler housing walls. This sample air inlet gap should be approximately uniform on all sides of the sampler. The area of the sample air inlet gap must be sized to provide an effective particle capture air velocity of between 20 and 35 cm s⁻¹ at the recommended operational flow rate. The capture velocity is the sample air flow rate divided by the inlet area measured in a horizontal plane at the lower edge of the cover. Ideally, the inlet area and operational flow rate should be selected to obtain a capture air velocity of 25 ± 2 cm s⁻¹.”

PM head design is quantified basically by two important characteristics. The first is the maximum particle size diameter drawn inside the sampler, and the second is its aspiration efficiency. Louvered PM inlet and PM₁₀ size-selective inlet design can intake particles of a maximum of 100 µm at intake velocity of 25 cm s⁻¹ in calm air (Kenny et al. 2005). The aspiration efficiency is defined as the mass (or number count) of the particle of a given size in ambient air to the mass of the particle (or number count) of that size inside the sampler’s head. In short, the ratio of these should be close to 1 for a particle size of interest (Liu and Pui 1981). It should be noted that after PM head, a size fractionator in the form of impactor or cyclone is present, which can be affected by irregularity in aspiration efficiency of sampler head and consequently the final mass determination of given air samples.

Several studies conducted on louvered PM inlet in Figure 3 show that in the calm wind where the ambient air velocity is lower than PM inlet intake velocity, the aspiration efficiency for bigger size fraction (>10 µm) drops to 80–90% (Lai and Chen 2000). While, when the ambient air velocity in the windy condition is higher than sample intake velocity, the aspiration efficiency for 10 µm increases to 120% and decreases gradually with bigger particle size in louvered TSP inlet (Vanderpool et al. 2018). The US standard mentioned the requirement of aspiration efficiency of 97% for PM head (40 CFR part E:53.59). Other countries’ standards or guidelines for PM heads lack any such criteria. Many studies on TSP (HVS) gabled roof inlets also show irregularity in particle intake. Studies conducted by Wedding, McFarland, and Cermak (1977) and Krug et al. (2017) explored various factors (wind speed, sampler orientation with respect to wind direction, active or passive sampling state) that can affect TSP (HVS) size-selective performance. Their findings indicate that overall effectiveness decreases as particle size increases and depends on wind direction.

Size fractionators

The core device part of any PM sampler is impactor or cyclone, or both, which primarily governs PM sampler type and its measurement accuracy. Impactors or cyclones are mechanical devices that fractionate the incoming particulate matter in different sizes based on particle aerodynamic diameter. The cutoff size of the impactor or cyclone decides the PM sampler types, and it is generally classified as PM₁₀ or PM_2.5 sampler, where the D₅₀ cutoff (Table 1) is 10 µm or 2.5 µm aerodynamic diameter of particles, respectively (Hinds 1999; Marple and Willeke 1976). The D₅₀ cutoff is relative to the flow rate and can be changed by varying the flow rate of the PM sampler (Wedding et al. 1987). Therefore, the sampler used for size-segregated PM sampling has a fixed and constant sample flow rate. Besides the D₅₀ cutoff, the collection/penetration efficiency curve is defined in terms of GSD (Table 1) (Peters et al. 2001b) and is also an important parameter on which particle mass measurement is dependent. The lower is the GSD, the sharper is the cutoff of impactor and cyclone and thus, the accuracy is better in the mass measurement of that size segregated sampling.

In ambient air monitoring, especially for SPM, PM₁₀, and PM_2.5, the collection efficiency curve is defined in ISO (ISO 7708 (1995)) standards. The standard is linked with inhalable particle deposition in the respiratory tract. The deposition of particles depends on particle physical properties, especially their size. ISO 7708 (1995) acts as a bridge between particle size and its collection efficiency in the human respiratory tract to particle measurement sampler/device. Therefore, PM fraction is classified into 3 (sometimes 4) types based on particle penetration into human respiratory system studies. It is classified into highly respirable (PM_2.5), respirable (PM₄), thoracic (PM₁₀), and inhalable, as shown in Figure 2. In standard, besides D₅₀ cutoff, the shape, bigger and smaller particle diameter, and corresponding collection efficiency are also important, which sampler designer needs to consider. However, it should be noted that only a few countries have defined the curve according to ISO standards in their NAAQS standards. The U.S. EPA has different conventions for PM₁₀, which differ from ISO 7708 (1995). The U.S. EPA theoretical penetration efficiency curves for ideal PM₁₀ samplers are shown in Figure 2. Their collection efficiency curve is narrow and reaches up to 15–16 µm at lower effectiveness as compared to ISO 7708 (1995) (thoracic convention). The louvered PM₁₀ (LVS) inlet certified by the U.S. EPA follows the ideal sampler collection efficiency curve (Figure 2). The European Union EN 12341 followed the ISO curves for the PM₁₀ sampler and explained the design for the PM₁₀ sampler, which is different from the reference design given in U.S. EPA. For PM_2.5, the sampler collection efficiency defined in other countries’ standards is the same as the ISO 7708 (1995) highly risk respirable sampling convention. The major designs of different PM size fractionators are reviewed below.

PM₁₀ size fractionator

Louvered PM₁₀ inlet (LVS)

This type of design is commonly used in most countries for PM₁₀ sampling. The design primarily comes in class 1 equivalent and a U.S. EPA reference design for PM₁₀ sampling, Figure 7. The outlet of PM₁₀ can be coupled with U.S. EPA reference PM_2.5 WINS type impactor (Figure 8) or VSCC cyclone (Figure 9). The PM₁₀ impactor is connected down to a louvered PM head outlet, Figure 3. The design of the PM₁₀ impactor consists of one accelerating tube which carries particle-laden air. The bigger size particle fraction because of impaction is deposited on the surface below the accelerating tube, while the smaller size particle remains in air trajectory. The smaller size particle lifts and moves out of the three long tubes, extending upward from the impaction surface to the accelerating tube.

The reference design of the PM₁₀ impactor is mentioned in EPA (1998), Figure 7. It meets two essential criteria set in 40 CFR part 53.40, i.e. D₅₀ cutoff of 10 ± 0.5 µm, and the sampler effectiveness curve should be closed to the ideal sampler or follow PM₁₀ U.S. EPA convention (EPA 1998). The GSD is not given as it follows the ideal sampler effectiveness curve. The limit of correct mass measurement on PTFE filters is 200 µg m⁻³. The field intercomparison of PM₁₀ with other collocating real-time and manual measuring instruments such as optical scattering-based instruments, high-volume PM₁₀ sampler, etc. shows a good linear relationship (CEN 2006; Guo et al. 2009; Rodes et al. 1985; Walden et al. 2017). Many field comparisons under different weather conditions and particle concentration in ambient air have been conducted in the past. Some of the unique studies are listed in Table 2. McFarland and Ortiz (1984) and Tolocka et al. (2001) conducted detailed studies of this inlet as per the testing protocol of U.S. EPA at different wind speeds with different monodisperse liquid aerosols as test aerosols. Other researchers (Chen and Shaw 2007; Faulkner, Smith, and Haglund 2014; Lai and Chen 2000) have conducted similar studies and found that the D₅₀ cutoff point lies in the range of 9.5–10.5 µm (Figure 10). The sampler effectiveness curve is close to the U.S. EPA PM₁₀ ideal sampler.

Table 2. Field co-comparisons of various PM samplers.

S. No.	Instrument (X)	Classification method	Flow rate (m^3 h^−1)	Instrument(Y)	Classification method	Flow rate (m^3 h^−1)	No. of Samples (N)	R2	Slope	Intercept	Concentration (min-max) µg m^−3	RH (%)	T (°C)	References
1	BGI PQ 200 (EPA- FRM)	WINS-PM2.5	1	TSI-3320 Aerodynamic Particle Sizer	Time of Flight (TOF)	0.3	16	0.592	0.83	−2.21	5–20.4			Yanosky, Williams, and MacIntosh 2002
2	BGI PQ 200 (EPA- FRM)	WINS-PM2.5	1	TSI-8520 Dusttrack	Optical Scattering	0.1	17	0.859	2.57	−1.73				Yanosky, Williams, and MacIntosh 2002
3	Thermo Partisol 2000 (EPA-FRM)	WINS-PM2.5	1	Thermofischer SHARP 5030	PM2.5 VSCC + Light Scattering	1	207	0.88	0.92	1.2	3–41			Su et al. 2018
4	Thermo Partisol 2025 (EPA-FEM)	WINS-PM2.5	1	Thermofischer SHARP 5030	PM2.5 VSCC + Light Scattering	1	1171	0.84	0.94	1.4	3–48			Su et al. 2018
5	Metone SASS or Partisol 2300 (EPA-FEM)		1	Thermofischer SHARP 5030	PM2.5 VSCC + Light Scattering	1	1298	0.82	0.88	0.92	3–38			Su et al. 2018
6	Metone 1020 BAM (EPA-FEM)	WINS-PM2.5	1	Reference annular denuder system	PM2.5 Cyclone (URG 2000–30EH)	1	153	0.9	1.23	−3.25	1.3–260.2	29–96	8.1–35	Young et al. 2016
7	Metone 1020 BAM (EPA-FEM)		1	Reference annular denuder system	PM10 Inlet (URG 2000–30DBQ)	1	144	0.91	1.11	0.2	5.2–284.9	29–96	8.1–35	Young et al. 2016
8	PM2.5 Sampler	WINS-PM2.5	1	PM2.5 Sampler	PM2.5 Cyclone (URG-2000-30EH)	1	151	0.92	1.22	5.64	2–100			Ghim et al. 2017
9	PM2.5 Sampler	WINS-PM2.5	1	OPC Grimm 1.109	Optical Scattering	0.07	219	0.88	0.95	6.79	2–100			Ghim et al. 2017
10	RAAS 2.5–300 Sequential Sampler (EPA-FRM)	WINS-PM2.5	1	RAAS 2.5–300 Sequential Sampler (EPA-FRM)	WINS-PM2.5	1		0.98	1	0.83	5–130	21–99	−0.7–22	Chung et al. 2001
11	Sierra Anderson 1200 -HI-VOL SSI (EPA-FRM)	Size selective inlet (PM10)	67.8	Sierra Anderson 1200 -HI-VOL SSI (EPA-FRM)	Size selective inlet (PM10)	67.8		1	1	0.01	10–150	21–99	−0.7–22	Chung et al. 2001
12	Metone 1020 BAM (EPA-FEM)	WINS-PM2.5	1	Metone 1020 BAM (EPA-FEM)	PM2.5-SCC (Sharp Cut Cyclone)	1		1.06	0.99	2.92	5–130	21–99	−0.7–22	Chung et al. 2001
13	RAAS 2.5–300 Sequential Sampler (EPA-FRM)	WINS-PM2.5	1	Metone 1020 BAM (EPA-FEM)	WINS or VSCC -PM2.5	1		0.99	0.95	1.36	5–130	21–99	−0.7–22	Chung et al. 2001
14	Sierra Anderson 1200 -HI-VOL SSI (EPA-FRM)	Size selective inlet (PM10)	67.8	Rupprecht and Patashnick -TEOM-1400(EPA-FEM)	Size selective inlet (PM10)	1		0.95	0.37	6.63	10–150	21–99	−0.7–22	Chung et al. 2001
15	Sierra Anderson 1200 -HI-VOL SSI (EPA-FRM)	Size selective inlet (PM10)	67.8	Metone 1020 BAM (EPA-FEM)	Size selective inlet (PM10)	1		0.99	1.01	1.55	10–150	21–99	−0.7–22	Chung et al. 2001
16	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	BGI PQ −200 (EPA-FRM)	M-WINS(Oil wetted)	1	76	0.99	1	0.12	0.65–42.2	77.5 ± 4.9	24.8 ± 2.8	Le et al. 2020a
17	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	BGI PQ −200 (EPA-FRM)	M-WINS(Oil wetted)	1	30	0.99	0.99	0.02	6.9–29.5	72.2 ± 8.6	20.1 ± 2.1	Le et al. 2020a
18	Reference samplers	WINS-PM2.5	1	Reference samplers	VSCC-PM2.5	1	33	0.999	1.01	0.1	2.5–41		−16.5–12.5	Vanderpool et al. 2007
19	Thermo Partisol 2000, BGI PQ-200 (EPA-FRM)	WINS-PM2.5	1	Metone 1020 BAM (EPA-FEM)	FEM	1	265	0.984	1.06	0.6	3.6–69.1	54.9–96.1	11.5–31.4	Le et al. 2020b
20	Thermo Partisol 2000, BGI PQ-200 (EPA-FRM)	WINS-PM2.5	1	TEOM-FDMS (Model 1405-DF) (EPA-FDMS)	FEM	1	265	0.982	1.03	1.62	3.6–69.2	54.9–96.1	11.5–31.5	Le et al. 2020b
21	Graseby Andersen SA −241 (EPA-FRM)	WINS-PM2.5	1	Graseby Andersen SA −241 (EPA-FRM)	VSCC-PM2.5	1		0.997	1.03	−0.97	7.9–34.8			Le at al. 2017
22	Graseby Andersen SA −241 (EPA-FRM)	WINS-PM2.5	1	TEOM-FDMS (Model 1405-DF) (EPA-FDMS)		1		0.991	1.05	0.04	7.9–34.8			Le and Tsai 2017
23	BGI PQ −200 (EPA-FRM)	WINS-PM2.5	1	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	11	0.9993	1.013	0.17				Kenny et al. 2004
24	BGI PQ −200 (EPA-FRM)	WINS-PM2.5	1	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	13	0.9991	1.005	0.23				Kenny et al. 2004
25	BGI PQ −200 (EPA-FRM)	WINS-PM2.5	1	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	15	0.97	1.04	0.859				Kenny et al. 2004
26	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	Metone 1020 BAM (EPA-FEM)	VSCC-PM2.5	1	30	0.9943	0.9404	−0.9552	~4-45			Gobeli, Schloesser, and Pottberg 2008
27	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	Metone 1020 BAM (EPA-FEM)	VSCC-PM2.5	1	42	0.9801	0.9598	−0.8627	~3-46			Gobeli, Schloesser, and Pottberg 2008
28	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	Metone 1020 BAM (EPA-FEM)	VSCC-PM2.5	1	50	0.9916	0.962	−0.761	~5-100			Gobeli, Schloesser, and Pottberg 2008
29	BGI PQ −200 (EPA-FRM)	VSCC-PM2.5	1	Metone 1020 BAM (EPA-FEM)	VSCC-PM2.5	1	29	0.9977	1.02	0.585	~3-52			Gobeli, Schloesser, and Pottberg 2008
30	Sven/Leckel LVS3	Reference design-PM10 EN-12341(2014)	2.3	Turnkey OSIRIS Particle Monitor (Model 2315)	Optical scattering	0.036	60	0.87	0.875	−0.968	~8.8–75			Tasić et al. 2012
31	Sven/Leckel LVS4	Reference design-PM2.5 EN-12341(2014)	2.3	Turnkey OSIRIS Particle Monitor (Model 2315)	Optical scattering	0.036	60	0.71	0.367	2.72	~8.2–65			Tasić et al. 2012
32	Leckel SEQ47/50	Reference design-PM10 EN-12341(2014)	2.3	Metone 1020 BAM	EPA Reference Inlet (PM10)	1	211	0.993	0.942	0.437	~0-290			Waldén et al. 2017
33	Leckel SEQ47/50	Reference design-PM10 EN-12341(2014)	2.3	Thermo Fisher FH −62	Reference design-PM10 EN-12341(2014)	1	195	0.99	1.3	−0.904	~0-290			Waldén et al. 2017
34	Leckel SEQ47/50	Reference design-PM10 EN-12341(2014)	2.3	Grimm EDM-180	Light Scattering	0.07	179	0.946	0.855	2.139	~2-100			Waldén et al. 2017
35	Leckel SEQ47/50	Reference design-PM10 EN-12341(2014)	2.3	Environnement s.a MP101		1	201	0.995	0.811	2.311	~0-290			Waldén et al. 2017
36	Leckel SEQ47/50	Reference design-PM10 EN-12341(2014)	2.3	OSIRIS PM10	Nephlometer	0.036	284	0.988	1.401	−0.153	~0-290			Waldén et al. 2017
37	Leckel SEQ47/50	Reference design-PM10 EN-12341(2014)	2.3	Thermofischer SHARP 5030	Reference design-PM10 EN-12341(2014)	1	203	0.993	1.415	−2.233	~0-290			Waldén et al. 2017
38	Leckel SEQ47/50	Reference design-PM10 EN-12341(2014)	2.3	Thermofischer TEOM 1405	Reference design-PM10 EN-12341(2014)	1	194	0.992	0.868	−2.068	~0-290			Waldén et al. 2017
39	Leckel SEQ47/50	Reference design-PM2.5 EN-12341(2014)	2.3	Metone 1020 BAM	VSCC-PM2.5	1	195	0.832	1.1	0.733	~2-15			Waldén et al. 2017
40	Leckel SEQ47/50	Reference design-PM2.5 EN-12341(2014)	2.3	Thermo Fisher FH −62	Reference design-PM2.5 EN-12341(2014)	1	195	0.912	0.85	1.709	~2-15			Waldén et al. 2017
41	Leckel SEQ47/50	Reference design-PM2.5 EN-12341(2014)	2.3	Grimm EDM-180	Light Scattering	0.07	173	0.907	0.747	0.532	0–45			Waldén et al. 2017
42	Leckel SEQ47/50	Reference design-PM2.5 EN-12341(2014)	2.3	Environnement s.a MP101	Beta guage +optical	1	196	0.831	0.812	−0.306	~2-15			Waldén et al. 2017
43	Leckel SEQ47/50	Reference design-PM2.5 EN-12341(2014)	2.3	Turnkey OSIRIS Particle Monitor	Optical (Nephlometer)	0.036	195	0.383	3.324	−1.073	~2-15			Waldén et al. 2017
44	Leckel SEQ47/50	Reference design-PM2.5 EN-12341(2014)	2.3	Thermofischer SHARP 5030	Reference design-PM2.5 EN-12341(2014)	1	196	0.929	0.971	−0.003	~2-15			Waldén et al. 2017
45	BGI PQ 200 (EPA- FRM)	Reference design-PM2.5 EN-12341(2014)	2.3	Thermofischer TEOM 1405	VSCC-PM2.5	1	196	0.794	1.009	−1.681	~2-15			Waldén et al. 2017
46	BGI PQ 200 (EPA- FRM)	Reference design-PM2.5 EN-12341(2014)	2.3	Digitel DHA-80 HVS sampler	Reference design-PM2.5 EN-12341(2014)	30	525	0.946	0.962	0.47	1–115		−11-31	CEN 2006
47	Thermo Partisol 2000 (EPA-FRM)	Reference design-PM2.5 EN-12341(2014)	2.3	Opsis SM 200 ß-ray attenuation monitor	Reference design-PM2.5 EN-12341(2014)	1	426	0.795	0.942	1.83	1–115		−11-31	CEN 2006
48	Thermo Partisol 2025 (EPA-FEM)	Reference design-PM2.5 EN-12341(2014)	2.3	Thermo Fisher FH −62	PM2.5- SCC	1	585	0.66	0.858	2	1–115		−11-31	CEN 2006
49	Metone SASS or Partisol 2300 (EPA-FEM)	Reference design-PM2.5 EN-12341(2014)	2.3	Metone 1020 BAM (EPA-FEM)	Reference design-PM2.5 EN-12341(2014)	1	460	0.909	0.967	1.01	1–115		−11-31	CEN 2006
50	Metone 1020 BAM (EPA-FEM)	Reference design-PM2.5 EN-12341(2014)	2.3	Rupprecht and Patashnick -TEOM	EPA Reference Inlet (PM10)	1	538	0.813	0.742	3.56	1–115		−11-31	CEN 2006
51	Metone 1020 BAM (EPA-FEM)	Reference design-PM2.5 EN-12341(2014)	30	Thermo fisher 1400a EPA-FEM	SCC PM2.5 Inlet (R&P)	1	311	0.85	0.78	1.57	7.9–29.3			Hauck et al. 2004
52	PM2.5 Sampler	Reference design-PM2.5 EN-12341(2014)	30	Thermo Fisher FH −62	WINS-PM2.5	1	303	0.85	0.7	−0.12	6–22.6			Hauck et al. 2004
53	PM2.5 Sampler	Reference design-PM2.5 EN-12341(2014)	30	Rupprecht and Patashnick -TEOM-1400ab (EPA-FEM)	SCC PM2.5 Inlet (R&P)	1	330	0.85	0.78	2.69	8.6–35.4			Hauck et al. 2004
54	RAAS 2.5–300 Sequential Sampler (EPA-FRM)	Reference design-PM2.5 EN-12341(2014)	30	Thermo Fisher FH −62	WINS-PM2.5	1	307	0.91	0.83	−0.21	8.6–35.4			Hauck et al. 2004
55	Rupprecht and Patashnick -TEOM 1400ab (EPA-FEM)	WINS-PM2.5	1	Greasby-Anderson RAAS 2.5–100	WINS-PM2.5	1	28	0.9489	0.93	1.6	6.8–49.6			Williams et al. 2000
56	Metone 1020 BAM (EPA-FEM)	Cyclone-PM2.5	0.6	Greasby-Anderson RAAS 2.5–100	WINS-PM2.5	1	28	0.9626	1.03	0.1	6.8–49.6			Williams et al. 2000
57	RAAS 2.5–300 Sequential Sampler (EPA-FRM)	PM10 (Dichotomous)	67.8	Sierra anderson −246 B	FRM-PM10	1	18	0.92	0.96	1.36				Rodes et al. 1985
58	Sierra Anderson 1200 -HI-VOL SSI (EPA-FRM)		67.8	Sierra Anderson- 321A	Size selective inlet (PM10)	67.8		0.997	1.15	−1.79	30–140			Purdue et al. 1986
59	Sierra Anderson 1200 -HI-VOL SSI (EPA-FRM)	Size selective inlet (PM10)	67.8	Thermofisher HV PM10 sampler (1130 lpm)	Size selective inlet (PM10)	67.8	60	0.97	0.98		7.4–355			Guo et al. 2009
60	BGI PQ −200 (EPA-FRM)	EPA Reference Inlet (PM10)	1	Thermo fisher 1400a (16.67 LPM)	EPA Reference Inlet (PM10)	1	28	0.97	1		3.9–181			Guo et al. 2009
61	BGI PQ −200 (EPA-FRM)	Size selective inlet (PM10)	67.8	Thermo fisher 1400a (16.67 LPM)	EPA Reference Inlet (PM10)	1	208	0.94	1.15		10–505			Guo et al. 2009
62	Reference samplers	Size selective inlet (PM10)	67.8	Energetica HV PM10 sampler	Size selective inlet (PM10)	67.8	15	0.998	0.997	0.133	10.73–54.04			Rajoy et al. 2015

Very little research has been conducted on bouncing problems of bigger size particle fraction on impaction, which is a common problem in any impaction type particle size classifier (John, Winklmayr, and Wang 1991). The bouncing problem is real in an ambient field condition, where the aerosol is polydisperse and consists of both solid and liquid type aerosol. Le et al. (2019) conducted research on coating and new mesh-type material, to resolve the bouncing issue and reduce the need for regular cleaning or maintenance of the impactor. The finding suggests that the bouncing problem can be minimized using the grease/oil coating of the impaction surface up to a certain limit. The reference design of this inlet is also mentioned in other countries’ NAAQS for PM₁₀ sampling and monitoring purposes (HJ 618 2011; IS 2019; JIS Z 2008).

PM₁₀ size selective inlet (HVS)

As per U.S. EPA, a high-volume sampler (HVS) should have PM sampling flow rate greater than 0.5 m³ min⁻¹ (EPA 1999a). This PM₁₀ HVS designed at a flow rate of 1.13 m³ min⁻¹ (40 cfm). The size of the filter on which PM₁₀ gets collected is 8-inch × 10-inch. The design consists of a tube-like nozzle impactor as given in EN 12341 (2014) and reproduced in Figure 5. The particles laden air entered the PM head and distributed to nine-tube inlet facing downwards to the impaction plate, and then the PM₁₀ fraction is carried by 16 tubes facing upward opposite to the impaction plate. The D₅₀ cutoff and sampler effectiveness curve match the ideal sampler criteria defined by U.S. EPA. An experimental D₅₀ cutoff (Ranade et al. 1990) comparison relative to other PM₁₀ samplers is shown in Figure 10. Several manufacturers and researchers have recommended the oiling of the impaction surface to minimize the bouncing problems. This kind of PM₁₀ HVS was thoroughly studied in the early 70s to date. The linearity with other PM₁₀ samplers is good during field comparisons (Chung et al. 2001; Guo et al. 2009; Purdue et al. 1986; Rajoy et al. 2015; Rodes et al. 1985) as summarized in Table 2.

PM₁₀ cyclone (HVS)

In the early 1990s, PM₁₀ high-volume cyclonic sampler designed in India was introduced (NEERI 1992), as shown in Figure 11. The technical specification of the HVS is similar to US PM₁₀ HVS. The sample flow rate of 1 −1.13 m³ min⁻¹ and the accommodated filter size is 8-inch × 10-inch size  to collect PM₁₀. The sampler design consists of a single inlet and outlet cyclone. The particle-laden air enters through PM head to cyclones by an L type bend tube which takes the shape of rectangular cyclone inlet from circular PM head inlet tube, Figure 11. After segregation by the cyclone, the smaller particle lifts and moves out of the cyclone’s circular outlet to the filter. The cyclone outlet and filter are designed in a way that the top cover of the PM sampler needs to be closed to draw sampled PM from the cyclone outlet to the filter. The arrangement of such a PM₁₀ sampler is prescribed in the CPCB technical manual to monitor PM₁₀ under NAAQS and also in Indian standards published by BIS (CPCB 2012b; IS 2006).

It is widely used in India and, used by many industries and research organizations for different purposes, such as source apportionment studies (Bhuyan et al. 2018; Gupta et al. 2018; Jain et al. 2020; Karar and Gupta 2007). The QA and QC, D₅₀ cutoff check, sampler effectiveness curve, field measurement, and comparison with other equivalent samplers are not available in the public domain or peer-reviewed journals so far, and hence open to further evaluating the efficacy of this PM₁₀ cyclone HVS design. Moreover, even if the cyclonic separation is defined, the PM head is very small (diameter = 185 mm) compared to that of U.S. EPA designated PM₁₀ HVS (diameter = 700 mm), while the flow rate is the same. It results in a higher suction velocity (sample velocity) for drawing particle-laden air inside the PM head in PM₁₀ cyclone (India) than PM₁₀ U.S. EPA. Hence, the sampler draws more particles that follow air trajectory (sample flow) inside the sampler, and it results in oversampling of smaller-sized particles and subsequently undersampling of comparatively bigger-sized particles that do not follow air trajectory (because of inertia) (Kulkarni, Baron, and Willeke 2011). In other words, the ratio of fine and coarse particles in sample air does not remain representative of ambient air. Therefore, the PM₁₀ head of the cyclone lacks factors that need to be taken care of while designing the PM inlet. These designing parameters affect the aspiration efficiency of a sampler, such as the size and geometry of the inlet, the velocity of sample flow, etc.

PM₁₀ reference impactor (EN 12341)

EN 12341 acts as the base standard for NAAQS in European Union. The standard mentioned a wide range of PM₁₀ measurement reference design of size-selective fractionators at different flow rates (1 m³ h⁻¹, 2.3 m³ h⁻¹, 30 m³ h⁻¹, and 68 m³ h⁻¹), which cover all three classes of sampler (LVS, MVS, and HVS) (EN 12341 2014). The design of PM₁₀ HVS is similar to the U.S. EPA PM₁₀ sampler, as discussed earlier. The LVS (1 m³ h⁻¹ and 2.3 m³ h⁻¹) is widely used all over Europe. The design of PM₁₀ LVS is different from the U.S. EPA reference design. The fractionator is an impactor that consists of an ambient particle inlet, eight long accelerating tubes, and an impaction plate at the bottom of accelerating tubes, Figure 4. It together forms the PM₁₀ fractionator. The sampler effectiveness curve is aligned to ISO 7708. These curves were broader (EN 12341 2014) (Figure 10) and different from U.S. EPA PM₁₀ sampler effectiveness curves (Tolocka et al. 2001).

Figure 6. TSP high-volume louvered inlet and its dimension in commercially available models.

Figure 7. U.S. EPA low-volume PM₁₀ impactor (reference design).

Very few studies were conducted simultaneously on both EU and U.S. EPA PM₁₀ field sampling (Walden et al., 2017). (Tasić et al. 2012) performed a field comparison with an optical scattering-based dust monitor. The trend line of co-comparison sampling in ambient PM₁₀, which ranged from 8.8 to 75 µg m⁻³, is linear between samplers. However, some reports suggest differences in mass measurement as each sampler has a different design and follows a different sampler effectiveness curve (EPA., 2012; Helmink et al. 2011). Moreover, studies related to its size segregation capabilities (D₅₀ cutoff size) under a high mass loading environment and in different wind speed conditions are limited. Also, as discussed above, none of the standards includes the information and criteria of sampler applicability in a given range of wind velocity, which is an essential factor to maintain aspiration efficiency > 97% and consistency of a PM head performance.

PM_2.5 size fractionators

WINS impactor

This is the most commonly used PM_2.5 size-selective fractionator. U.S. EPA introduced the reference design in 1996 and called it, Well Impactor Ninety-Six (WINS). The design is simple, rigorously tested, and straightforward (reproduced as shown in Figure 8). The inlet is configured to accommodate a PM₁₀ pre-separator to allow only particles smaller than PM₁₀ to be drawn inside the accelerating nozzle of the WINS impactor. The impaction plate is circular well-shaped, and contains an oil-soaked 37 mm filter to trap bigger fraction particles (Vanderpool et al. 2007) and prevent particle bouncing and re-entrainment to some extent (EPA 1998). Vanderpool et al. (2001a), Vanderpool et al. (2001b), Peters, Vanderpool, and Wiener (2001a) and Peters et al. (2001b) have extensively studied the design and calibration of WINS impactor (Figure 10).

Figure 8. U.S. EPA low-volume PM_2.5 WINS impactor (reference design).

Figure 9. U.S. EPA low-volume PM_2.5 VSCC cyclone.

The D₅₀ cutoff size as notified in the U.S. EPA is 2.5 ± 0.2 µm (Kenny et al. 2004). This uncertainty in measuring D₅₀ is dependent mainly on WINS impactor development and flow rate fluctuation during measurement. The monodisperse aerosol and reference PSL test aerosol were used to estimate the GSD from the cutoff curves. The GSD estimates were less than 1.2, which is considered the sharp cut impactor category (Vanderpool et al. 2001b). The design is widely accepted in the NAAQS standard of China (HJ 618 2011), India (CPCB 2012b) and Japan (JIS Z 2008). It is interesting to note that the U.S. EPA, which has developed this design, has not mentioned the GSD in its federal regulation. At the same time, countries that adopted the same design have mentioned the GSD (HJ 618 2011) except the Indian standard, where both the term D₅₀ cutoff and GSD are missing. The intercomparison with other equivalent samplers (Table 2) shows good linearity (Chung et al. 2001; Hauck et a. 2004; Su et al. 2018; Vanderpool et al. 2007; Williams et al. 2000; Yanosky, Williams, and MacIntosh 2002; Young et al. 2016). However, the linearity starts decreasing when particle loading is high for a longer period of time. The possible causes determined are the D₅₀ cutoff shift to lower size (Figure 10) because of a change in nozzle end to plate distance ratio (Le and Tsai 2017). This D₅₀ cutoff shift can be easily enhanced when fine size particle concentration in ambient air is more than a coarser particle fraction (Chen and Huang 1999).

The U.S. EPA has recommended the upper limit to measure PM_2.5 concentration in one PTFE filter sample should not exceed 200 µg m⁻³ (EPA 1998). It could be an issue in a region like India where the PM loading is high for most days in a year, primarily in cities that lie in Indo-Gangetic plains (Kaskaoutis et al. 2014; Murari et al. 2017; Sen et al. 2017; Shukla and Aggarwal 2021), which adopted the same guidelines and WINS impactor-based PM_2.5 sampler to meet the compliance set in NAAQS. Particle loading problems in WINS are investigated in several studies (Kenny 1998; Kenny et al. 2004; Le and Tsai 2017). Few researchers tried to reduce the particle loading effect (accumulation as a tip at the point where nozzle directed the particles on impactor plate) by introducing an injection system that allows continuous cleaning and wetting impaction surface with silicone oil (Le et al. 2020a). In contrast, others introduced a cyclonic size separator (e.g. VSCC) that has the same performance (Kenny and Thorpe 2001) and requires low maintenance with minimum particle loading effect compared to that of WINS impactor during sampling.

VSCC Cyclone

The Very Sharp Cut Cyclone (VSCC) is another widely used PM_2.5 size fractionator. It operates mainly with continuous monitors (Beta Attenuation Method (BAM)) and gravimetric measurement systems. The cyclone is designed at 1 m³ h⁻¹ of flow rate, Figure 9. The inlet and outlet are configured so that it can replace the WINS impactor and accommodate the PM₁₀ reference inlet at the top. The configuration of the cyclone is horizontal, which means the core cyclone body is perpendicular to the downward airflow direction (Kenny, Thorpe, and Stacey 2017). The VSCC cyclone is thoroughly studied by Kenny et al. (2004) for its D₅₀ cutoff and GSD, almost equivalent to the WINS impactor. It resolves the problems in a WINS impactor due to the particle loading effect. Study conducted by Le and Tsai (2017) compared to the WINS impactor shows that it can maintain the D₅₀ cutoff for a longer time. However, the D₅₀ maintenance in some environmental conditions requires further evaluation, where the dew point is significantly lower than the ambient temperature. Under this condition, the particle tends to stick on the inner surface of cyclones (Lin et al. 2018), resulting in the impacted particle does not fall/slip in the dust well of the cyclone. This problem can be enhanced when particle loading in the atmosphere is high. Other issues which are reported in VSCC cyclones are purely designing/fabrication related, which causes particle accumulation inside the cyclone during a longer period of sampling (Le et at. 2020b; Lin et al. 2018). Also, fabrication is complex compared to WINS and needs special machining tools.

Figure 10. Sampler effectiveness curve of various reference PM sampler.

Figure 11. PM₁₀ cyclonic high-volume sampler (India).

Reference PM_2.5 sampler (EN 12341)

The reference PM_2.5 size fractionator commonly used in Europe has a flow rate of 1 m³ h⁻¹ and 2.3 m³ h⁻¹. Design-wise, it is very similar to the PM₁₀ reference design mentioned in EN 12341 (2014) and discussed earlier in Figure 4. It differs only in acceleration nozzle diameter, which is considerably smaller, and hence, it gives a D₅₀ cutoff of 2.5 µm. The sampler effectiveness curve and D₅₀ cutoff are shown in Figure 10. The design lacks pre-separators like PM₁₀ inlet or head and directly separates PM_2.5 from ambient aerosol concentration. Therefore, the impactor is not efficient enough to sample the air for a longer period and under high particle loading conditions. Several field studies with other equivalent devices were also conducted to check the linearity. The linearity (consistency) is good under low PM loading concentration (CEN 2006; Hauck et al. 2004; Tasic et al. 2012; Walden et al. 2017) (Table 2) i.e., up to the concentration of 120 µg m⁻³ as mentioned in EN 12341 (2014), a limit of measurement of this sampler. However, under high ambient PM mass loading conditions, its response requires further investigation (in both contexts, consistency and agreement level with a reference).

Sampling filters

Filters play an important role not only in the collection of ambient aerosols and mass measurement but also to perform chemical characterization of deposited materials. Besides TSP, PM₁₀, and PM_2.5, the NAAQS of many countries has notified chemical parameters too such as Pb, Ni, Cd, As, and polyaromatic hydrocarbon (PAH) that also require filters for collection and the final analysis (CPCB 2009; EPA 2021; Kuklinska, Wolska, and Namiesnik 2015). Therefore, it is crucial to choose the filter decisively. Filters also contribute to an error during gravimetric measurement of PM such as loss of volatile particles, artifacts caused by retention of gaseous species on filters such as SO₂ and HNO₃ in various degrees in filter types like glass fiber, cellulose ester, quartz filter, Teflon filter, etc. (Appel et al. 1984). Some filters quality can be directly correlated to its properties, such as its alkalinity (Holland et al. 1989). Other measurement errors were caused by ambient temperature and RH, filter handling, and conditioning described in various studies (Brown et al. 2006; Lipfert 1994; Witz 1985; Witz et al. 1990). The filter types used for measuring PM classes in multiple countries’ standards show the relative differences. The U.S. EPA for PM₁₀ (HVS) and PM_2.5 (LVS) notifies glass fiber filters and Teflon filters with a supportive ring, respectively (EPA 1998), which are commonly practiced in other countries’ standards as well. It should also be noted that membrane-type filters such as ringed PTFE consist of structured mesh compared to other filters (glass or quartz), where fiber orientation is random in a filter sheet (Watson et al. 2017). Therefore, membrane-type filters have limited loading capacity because of clogging of pores compared to fiber-type filters.

Filters are also prone to electrostatic charging. It can be caused by various actions during filter handling and processing, either by friction, contact, or stripping between materials. This affects the mass of the filter measured initially (clean filter) and finally (particle deposited filter). The filter mass can also be affected by weighing under an uncontrolled environment where humidity and temperature levels vary (Engelbrecht, Cahill, and Feeney 1980; Tsai et al. 2002). Cellulose ester filters may absorb moisture and increase its weight. The controlled environment and electrostatic charge neutralizer such as Polonium-210 (²¹⁰Po) (Niemeyer, Case, and Cutshall 1978), bipolar or Corona discharge bipolar air ionizer (Tsai et al. 2002; Watson et al. 2017) is recommended for filter weighing.

Others NAAQS parameters associated to PM sampling

NAAQS of different countries has mentioned other parameters that need to be within the limit. These chemical parameters are basically determined by analyzing the PMs collected on filters, Table 3. Their sampling and analytical techniques are mentioned in country-specific standards. In most of the NAAQS, Pb is a common pollutant that is determined in filter samples. However, there is a difference in the method of sampling, filter types for collection, and subclasses of PM for Pb analysis. Earlier, U.S. EPA has mentioned Pb analysis using TSP samples sampled by HVS. Later, PM₁₀ LVS samplers on PTFE filter are also recommended under specific scenarios, as mentioned in 73FR67052 (EPA 2008). China recommends mid-volume samplers of flow rate 3–9 m³ h⁻¹ for Pb analysis in TSP glass fiber filter samples. In comparison, EU and Indian regulation for Pb recommends a PM₁₀ filter for analysis. India recommended PM₁₀ HVS to collect particles on a glass fiber filter (EPM-2000) only.

Table 3. Airborne chemical parameter sampled in various PM classes.

Countries	Chemical Parameter	PM class	Sampler type (Flow rate,m^3 h^-1)	Filter type	References
US	Pb	TSPM and PM10	HVS/LVS	Glass or PTFE	EPA 2008
EU	Pb	PM10	HVS/LVS	Glass or Quartz fiber filter, PTFE, Cellulose acetate etc	EN 14902 2005
	As	PM10	HVS/LVS	Glass or Quartz fiber filter, PTFE, Cellulose acetate etc	EN 14902 2005
	Cd	PM10	HVS/LVS	Glass or Quartz fiber filter, PTFE, Cellulose acetate etc	EN 14902 2005
	Ni	PM10	HVS/LVS	Glass or Quartz fiber filter, PTFE, Cellulose acetate etc	EN 14902 2005
	BaP	PM10	HVS/LVS	Glass or Quartz fiber filter	EN 15549 2008
China	Pb	TSP	MVS (3–9)	Glass fiber	GB/T 15264 1994
	BaP	PM10	HVS (63)	Glass or Quartz	HJ956 2018
India	Pb	PM10	HVS (60)	Glass fiber (EPM- 2000)	CPCB 2012b
	As	PM10	HVS (60)	Glass fiber (EPM- 2000)	CPCB 2012b
	Ni	PM10	HVS (60)	Glass fiber (EPM- 2000)	CPCB 2012b
	BaP	PM10	HVS (60)	Glass fiber (EPM- 2000)	IS 2004

Other chemical pollutants in NAAQS are As, Cd, Ni, and BaP. For all these pollutants, different countries have different standards for sample collection. EU notifies As, Cd, Ni and BaP, while India has reported only As, Ni, and BaP determined on the PM₁₀ filter. The only difference is the sampler type used, EU gives the option to use both PM₁₀ HVS and LVS, while India’s regulation recommends PM₁₀ HVS only. China has notified PM₁₀ HVS at a flow rate of 63 m³ h⁻¹ for BaP analysis on glass or quartz filters.

Flow measurement and calibration standards

An important parameter of different size segregated PM samplers is the airflow rate, which should be controlled at a recommended value for a particular sampler throughout the sampling period. This is to maintain a consistent D₅₀ cutoff of the impactor and cyclone and thus measure both PM mass, and total air volume sampled accurately. However, most of the standards lack controlling techniques. Still, all have mentioned a limit of variation from set flow (e.g., U.S. EPA (EPA 2016) and EU (EN 12341 2014) allowed 5% variation in the instantaneous flow of LVS, which implies flow control is requisite for any PM sampler.

The sampling flow rate can be measured in two forms using different devices, i.e. (i) volumetric flow and (ii) mass flow. The differences between them are well known. The volumetric flow varies with actual temperature and pressure, while the mass flow rate does not vary with temperature and pressure. The impactors and cyclones work on the volumetric flow of air to maintain their D₅₀ cut off and for the final measurement of PM mass concentration. Hence, measuring and controlling the flow inside the sampler volumetrically is necessary and taking it as actual flow. Even though the mass flow meter and controller are employed to maintain the fixed flow, it should also be converted into the volumetric flow (actual flow). The standards and manuals of U.S. EPA (EPA 1999b), Japan (JIS Z 2008) and EU (EN 12341 2014) give an insight of conversion of mass flow to actual volumetric flow and vice versa. It is also important to note that first, the flow reads by a flow meter should be converted to real value by considering the conditions at which it was calibrated (temperature and pressure). Then this value is further converted to flow at actual conditions (Hinds 1999). However, this two-step calculation is not clearly mentioned in any standards.

The flow calibration method of PM sampler varies with sampler types, i.e. HVS and LVS. The US and other countries’ standards mention the calibration of HVS with orifice or resistance plate (old method), which is directly based on pressure difference in flow caused by orifice deployed at the inlet of HVS. The volumetric flow meter of HVS is calibrated using this method. It is also known as transfer standards for field calibration of HVS. The primary method to calibrate the transfer standards is by using a direct volumetric flow meter, i.e., Roots meter or positive displacement flowmeters (EPA 1999b).

The LVS requires a different approach for the calibration of flow. The standards employ different types of flow meters. The preferable ones are the flow meters that do not obstruct the operations of LVS. It includes bubble, piston, mass, orifice, laminar, wet test and dry gas type flow meters described in the U.S. EPA quality assurance guidance document (EPA 2016) and Japanese PM_2.5 sampling standards (JIS Z 2008). In both types of samplers, the temperature and pressure need to be monitored during sampling and perform calibration procedures for correction and conversion to standard and actual volumetric flow. Therefore, calibration of supporting devices is also necessary. Other essential parameters requiring calibration are the timer (sampler clock), power supply, etc. However, the procedure/guidelines on these calibrations are not given in the standards except U.S. EPA standards.

Summary and conclusion

National ambient air quality standards (NAAQS) differ in most parts of the world in terms of criteria pollutant’s types, their regulation limits, monitoring methods, techniques used, etc. This is because the criteria pollutants and their ambient concentration are region-specific. However, one of the most common criteria pollutants in all NAAQS is particulate matter (PM). Nevertheless, the instrument/device used for the measurement of PM varies significantly in terms of design and operating parameters in the NAAQS and related standards. Therefore, here we reviewed several NAAQS and also standards (U.S. EPA, JIS, CN, CEN, ISO, IS, etc.) of different countries in context to PM measurements.

We observed that in country-specific PM₁₀ measurement practices, where samplers differ a lot in terms of design, operating flow rate and sampler effectiveness curve, etc. but in some cases, it uses the same techniques, e.g. in PM_2.5 measurements, Japan, China and India follow U.S. EPA standards. The QA/QC in PM measurement is mostly validated by D₅₀ cutoff calibration and field intercomparisons (collocating measurement using different instruments). In addition to this, the filter used for the chemical analysis of some criteria pollutants and requirement of sampling type differs widely in countries specific standards. Issues which are needed to be taken care of in measurement standards are summarized as follows;

1. Aspiration efficiency of PM samplers is one of the designing parameters that is ignored in all standards. It is one of the essential parameters required for the evaluation of PM heads that draw representative PM samples from the air for the accurate determination of SPM and PM₁₀ mass. It is known that apart from inlet design, the aspiration efficiency of a sampler inlet could be affected by ambient wind speed and sampling flow rate, but no standards talk about the criteria of applicability of a sampler under the defined wind velocity conditions. Also, very limited research has been reported on this, especially co-comparison study of different countries’ sampler inlets.

2. D₅₀ cutoff defines the class of PM sampler. However, only a few standards have mentioned the D₅₀ cutoff calibration method, while others implicitly mentioned the D₅₀. The test material used for calibration is still a matter of discussion. Standards suggest the use of monodisperse aerosol (mostly liquid aerosol) for cutoff calibration which is not only time consuming (generating specific size test aerosols) but also requires capital to set the testing facility. A polydisperse test aerosol (e.g. Arizona road dust or other ambient aerosol reference materials) could be the alternate option (as secondary standard) for calibration, which not only can give quick replicative tests but also requires comparatively less capital to set a testing facility. Lastly, the cutoff size and factors that contribute to its uncertainty along with calculations should also be incorporated in standards.

3. Consistently high ambient PM loading is also an issue that may not be an issue for countries like the US, EU, and Japan, where ambient PM levels are comparatively low in comparison to China and India. These countries adopted similar PM measurement standards as mentioned in U.S. EPA. The scope of the U.S. EPA standards is clearly defined, e.g. for PM_2.5 measurements, it sets the limit of 200 µg m⁻³ in 24 hours sampling. But in India and China in some seasons, it crosses the set limit for a consistent period of days, which could change the D₅₀ cutoff size to a lower size and results in large uncertainties in measuring the PM₁₀ and PM_2.5 unintentionally. Therefore, it is important to implement a method (inlet design) that can withstand a longer duration of sampling and have the minimum effect of particle loading under prevailing ambient conditions.

4. For PM_2.5 sampling, the EU has given its own design at a low flow rate of 1 and 2.3 m³ h⁻¹. The design does not consist of any PM₁₀ pre-separators like U.S. EPA reference louvered PM₁₀ inlet before PM_2.5 WINS impactor. This may cause frequent shifts in D₅₀ cutoff in EU reference design because the PM_2.5 impactor has to segregate the PM_2.5 from ambient TSP directly and hence more possibility of deposition of bigger size fraction (greater than PM_2.5) on the impaction plate and results in a frequent shift in D₅₀ cutoff, that is minimized in U.S. EPA PM_2.5 reference samplers. However, the PM_2.5 impactor of EU design has multiple nozzles which are preferable to reduce the particle bouncing and re-entrainment problem that can be an issue in a single nozzle design (like WINS), especially under high particle mass loading conditions.

5. Flow measurement and controlling of flow is another aspect of PM samplers. Indirect flow measuring devices, e.g. anemometer, mass flow meter, flow pressure sensing devices are widely in use to control the flow in a sampler. Sometimes, during the long and unattended run, the biases in flow sensing devices result in fluctuation in flow rate. Therefore, standards also need to clarify the mechanism for flow control and also the procedure to check the quality of flow measuring devices under laboratory (standard) and field conditions. Other issues are with the clarity in representing the flow (volumetric or mass) to the actual flow for the final calculation of PM concentration in samplers.

Acknowledgment

This is a part of internal project funding, OLP 183832. Director, CSIR-NPL is acknowledged for providing all support. PP thanks to DST, New Delhi for providing JRF/SRF fellowship under a project (IDP/IND/17/2013). All members of the Gas Metrology group are also acknowledged for their all support and help.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data available on reasonable request from the corresponding author.

Additional information

Notes on contributors

Prashant Patel

Prashant Patel is a Senior Research Fellow (SRF) in Gas Metrology, Environmental Sciences & Biomedical Metrology Division at CSIR-National Physical Laboratory, New Delhi and pursuing Ph.D. degree from the Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India. His current focus of research is the designing of particulate matter samplers and their cutoff size measurement.

Shankar G. Aggarwal

Shankar G. Aggarwal is a Senior Principal Scientist at CSIR-National Physical Laboratory, New Delhi. His current research interests are particle and gas metrology, aerosol chemistry and physics, air quality measurement and instrumentation, control technology development and measurement.