ORIGINAL RESEARCH

https://doi.org/10.4209/aaqr.220436

Design of Round-Nozzle Inertial Impactors Review

with Updated Design Parameters
Aerosol and Air Quality
Research
Francisco J. Romay1*, Estíbaliz García-Ruiz2
Special Issue: 1
In honor of Prof. David Y.H. Pui for
Mechanical Engineering Department, College of Science & Engineering, University of Minnesota,
his “50 Years of Contribution in Minneapolis, MN, USA
2
Aerosol Science and Technology” Department of Chemical & Environmental Engineering, University of the Basque Country,
(III) UPV/EHU, Bilbao, Spain

ABSTRACT
Round-nozzle inertial impactors are widely used aerosol measuring instruments to characterize
the mass and chemical composition of airborne aerosol particles as a function particle aerodynamic
diameter. This article summarizes the most important design considerations with updated
recommended design parameters taken from our review of published research articles and
discusses some of the more common non-idealities seen in the operation and performance of
inertial impactors. With this information, it is now possible to design a cascade impactor with
near-ideal particle separation performance, and with stage cutpoints that can be predicted with
excellent accuracy and verified experimentally using state-of-the-art calibration techniques.

Keywords: Inertial impactor, Cascade impactor, Impactor design parameters, Stokes number

1 INTRODUCTION
Inertial impactors continue to be used today for the collection of size segregated aerosol
particles, for mass and chemical composition characterization as a function of particle aerodynamic
diameter. The first impactors date back to the 1860s, and Marple (2004) provides a comprehensive
OPEN ACCESS review of the history of impactors from 1860s to the early 2000s. Impactors are used in many
important applications such as indoor and outdoor air pollution (Cass et al., 2000), industrial
process aerosols (Kero et al., 2015), pharmaceutical inhalers (Taki et al., 2010), combustion aerosols
Received: November 30, 2022 from engines (Schauer et al., 2008), forest fires (Costa et al., 2022), nanoparticle characterization
Revised: January 12, 20223 (Curwin and Bertke, 2011), biological aerosols (Appert et al., 2012), and others. The beauty of
Accepted: February 6, 2023 inertial impactors is that users can easily understand how they work because they are simple
* Corresponding
devices that rely solely on the ability of airborne particles to deviate from fluid streamlines in the
Author:
romay001@umn.edu
vicinity of an obstacle. This inertial separation principle led to the development of the single
nozzle impactor, where particles are accelerated by a nozzle towards a flat plate (Fig. 1(a)). This
Publisher: basic geometry can result in a very sharp separation between particles that impact the plate,
Taiwan Association for Aerosol and particles that are able to go around the plate (Fig. 1(b)). Marple (1970) conducted the first
Research fundamental study of inertial impactors by numerical flow field simulation and particle trajectory
ISSN: 1680-8584 print calculations to determine the collection efficiency curve of impactors, as a function of several
ISSN: 2071-1409 online parameters such as Stokes number, Reynolds number, jet-to-plate distance, and nozzle throat
length (see Fig. 1). Rader and Marple (1985) updated the original numerical simulations (Marple
Copyright: The Author(s). and Liu, 1974) by adding ultra-Stokesian drag and particle interception to the collection efficiency
This is an open access article
distributed under the terms of the predictions. This work has led to the proper design of several cascade impactors (Chen et al.,
Creative Commons Attribution 2018; Chien et al., 2015; Marjamäki et al., 2000; Marple et al., 2003a, 1995, 1991) by selecting
License (CC BY 4.0), which permits design parameters that resulted in experimentally calibrated collection efficiency curves that are
unrestricted use, distribution, and usually in excellent agreement with Marple’s theoretical predictions. In the past 30 years, a lot
reproduction in any medium,
provided the original author and of work has been done by researchers to better understand the performance of inertial
source are cited. impactors, particularly to include the effect of gravity on the collection of large particles, the

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Fig. 1. (a) Single nozzle inertial impactor; (b) Ideal and actual collection efficiency curve as a function of Stokes number (i.e., √Stk).

effect of the cross-flow in the collection efficiency of multi-nozzle impactors, the particle bounce
that occurs when sampling solid particles and how to avoid it, the unwanted deposition of particles
on the back side of the nozzles, the formation of secondary particle deposits between nozzles,
the formation of halo-type particle deposits away from the primary impaction zone, and other
non-ideal behavior of impactors. A recent paper on inertial impactors (Le and Tsai, 2021) provides
a comprehensive review of design and practical aspects such as particle bounce, re-entrainment,
and overloading effects, as well as some of the typical applications.
This paper attempts to summarize and consolidate all these aspects that characterize the
realistic performance of conventional inertial impactors with round nozzles, and provides a list
of the main design parameters with its recommended ranges (Table 1), that will result in the
near-ideal performance of a multi-jet cascade impactor with predictable collection efficiency
curves that can be verified experimentally using conventional modern calibration techniques. We
are excluding in this paper a discussion on virtual impactors, where the impaction plate is replaced
by a minor flow tube, and a fraction of the flow is used to separate the particles by inertia.

2 MAIN DESIGN PARAMETERS

This paper will consider the design of multi-jet cascade impactors with round nozzles. While
the analysis can be extended to impactors with rectangular nozzles, the great majority of
commercially available impactors use circular nozzles, so we are purposely discussing impactors
with round nozzles.

2.1 Stokes Number

The Stokes number is the main parameter that characterizes the performance of inertial
impactors, defined as the ratio of the particle stopping distance calculated at the average nozzle
exit velocity V0, to the nozzle radius W/2, where W is the nozzle diameter. For a single or multiple
nozzle impactor the Stokes number is calculated as (Marple and Liu, 1974):

C ( dp ) dp2 ρ pV0
Stk = (1)
9 µW

4Q
V0 = (2)
nπ W 2

where C(dp) is the slip correction factor, dp is the particle diameter, ρp is the particle density, µ is

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Table 1. Recommended impactor design parameters.

Impactor Design Parameter Recommended Value Comments
Stokes Number (√Stk50) 0.489 for Re = 500 These values come from numerical
0.498 for Re = 1,500 simulations for laminar and turbulent
0.507 for Re = 2,200 flow models (García-Ruiz et al., 2019b).
Reynolds Number (Re) 500 to 3,000 For stages with cutpoints below 100-nm Re
can be as low as 200.
Jet-to-Plate Distance (S/W) 1 to 5 (for sharper collection efficiency For stages with cutpoints below 100-nm
curves). S/W can be as high as 15, preferably
5 to 10 (for nanometer stages where around 10 to maintain √Stk50 close to
larger values of S are required). 0.50.
Cross Flow Parameter (Wn/4Dc) < 1.2 This parameter applies mainly to submicron
impactor stages with multi-jet nozzles.
Nozzle Spacing Parameter (A/W) ≥4 This parameter applies mainly to super-
micron impactor stages with mm sized
nozzles.
Nozzle Geometry and Configuration Conical tapered nozzles followed by For stages with cutpoints below 100-nm
straight throat. nozzles with a gradual contraction are
Hexagonal, Radial, Concentric Rings, or preferred.
Random nozzle patterns. For a small number of nozzles the pattern
can be a single ring.
Gravitational Parameter (Fr) Fr > 500 and Re > 1,500 Important for super-micron impactor
stages; Values of √Stk50 are lower when
gravity plays a role.
Pressure Drop (∆P) Pressure drop should be optimized by The impactor pressure drop is dominated by
increasing the number of nozzles the lower stages with submicron
while maintaining other design cutpoints. Avoid square cylindrical nozzles
parameters within acceptable values. with lower discharge coefficients.

the absolute gas viscosity, Q is the total volumetric flow rate, and n is the number of nozzles. The
square root of the Stokes number is typically considered as a dimensionless particle diameter.
Considering a typical sigmoidal collection efficiency curve, as shown in Fig. 1(b), the diameter
corresponding to the 50% collection efficiency is defined as the cutpoint of the impactor.
Therefore,

9 µW
dp ,50 = Stk50 (3)
C ( dp ) ρ pV0

where dp,50 is the nominal cutpoint, and Stk50 is the Stokes number for which the collection
efficiency is 50%. It is important to point out that dp,50 is usually converted to the corresponding
aerodynamic diameter based on particles with the density of water. In this case, Eq. (3) becomes

9 µW
da ,50 = Stk50 (4)
C ( da ) ρ0V0

where da,50 is the aerodynamic cutpoint diameter, and ρ0 is the density of water (i.e., 1 g cm–3).
The slip correction factor in Eq. (4) is calculated for the aerodynamic diameter da.
Rader and Marple (1985) recommend using a √Stk50 of 0.495 for the design of impactors
assuming that the Reynolds number Re, and the dimensionless jet-to-plate diameter S/W are
within the recommended ranges discussed later. A more recent numerical study (García-Ruiz et
al., 2019b) using modern CFD models with a much finer computational domain resulted in a small
variation of √Stk50 with the Reynolds number. These √Stk50 values are 0.489, 0.508 and 0.518 for
Reynolds of 500, 1500 and 3000 respectively and for laminar flow, while for a (Wilcox) k-w

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Fig. 2. Collection efficiency curves for a single nozzle impactor as a function of Reynolds number
using laminar flow with a fine computational mesh, and turbulent flow (KW model) with a
medium computational mesh (García-Ruiz et al., 2019b).

turbulence flow model, the √Stk50 are 0.498 and 0.507 for Reynolds 1380 and 2210 respectively
(see Fig. 2). Therefore, we recommend using these more refined √Stk50 values when designing
new impactors.

2.2 Reynolds Number

The Reynolds number has an important effect on the performance of inertial impactors. The
Reynolds number defines the flow regime in the region of particle separation. Reynold number
is defined as

ρgV0W
Re = (5)
µ

where ρg is the gas density. The Reynolds number affects the shape of the collection curve and
the values of √Stk50, particularly for low and high values. However, Marple and Liu (1974) found
that if the Reynolds number was between 500 and 3000, the effect of Reynolds number was
minimum, and that the collection efficiency curves had sharp characteristics ideal for particle
separation with a nearly constant value of √Stk50. For smaller values of Reynolds number, the
collection efficiency curves become shallower and shift to the right due increased viscous effects.
For larger Reynolds numbers, a more pronounced tail appears at larger particle sizes due to
ultrastokesian drag, leading to a lower sharpness of cut (Rader and Marple, 1985). Therefore, to
simplify the impactor design, particularly for cascade impactors with several multi-nozzle stages,
it is recommended to maintain Reynolds numbers between 500 and 3000, so that the collection
efficiency curves and corresponding cutpoints can be predicted from conventional impactor
theory based on numerical simulation by CFD models. It is important to mention that when trying
to extend the dynamic sizing range of cascade impactors, both for large (i.e., > 10 µm) and very
small (i.e., < 50 nm) particles, it is possible to see some impactor stages with Reynolds numbers
slightly outside this recommended range (e.g., NanoMOUDI impactor, Marple and Olson, 1999).
In these cases, it becomes imperative to determine the cutpoint of these stages experimentally,
since deviations from theory can be expected. Therefore, it is recommended to stay within the
range of Re between 500 and 3000, to avoid additional costly trial and error calibrations to
determine the cutpoint and sharpness of cut. The Reynolds number also affects the formation of
secondary deposits and particle losses on the backside of the nozzles, as well as the influence of
gravity, but these effects will be discussed later in this paper.

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2.3 Dimensionless Jet-to-Plate Distance (S/W)

The dimensionless jet-to-plate distance affects the impactor sharpness of cut, but the value of
√Stk50 is nearly independent of S/W, if it is maintained between 1 and 5 (Rader and Marple, 1985).
A value of S/W = 1 results in the sharpest collection efficiency curve with a typical geometrical
standard deviation or GSD < 1.2, where the GSD is typically calculated as (Marple and Olson,
2011):

da ,84
GSD = (6)
da ,16

where da,84 and da,16 are the aerodynamic diameters for 84% and 16% collection efficiencies
respectively. In practice, it is not always possible to maintain S/W values of 1, particularly as the
nozzle diameters become very small for collecting finer particles. The minimum value of S that
can be maintained in a precisely machined impactor is 0.5 to 1.0 mm, so as the nozzles reach
sub-millimeter dimensions, larger jet-to-plate distances are required. For example, in the MOUDI
impactor, the values of S/W vary from 1 for upper stages, to a value of 11 for the last 2 stages
with cutpoints of 100 nm and 56 nm. Using S/W values larger than 1 result in lower sharpness of
cut on the collection efficiency curves, as clearly seen in the published MOUDI efficiency curves
for stages 6 to 10 (Marple et al., 1991). Also, when using S/W values larger than 4, it is possible
to see a change in the √Stk50 values. Chien et al. (2015) reported a linear increase of √Stk50 with
S/W for stages 8, 9 and 10 of the NMCI impactor for S/W values between 4 and 20. Therefore,
when designing impactor stages with nanometer cutpoints (< 100-nm) that require small nozzles
(i.e., < 0.10-mm) and larger S/W values, experimental calibration is necessary to determine the
stage cutpoints accurately.
The design parameters discussed so far, and their recommended ranges come from flow and
particle trajectory simulations for a single nozzle impactor. However, they can also be used for
designing multi-nozzle impactors as long these nozzles act independently of each other, which
means that the acceleration jets do not interact with each other. This can be accomplished by
ensuring that the nozzles are sufficiently separated to prevent jet-to-jet interactions. The following
two parameters address this issue in the design of cascade impactors.

2.4 Cross-Flow Parameter

The cross-flow parameter (Fang et al., 1991) is defined as:

Wn
CFP
= < 1.2 (7)
4Dc

where n is the number of nozzles, and Dc is the cluster diameter (i.e., the diameter where all the
nozzles are clustered). In the analysis by Fang et al. (1991), it was demonstrated experimentally
that if the CFP was above 1.2, the cross flow exiting the stage interferes with some of the jets,
leading to incomplete impaction, and collection curves not reaching 100% efficiency. But for CFP
< 1.2, the collection efficiency curves are sigmoidal and reach 100% collection efficiency. The
cross-flow parameter becomes critical when designing impactor stages with very low cutpoints
smaller than 100-nm, requiring a large number of small nozzles needed to reduce the larger
pressure drop associated with impacting very small particles. The design of the micro-orifice collector
in the Next Generation Pharmaceutical Impactor (Marple et al., 2003a) required to increase the
nozzle cluster diameter to 75-mm, to accommodate a total of 4032 nozzles of 70-µm diameter,
and ensuring the CFP was below the maximum recommended value of 1.2.

2.5 Dimensionless Nozzle Spacing Parameter

The dimensionless nozzle spacing parameter (i.e., NSP) can be defined as the ratio of the
nozzle-to-nozzle distance A (measured from center to center), divided by the nozzle diameter W:

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A
NSP
= ≥4 (8)
W

This parameter assumes that all the nozzles are equally spaced, even though in many impactor
designs this is not always the case. The importance of this parameter becomes obvious when
designing impactor stages with large super-micron cutpoints (typically larger than 5-µm aerodynamic
diameter), which require nozzles of several millimeters in diameter. In this case, it is tempting to
cluster the nozzles too close to each other (to save space), which leads to the formation of
secondary line deposits (García-Ruiz et al., 2019a; Rocklage et al., 2013) observed in many impactor
designs. Until recently, the nozzle spacing between large millimeter-sized impactor nozzles has
been treated rather informally. García-Ruiz et al. (2019a, 2019b) conducted a detailed numerical
and experimental study using a single stage impactor with 3 nozzles equally spaced in an
equilateral triangle, with A/W values ranging from 2.5 to 8, and Reynolds numbers between 465
and 2210. For the lowest nozzle spacing of 2.5W, secondary line deposits and premature half-moon
primary deposits were observed for particles smaller than the impactor cutpoint, leading to
collection efficiency curves with a pronounced tail on the low end of the curve (Fig. 3), and with
a lower value of √Stk50. The use of a Shear Stress Transport turbulence model to predict the
strong recirculation induced by the jet-jet interactions, provided excellent agreement with the
experimental results (Fig. 3). Therefore, this study recommends to use a minimum nozzle spacing
parameter (i.e., NSP) of at least 4 to minimize the effects of the jet-to-jet interactions. While at
NSP = 4 it was possible to still see some line deposits, their contribution to the total deposited mass
was very small (under 2%, García-Ruiz et al., 2019a).

2.6 Nozzle Geometry and Configuration

The typical nozzle shape of most impactors has a tapered conical entrance followed by a straight
throat section with a length T equal to the nozzle diameter W (see Fig. 1). This nozzle geometry
gives sufficient time for the particles to accelerate to the fluid velocity exiting the nozzle (Marple
and Willeke, 1976), and is typical for the majority of commercially available impactors with some
exceptions (e.g., the Andersen-type impactor and the ELPI impactor have cylindrical nozzles).
Tapered nozzle entrances also have a lower pressure drop due to their higher discharge coefficient
(see Section 2.9 on pressure drop). Impactor stages for nanometer particles (typically < 100-nm)
are difficult to make with a tapered conical nozzle, so in this case it is common to see an abrupt

Fig. 3. Experimental and numerical collection efficiency curves at nozzle-to-nozzle spacings of

2.5W, 6W and 8W for Reynolds of 2210 (data from García-Ruiz et al. (2019a, 2019b)).

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contraction, such as in the stages 9 and 10 of MOUDI impactor. This can lead to faster nozzle
clogging and associated particle losses as reported by Liu et al. (2013). By making the nozzles with
a smooth transition, Liu et al. (2013) found that the lower stages of the NMCI cascade impactor
were less prone to clogging when sampling high concentration combustion aerosols.
The nozzle configuration pattern in multi-nozzle impactors is normally left open to the designer,
provided the nozzle spacing is adequate according to the discussion above. Kwon et al. (2002)
conducted an experimental study using a 20-nozzle stage with 5 different nozzle configurations.
The models had a conventional nozzle cluster design with either concentric rings or randomly
spaced nozzles. The experimental values of √Stk50 were in good agreement with theory, even though
in some cases the nozzles were too close to each other, resulting in secondary line deposits and
shallower efficiency curves. The nozzle configuration in stages 5 to 10 of the MOUDI impactor have
nozzles in a randomly generated pattern. The NGI impactor (Marple et al., 2003a, 2003b), has nozzles
with different nozzle patterns (see Fig. 4) depending on the stage (a single ring for stage 2, a
symmetric hexagonal pattern for stages 3, 4 and 5, and a radial pattern for stages 6 and 7). The
values of √Stk50 for stages 2 to 7 of the NGI stages fall within the range predicted by theory, an
indication of a proper impactor design with collection efficiency curves that are reasonably sharp
(GSD values of 1.11 to 1.21 at 30 L min–1), and with minimum overlap between them.

2.7 Gravitational Parameter

Until now, we have not mentioned the effect of gravity in impactors. The reason is that gravity
plays a role mainly for stages with large super-micron cutpoints (i.e., > 5 µm), The influence of
gravity on the impactor performance can be inferred using the dimensionless Froude number
defined as:

V02
Fr = (9)
gW

Computational fluid dynamics models have been used to predict the effect of gravity as a
function of Froude number and Reynolds number (Huang and Tsai, 2001; Rader and Marple,
1984). Gravity reduces the value of √Stk50 and the steepness of the efficiency curves, particularly
for values of Fr < 500 and Re < 1,500. However, experimental data from real impactors do not
generally agree very well with these numerical models, so in cases where gravity plays a factor,
experimental calibration is required to determine the cutpoint of the impactor. In most impactors
that have a large cutpoint stage of about 10-µm, the efficiency curve is affected by gravity and

Fig. 4. Nozzle configuration of Next Generation Pharmaceutical Impactor (The Next Generation Pharmaceutical Impactor User
Guide, Rev C, MSP Corporation, Shoreview, MN, 2008).

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the value of √Stk50 is below 0.50. For example, in the NGI impactor, stage 1 with a 11.7-µm
cutpoint at 30 L min–1 has a √Stk50 of 0.425 (Marple et al., 2003b).

2.8 Particle Losses in Cascade Impactors

Proper design of a cascade impactor requires to minimize its internal particle losses, since
particles that are lost on surfaces other than the impaction plates are typically not quantified,
and therefore ignored in the mass size distribution determined from the impaction plate deposits.
Particle losses depend on the type of particles being collected, being worse for liquid particles
when compared to solid particles. A common type of particle losses is on the backside of the nozzle
plates. These losses are dependent on the Reynolds number and on the nozzle spacing (García-
Ruiz et al., 2019a). They typically occur at higher Reynolds numbers and increase as the nozzle
spacing decreases. For S/W = 1, García-Ruiz quantified these nozzle backside losses to be under
6% for liquid particles. In addition to the particle losses on the backside of the nozzles, particles
can also be deposited on other surfaces, particularly on the inlet of the impactor where large
particles can be lost by turbulent deposition in recirculation zones. For example, in the MOUDI
impactor particles losses at the inlet can be as high as 35% and 20%, for liquid and solid particles
respectively, and for particles larger than 10-µm in aerodynamic diameter (Marple et al., 1991).
Therefore, particular attention must be given to large particle losses near the inlet of impactors
and in stages with ~10 µm cutpoints. One way to address the losses of large particles at the
entrance of the impactor is to add a pre-separator with a large aerodynamic cutpoint. For the
NGI impactor, a large particle pre-separator with two internal stages was designed for sampling
dry powder inhaler formulations (Marple et al., 2003a). This preseparator has a large enough
volume and a special impaction well that can be filled with a solvent for the first coarse internal
stage. This feature improves the performance of the impactor to remove particles larger than
about 15-µm. The second internal stage has a conventional flat impaction plate with a sharper
collection efficiency curve (Marple et al., 2003b).
For particles between a few microns and 100-nm, interstage particle losses tend to be low in
most impactors, if they are well designed. However, cascade impactors with stages for ultrafine
particles below 100 nm can have significant losses by convection-diffusion (Durand et al., 2014;
Liu et al., 2013). Liu et al. (2013) measured the particle losses by convection-diffusion in the MOUDI
impactor, and found that for particles smaller than 40nm, particle losses can exceed 10%. These
losses occur on all the internal surfaces of the impactor including the upper stages. To reduce
particle losses by diffusion in a cascade impactor, the internal volume can be optimized to reduce
the total residence time. Also, to collect nanoparticles by impaction low pressures are typically
required, and this may enhance the evaporation of volatile and semivolatile particles. Geller et
al. (2002) conducted tests to measure ammonium nitrite particle losses by volatilization in the
10-nm stage of the nanoMOUDI, and determined that they were negligible compared to the losses
of the same ammonium nitrate particles collected in filters. This was explained by the reduced
surface area and the formation of a stagnation flow boundary layer for the impactor deposits,
resulting in lower losses of volatile particles collected in impactors as compared to filters. However,
to reduce the extent of possible evaporation of collected semivolatile particles, it is recommended
to optimize the pressure drop as explained in Section 2.9.

2.9 Impactor Pressure Drop

The total pressure drop of a cascade impactor is an important consideration during the design
process, since the flow for the impactor is typically provided by a vacuum pump, whose capacity
and minimum absolute pressure must be sufficient to draw the design inlet flow rate through the
impactor. The pressure drop for a stage of a multi-nozzle impactor can be calculated as:

2 2
1 ρgV0 1 ρg  4Q  8 ρ g Q2
∆P
= =  =  (10)
2 Cd2 2 Cd2  nπ W 2  n2π 2Cd2W 4

where Cd is the discharge coefficient of the nozzles, which is related to the shape of the nozzle,
the formation of the “vena contracta”, and the frictional losses associated with the acceleration
of the jet. The value of Cd is less than 1, and for typical tapered nozzles it varies between 0.7 and

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0.9, while for a straight nozzle is 0.6. It is important to note that in the Eq. (10), the volumetric
flow rate and density of the gas are at the actual pressure of the impactor stage. To reduce the
magnitude of the pressure drop in the lower stages of a cascade impactor, the number of nozzles
is increased as needed, while maintaining other parameters such as Reynolds number, cross-flow
parameter, and jet-to-plate distance within recommended ranges. This optimization has resulted
in the design of impactor stages with a cut-point as low as 10-nm, with nozzle diameters of about
50-µm, while operating at an absolute pressure of 10 kPa (Marple and Olson, 1999).

3 OTHER NON-IDEAL BEHAVIOR OF IMPACTORS

It is worth mentioning some other common observations on non-ideal behavior of impactors.
Several researchers have mentioned or studied the formation of halo particle deposits around
the primary deposits (García-Ruiz et al., 2019a; Rocklage et al., 2013; Štefancová et al., 2011).
García-Ruiz et al. (2019a) explained that the halo is formed due to the boundary layer separation
from the impaction plate, occurring while the air flow runs radially towards the outlet. That way,
the particles with sufficient inertia, which are not able to follow the separation of the boundary
layer, are inertially separated and deposited by gravity (Fig. 5).
Therefore, the analysis of the formation of the halo must consider both inertial and gravitational
effects and the balance between them. The net effect of the halo on the collection efficiency
curve is to reduce its slope and displace it towards smaller values of √Stk50. That effect is increased
by decreasing the Reynolds number and the Froude number. García-Ruiz et al. (2019a) concluded
that halos are minimized for Reynolds numbers greater than 1000 and Froude numbers of about
104. If these requirements cannot be met, the halo can be avoided with impaction plates small
enough that prevent flow detachment. Therefore, the diameter of the impaction plate should be
slightly larger than the nozzle cluster, ensuring that particles from all the jets can be collected on
the impaction plate.
Particle bounce is one of the most common occurrences when using impactors, where particles
that impact the collection surface bounce off, resulting in collection of these particles in the lower
stages of the impactor, and skewing the mass size distribution towards smaller sizes.
Particle bounce in impactors can be effectively controlled by the application of anti-bounce
coatings such as high-viscosity silicone oil to the impaction plates (Pak et al., 1992), and this is

Fig. 5. Halo formation on impaction plate for 3-nozzle impactor at A/W = 6 and Re = 1380 (García-
Ruiz et al., 2019a).

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the most common way to control impactor bounce. The application of such coatings is not always
a desirable option, particularly when determining the composition of the collected particles, and
there is a risk of interference in the chemical analysis of the collected particles (Fujitani et al.,
2006). Another way to reduce particle bounce is by conditioning the relative humidity of the
sampled aerosol to near 70–80% RH (Vasiliou et al., 1999), where it was found that RH conditioned
ambient aerosols were correctly sampled in the impactor with minimal particle bounce. This
particle bounce reduction is attributed to the increased adhesion by capillary force between the
adsorbed water on the particle and the impaction plate (Bateman et al., 2014). Chen et al. (2016)
devised a humidity conditioning system using Nafion® tubes to maintain the RH of the sampled
aerosol to 65% RH on the QCM (quartz crystal microbalance) MOUDI impactor. Finally, the
surface properties of impaction plates can also be modified by increasing its surface roughness
or porosity (Huang et al., 2001; Le et al., 2019; Marjamäki and Keskinen, 2004), to reduce bounce
and resuspension of particles after being collected. While these surface modification methods can
be highly effective, the collection efficiency curves for these modified impaction substrates may
deviate significantly from the theory presented in this paper, due to increased particle deposition
by filtration mechanisms on the low end of the curve. Therefore, experimental calibrations are
necessary to determine the performance of the impactor when porous impaction plates or filters
are used for eliminating particle bounce.
Even when using sticky antibounce coatings to reduce particle bounce, excessive particle
loading on impaction plates can result in particle deposits being blown-off from the substrates,
and increasing the mass collected at lower stages or at the final filter. Tsai et al. (2012) designed
a nanoparticle sampler using a 100-nm impactor micro-orifice stage, with a silicone-oil coated
Teflon filter with internal rotation as the impaction substrate, eliminating particle bounce and
extending the total collected nanoparticle mass capacity. Le et al. (2022) recently developed a
novel single-stage nanoparticle impactor with a wetted impaction substrate designed to eliminate
the effect of particle loading on the impactor performance. While it is possible to reduce particle
bounce and re-entrainment by these specialized impactor designs, the easiest way to reduce the
incidence of particle blow-off in cascade impactors, even when using anti-bounce coatings, is to
limit the sampling time according to the expected mass concentration of the sampled aerosol, so
that the total mass collected per stage is maintained within reasonable limits, to meet the
requirements of the quantitative mass/composition analysis, and following guidelines provided by
the impactor manufacturer.

4 CONCLUSIONS
Inertial impactors with circular nozzles are basic aerosol instruments that can be designed
using fundamental physical principles and specific design parameters that have been refined by
many researchers during the last 50 years since the first study by Marple (1970) at the University
of Minnesota. The five main parameters that define an inertial impactor with a sharp collection
efficiency of a known aerodynamic diameter cutpoint are: (1) Stokes number, (2) Reynolds number,
(3) Dimensionless Jet-to-Plate distance, (4) Cross-flow parameter, and (5) Dimensionless Nozzle
Spacing parameter. By properly selecting appropriate values for these five parameters, several
multi-stage cascade impactors have been designed and commercialized worldwide, and these
impactors continue to be used for a wide range of applications encompassing the fields of air
pollution, indoor air quality, combustion and industrial aerosols, pharmaceutical inhalers, bio-aerosols
(bacteria, viruses and fungi), nanotechnology, and others. Like most aerosol instruments, inertial
impactors also exhibit non-ideal behavior effects during practical use, such as particle bounce
and re-entrainment of collected particles. These have also been studied and practical solutions
exist to avoid performance issues that can lead to erroneous results. This paper includes results
and recommendations from the most recent studies that enhance the understanding of inertial
impactors, and summarizes how to best approach new impactor designs.

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