ISO 19996:2024

International
Standard
ISO 19996
First edition
Charge conditioning of
2024-10
aerosol particles for particle
characterization and the generation
of calibration and test aerosols
Conditionnement de la charge (électrique) des particules
d'aérosols pour la caractérisation de particules et la génération
d'aérosols pour calibration et essais
Reference number
© ISO 2024
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ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 2
5 General principle . 2
5.1 General .2
5.2 Ionization sources .3
5.2.1 General .3
5.2.2 Sources with radioisotopes .3
5.2.3 Soft X-ray sources .5
5.2.4 Corona discharge .6
5.3 Charge conditioning .7
5.3.1 General .7
5.3.2 Bipolar charge conditioners .7
5.3.3 Unipolar charge conditioners .8
5.4 The charge distribution function .9
5.4.1 General .9
5.4.2 Charge distribution function for radioactive bipolar charge conditioners .9
5.4.3 Charge distribution functions for other bipolar and unipolar charge conditioners .9
6 Factors influencing the resulting charge distribution .10
6.1 General .10
6.2 Aerosol particle characteristics influencing the charge distribution .10
6.2.1 Particle size and surface area .10
6.2.2 Particle number and surface area size distribution and concentration . 12
6.2.3 Particle pre-charge . . 12
6.3 Aerosol carrier gas characteristics influencing the charge distribution . 13
6.3.1 Carrier gas composition . 13
6.3.2 Carrier gas pressure and temperature . 13
6.3.3 Carrier gas humidity . 13
6.4 Charge conditioner operating parameters influencing the charge distribution . 13
6.4.1 Aerosol flow rate . 13
6.4.2 Ion production rate .14
6.5 Others .14
6.5.1 Surplus ions downstream of device .14
6.5.2 Particle losses to the chamber wall .14
6.5.3 Aerosol dilution in the charge conditioner . 15
6.5.4 Generation of artefact particles . 15
7 Operational parameters for device specification .15
8 Test procedures for determining the suitability of charge conditioners .15
8.1 Guidance to test procedures in the annexes . 15
8.2 Charge conditioner performance verification .16
8.3 Particle losses in a charge conditioner . .16
8.4 Particle generation rate .16
8.5 Charge distribution of bipolar charge conditioners .16
9 Cleaning and maintenance including safety issues . 17
Annex A (informative) Implementation of bipolar steady-state charge conditioning .18
Annex B (informative) Performance test procedures for charge conditioners .24
Annex C (informative) Electrostatic precipitator to provide uncharged aerosol particles .37

iii
Annex D (informative) Concentration series test for charge conditioners .39
Annex E (informative) Example set of tests for bipolar charge conditioners .45
Annex F (informative) Test method for bipolar charge conditioners with ambient aerosols .53
Bibliography .55

iv
Foreword
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v
Introduction
Charge conditioning of aerosol particles is the crucial process of establishing a known, size-dependent charge
distribution on aerosol particles. Different designs for charge conditioners exist. In charge conditioners,
aerosol particles are exposed to a cloud of ions of either both positive and negative polarities (bipolar charge
conditioners) or a single polarity (unipolar charge conditioners).
The transport of the ions to the aerosol particles can either be driven by Brownian motion of the ions
(diffusion charging) or by an electrical field (field charging). Since field charging is strongly biased by a
particle’s electrical properties (namely the relative permittivity), diffusion charging is generally used to
condition aerosol particles:
— for particle size distribution measurement with the differential mobility analysing system (DMAS);
— for particle size classification with the differential electrical mobility classifier (DEMC).
Several parameters determine whether or not charge conditioning achieves its goal of either generating a
mathematically describable bipolar steady-state charge distribution or a quantifiable unipolar mean charge.
Examples for such parameters are the ion concentration, the particle concentration, the residence time of
the particles in the ion cloud, the ion mass distribution or the ion mobility distribution. However, there is no
standard methodology to specify the performance of charge conditioners.
The electrical mobility of aerosol particles is a physical particle property which is widely used for particle
characterization (e.g. size distribution measurement with DMAS) and for particle classification (e.g. by
DEMC). For a given particle size, the particles’ electrical mobility is proportional to the net number of
elementary charges on the particle. Therefore, the knowledge of particle charge distribution is an essential
requirement for particle size distribution measurements with the DMAS and for particle size classification
with the DEMC.
The purpose of this document is to provide a methodology to specify the performance of charge conditioners
and for adequate quality control when charge conditioners are used in particle size and number concentration
measurement or in particle size classification.
Other typical uses of charge conditioners which are not covered in this document are:
— conditioning of test aerosols for filter testing where particle charge has an influence on the test results;
— particle charge reduction during the droplet evaporation in an electrospray aerosol generator, where
the very high unipolar charge of the sprayed solution or dispersion droplets can lead to the unwanted
[34]-[36]
disintegration of the droplets due to exceeding the Rayleigh limit during droplet evaporation;
— diffusion chargers (DC) in particle number devices (PND) that are typically used as robust, compact
systems to measure particle number concertation in the exhaust emission of passenger cars, light and
heavy duty cars under real driving emissions (RDE) as well as under periodical technical inspections
(PTI) in Europe. The charging process in such a device is provided by a diffusion charger, which is
charging the aerosol in a positive unipolar diffusion state. Typically, a thin wire is used as a high voltage
electrode to generate positive ions. The ions are injected through a grounded grid into buffer volume
where they are mixed with the particles. Afterwards, the charged aerosols will be counted in a two stage
[37,38]
procedure by an pulsed precipitator and in an Faraday cup aerosol electrometer (FCAE);
— large-scale ionizers combined with electrostatic precipitators (ESP) for cleaning flue gases of waste
incinerators or power plants fired with solid fuels. In the ESP, a corona discharge generates ions which
charge the flue gas particles (usually fly ash) by diffusion and field charging (depending on the particle
size). Subsequently, the particles are deflected by electrophoresis in the ESP’s electrostatic field and
deposited on grounded collection electrodes. Industrial ESP are usually several tens of meters high and
consist of a multi-stage configuration to optimize the overall collection and gas cleaning efficiency.

vi
International Standard ISO 19996:2024(en)
Charge conditioning of aerosol particles for particle
characterization and the generation of calibration and test
aerosols
1 Scope
This document specifies requirements and provides guidance for the use of charge conditioners for aerosol
particles, especially for particle characterization and for the generation of calibration and test aerosols.
This document provides a methodology to specify the performance of charge conditioners and for adequate
quality control, with respect to their application in:
— particle size and concentration measurement with differential mobility analysing systems (DMAS);
—particle size classification with differential electrical mobility classifiers (DEMC).
For these applications, this document covers particle charge conditioning for particle sizes ranging from
approximately 1 nm to 1 µm and for particle number concentrations at the inlet of the charge conditioner up
7 -3
to approximately 10 cm .
This document does not address specific charge conditioner designs or other applications besides those
specified in Clause 1.
Radiation safety for charge conditioners with radioactive sources or x-ray tubes is not covered by this
document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 15900, Determination of particle size distribution — Differential electrical mobility analysis for aerosol
particles
ISO 27891, Aerosol particle number concentration — Calibration of condensation particle counters
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 15900, ISO 27891 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

3.1
charging probability
fd()
p
ratio of the number concentration of particles exiting a charge conditioner with p charges to that of particles
exiting the charge conditioner at all charge states, at particle size d
Note 1 to entry: p charges are 0, ±1, ±2, etc.
Note 2 to entry: The charging probability with respect to the number concentration entering (instead of exiting) the
charge conditioner is called "extrinsic charging probability".
3.2
charge distribution function
either mathematical or empirical, or both, description of a conditioned distribution of particle size dependent
charging probability (3.1)
3.3
electrostatic precipitator
ESP
device for removing charged particles from an airflow by electrophoresis to generate an uncharged aerosol
Note 1 to entry: More information on ESPs is given in Annex C.
3.4
ion mobility distribution
number density distribution with respect to the electrical mobility of the ionic molecular clusters that are
responsible for the charging of aerosol particles in a charge conditioner
4 Symbols and abbreviated terms
For the purpose of this document, the following symbols and abbreviated terms apply.
CPC condensation particle counter
DEMC differential electrical mobility classifier
DMAS differential mobility analysing system
ESP electrostatic precipitator
d particle diameter m
fd
()
charging probability dimensionless
p
-3
N number concentration of aerosol particles m
-3
N
number concentration of ions m
I
p number of net elementary charges on a particle dimensionless
t residence time of an ion in charge conditioner s
5 General principle
5.1 General
The function of the charge conditioner in this document is to establish a known size-dependent, steady-
state charge distribution on the sampled aerosol prior to the size classification process in electrical mobility
classifiers like the DEMC. The charge distribution on the particles can either be bipolar or unipolar.

All charge conditioners can be regarded as ionization sources because they generate ions of either one polarity
or both polarities in the carrier gas. These ions interact with the particles to generate a charge distribution.
The characteristics of ionization sources frequently used for charge conditioning are outlined in 5.2.
Since charge conditioners are used to achieve steady state charge distribution in the aerosol sample flow,
the charge conditioner shall, by design or by measurement, perform correctly and not produce artefact
particles.
In its simplest form, a charge conditioner, such as that used in a DMAS, consists of an aerosol inlet, aerosol
outlet, ionizing source, charging zone and enclosure.
5.2 Ionization sources
5.2.1 General
There are three common types of ionization sources for charge conditioning.
— Radioisotopes.
— Soft X-rays.
—Corona-discharges.
Other, less common ionization sources are included in Table 1.
5.2.2 Sources with radioisotopes
5.2.2.1 General
Radioisotope charge conditioners generally contain a sealed radioactive source. This device acts as a bipolar
diffusion charge conditioner. It produces both negative and positive ions in the carrier gas. The radiation
+ +
generates, so called, primary ions like N and O and free electrons in the carrier gas. These ions are short-
2 2
lived. Some of them attach themselves to neutral molecules, which then coagulate into relatively stable ion
clusters. Diffusion (Brownian movement) leads to collisions between these ions and the aerosol particles
and thus to charge transfer to the particles.
Either alpha or beta radiation can be applied for air ionization. Alpha radiation with its very high linear
energy transfer is able to produce high ion concentrations in a small charging volume. This is an advantage
over beta radiation, where the charging volume must be bigger. As a result, the particle residence time in a
radioactive charge conditioner with beta radiation is typically longer, which is a disadvantage with respect
to diffusion losses. On the other hand, alpha radiation sources can easily be shielded, e.g. by a very thin layer
of dust. Surface contamination can reduce the resulting ion concentration in charge conditioners with alpha
sources.
Figure 1 shows a schematic example of the design of a radioisotope charge conditioner.

Key
1 aerosol inlet 4 charging zone
2 aerosol outlet 5 enclosure
3 radioisotope source
Figure 1 — Schematic example of a radioisotope charge conditioner
The most commonly used radioactive isotopes are:
— Krypton 85 ( Kr).
— Americium 241 ( Am).
— Polonium 210 ( Po).
— Nickel 63 ( Ni).
Their properties are explained in 5.2.2.2 to 5.2.2.5.
[71]
NOTE Sealed radioactive sources are classified based on ISO 2919, which provides tests and a classification
system, e.g. for ranges of temperature, pressure, puncture, impact and vibration.
5.2.2.2 Krypton 85 ( Kr)
Kr is a beta emitter (with 0,43 % gamma radiation probability of 514 keV) with a half-life of 10,78 years.
The maximum beta energy is 687 keV. Krypton is a noble gas, substantially reducing the health risk in case
of leakage or damage to the source. In nearly all sources, the Kr gas is contained in a small-diameter,
sealed, stainless steel tube. This tube is contained inside a larger-diameter stainless steel or aluminium
housing. Aerosol passes axially through the housing that contains the Kr tube. Part of the beta radiation is
absorbed in the steel or aluminium that makes up the tube and the housing, thus producing Bremsstrahlung
that also contributes to ion production. It is recommended to use lead shielding if possible.
5.2.2.3 Americium 241 ( Am)
Am is an alpha emitter (with negligible additional beta and gamma radiation) with a half-life of 433 years.
Sealed sources of this metal are available as strips covered with a very thin gold, palladium, or gold and
palladium alloy film. The alpha energy is 5,5 MeV.
5.2.2.4 Polonium 210 ( Po)
210 210
Po is an alpha emitter with a half-life of 138 days. Due to their short half-life, Po sources should be
replaced annually or more often. The metalloid Po is available in the form of gold-coated, typically
embedded in a protective housing. Its alpha energy is in the range between 4 MeV and 5,3 MeV.
5.2.2.5 Nickel 63 ( Ni)
Ni is a beta emitter (100 %) with a half-life of 100,1 years. Its beta energy is 67 keV; the decay product is
63 63
stable Cu. Ni foils are also used, as ionisation source in GC-MS for example. Unsealed as well as sealed
(inactive Ni overplating) foils, with up to 100 MBq, are commercially available.
NOTE 100 MBq is the free limit in the EU.

5.2.2.6 Licensing and precautions for radioisotope sources
The use, transportation and disposal of radioisotopes is regulated by government authorities. Basic
international standards and guidelines are, for example, set by commissions of the United Nations, such as
IAEA, ICRP, ADR, etc. The licensing, shipping and disposal regulations that govern radioactive sources vary
from nation to nation.
5.2.3 Soft X-ray sources
5.2.3.1 General
Soft X-ray sources emit X-rays in the energy range below 10 keV. Soft X-rays are a very efficient source
for charge conditioning because they have energies that are much higher than the ionization threshold
of all molecules, thus creating an abundance of active ions. This device acts as a bipolar diffusion charge
conditioner, comparable to sources with radioisotopes. A stainless steel or aluminium housing is irradiated
with X-rays from a source. The aerosol flows through the housing from an inlet to an exit port. A radiation
window (e.g. beryllium) protects the X-ray source from particle impact and also attenuates the radiant flux
and radiation energy to adjust the ion concentration. X-ray blockers can prevent X-rays from exiting through
the aerosol ports. While radioisotope sources emit radiation continuously, X-ray sources can be turned on
and off.
Figure 2 shows a schematic example of the design of a soft X-ray charge conditioner. While in this example,
the aerosol flow is directed towards the attenuation window, other designs exist where the flow is reversed.
Key
1 aerosol inlet 5 x-ray source
2 aerosol outlet 6 charging zone
3 x-ray blocker (optional) 7 enclosure
4 attenuation window
Figure 2 — Schematic example for a soft X-ray charge conditioner
5.2.3.2 Licensing and precautions for soft X-ray sources
The use of soft X-ray sources can be regulated by international, national or local government authorities, or
all. Regulations can vary from nation to nation.
Users shall conform to manufacturers’ instructions.

5.2.4 Corona discharge
Corona discharge can function as a source for both negative and positive ions in the carrier gas. Either a
single corona electrode operated with DC-high voltage (for ions of one polarity) or with AC-high voltage (for
two ion polarities), or two separate corona electrodes (one for each ion polarity) can be used.
NOTE If an aerosol electrometer is used as a particle detector immediately downstream of the charge conditioner
(without the DEMC), an ion trap is possibly necessary as an additional element to eliminate any remaining free ions
from the charge-conditioned aerosol. Otherwise an aerosol electrometer will measure these free ions as an additional
current.
Figures 3 and 4 show schematic examples of the design of corona discharge charge conditioners.
Key
1 aerosol inlet 6 high voltage
2 charging zone 7 mesh electrode voltage
3 mesh electrode 8 aerosol outlet
4 corona wire 9 enclosure
5 ion generation zone
Figure 3 — Schematic example for a mesh corona discharge charge conditioner
Key
1 aerosol inlet 5 high voltage
2 turbulent charging zone 6 aerosol outlet
3 sheath air inlet 7 enclosure
4 corona needle
Figure 4 — Schematic example for a counter-flow corona discharge charge conditioner

5.3 Charge conditioning
5.3.1 General
In order to calculate the particle size distribution from the measured electrical mobility distribution, a
known particle size-dependent distribution of electrical charges shall be generated on the aerosol particles,
described by the charge distribution function, f (d). Charge conditioners upstream of a DEMC are used for
p
this purpose.
In a gaseous medium containing aerosol particles and a sufficient concentration of unipolar ions or ions
of both polarities, a charge distribution will develop on the particles. As the dominant driving forces are
the random thermal diffusion of the ions and the collision between ions and aerosol particles, the terms
bipolar or unipolar diffusion charging are frequently used for these types of charge conditioning. The main
advantage of diffusion charging over other methods is that it depends only weakly upon aerosol particle
[16]
material. Subclauses 5.3.2 and 5.3.3 describe the characteristics of bipolar and unipolar diffusion
charging.
In some charge conditioner designs, the ion transport is deliberately influenced by AC- or DC-electric fields
and sheath air flows.
The particle charging efficiency depends mainly on the so called N ·t product, which is the concentration
I
of either positive or negative ions, N , multiplied by their residence time, t, which is the interaction time of
I
aerosol particles with the ions.
The N ·t product reached in a radioactive charge conditioner depends on the type and energy of the radiation
I
of the isotope, on the activity and geometry of the sealed source, on the geometry of the housing, on the flow
rate and concentration of the aerosol through the housing and also on the composition of the carrier gas.
Similarly, the N ·t product reached in a soft X-ray charge conditioner depends on the X-ray energy and the
I
radiant flux, the radiation field geometry, the flow rate and concentration of the aerosol flow through the
housing and on the composition of the carrier gas (see Clause 6).
Table 1 gives an overview on charge conditioners. There is a list of literature provided at the end of the
document.
Table 1 — Overview on charge conditioners and selected references
Category Type Reference
a
Radioactive charge conditioner (RC) [1], [7], [49], [50]
Soft-X-ray charge conditioner (SXRC) [1], [31], [32], [49] – [53]
Bipolar charge condition-
ers
Bipolar corona ionizer (BCI) [54] – [59]
a
Surface-discharge microplasma aerosol charger (SMAC) [3], [60] – [64]
Positive unipolar corona discharge (PCD) charge conditioner
Unipolar charge condi- [5], [12], [49], [50], [65]
tioners – [67]
Negative unipolar corona discharge (NCD) charge conditioner
a
Can also be applied for unipolar charge conditioning.
5.3.2 Bipolar charge conditioners
Bipolar charge conditioners (also traditionally called aerosol neutralizers) produce ions of both polarities
(i.e. positive and negative ions). Neutral particles can acquire charge while highly charged particles can
discharge themselves by capturing ions of the opposite polarity. Bipolar charge conditioners differ by the
way the ions are generated.
— Radioactive bipolar diffusion charge conditioners generate ions in the carrier gas by α- or β-radiation
from a radioactive isotope.
— X-ray bipolar diffusion charge conditioners use soft-X-rays (< 10 keV) for ion generation in the carrier gas.

In these two charge conditioner types, the ions are produced directly in the carrier gas and diffuse to the
aerosol particles by Brownian motion.
— Bipolar corona ionizers (BCI) use an arrangement of two DC-corona ionizer stages (one for each polarity).
Ions of opposite charge are produced in separate sections and are subsequently mixed with the aerosol.
In another variant, bipolar ions are produced by AC-corona discharging.
5.3.3 Unipolar charge conditioners
Besides the widely used bipolar steady-state charge distribution, unipolar charge conditioning can also be
used to achieve a defined charge distribution. In a unipolar charge conditioner, ions of either positive or
negative polarity are produced (e.g., by a corona discharge process or separation of one ion polarity in an
electric field). Like in bipolar charging, diffusion charging is advantageous because variations caused by the
composition of the particles can be neglected for diffusion charging.
Unipolar charging can achieve higher charging probabilities than bipolar charging. This is an advantage if
small particles (d < 20 nm) are to be measured. Due to the higher charging probability, more particles are
classified by the DEMC. This leads to better counting statistics in a DMAS. On the other hand, larger particles
(d > 100 nm) carry significantly more multiple charges compared to bipolar charge conditioning. This makes
the data inversion more complex and reduces the size resolution of large particles. A variety of unipolar
charge conditioners for aerosol particles have been described and built; see, for example, References [44],
[45], [46], [47] and [48].
Corona discharge is produced by a strong nonuniform electrostatic field, such as that between a needle and
plate or a concentric thin wire and a tube. The electric field and space charge effects result in repulsion of
ions of polarity opposite to that of the wire which can lead to positively or negatively charged particles.
There are two designs for corona discharge charge conditioners:
— Negative corona discharge charge conditioner.
The discharge electrode is held at high negative potential. The free electrons are repelled from the
electrode and can attach to air molecules to form negative ions. Ozone is generated as a by-product
which makes this design not favourable for aerosol charging.
— Positive corona discharge charge conditioner.
In positive corona discharge charge conditioners, the discharge electrode (wire or tip) is held at
high positive potential. In this case the free electrons from the corona discharge are attracted to the
electrode and do not need to be absorbed. Most commercially available charge conditioners use positive
ions due to the fact that the process is stable by controlling the corona current and the emission of ozone
can be avoided.
Among the group of positive corona charge conditioners are indirect corona charge conditioners and
turbulent jet charge conditioners. Indirect corona charge conditioners shield the particle charging zone
from the corona discharging zone in order to reduce particle losses. A grounded electrode in the aerosol
flow can be applied as a trap for excess ions. Turbulent jet charge conditioners completely separate the
ion generation from the particle charging zone. This leaves the charging zone free of electrical fields
and reduces particle losses to a minimum. Ions are transported into the particle charging zone by an
additional flow, which dilutes the aerosol flow at the exit.
NOTE Corona charge conditioners that apply field charging, in contrast to diffusion charging, are not considered
for measurement purposes here because of their increased particle material dependence.
Other charge conditioning processes such as static electrification, photoionization, thermionic emission,
self-charging of radioactive particles and agglomeration are not considered because of their very restricted
controllability and usability to charge conditioning in measuring devices. However, some of these processes
should be taken into account as disturbances.

5.4 The charge distribution function
5.4.1 General
The particle size-dependent charge distribution function, f (d), shall be known in order to calculate the size
p
distribution of airborne particles classified in a DEMC. A charge conditioner is used at the entrance of a
DEMC to achieve a conditioned charge distribution which is independent of the initial charge state of the
aerosol particles. The conditioned charge distribution is, at least for typical aerosol residence times in a
DEMC, in a steady state or stable.
f (d) may then be given by a set of equations or tabulated data, approximating the size-dependent charge
p
distribution by either theoretical models or empirical data, or both.
5.4.2 Charge distribution function for radioactive bipolar charge conditioners
For commercially available radioactive bipolar charge conditioners, the charge distribution function under
standard conditions (spherical particles in air: 293,15 K, 101,3 kPa) is given by Formulae (A.10) and (A.11)
which are derived from an approximation to the theoretical models (see Reference [17] in combination
with the result from Reference [18]). Table 2 shows some numerical results. Unless explicitly specified
differently in the measurement report, Formulae (A.10) and (A.11) or values in Table 2 shall be used for the
determination of f (d) for radioactive bipolar charge conditioners.
p
Table 2 shows the bipolar charge distribution f (d) for spherical particles in air (293,15 K, 101,3 kPa),
p
produced by radioactive charge conditioners (see Formulae (A.10) and (A.11)).
Table 2 — Bipolar charge distribution f (d) produced by radioactive charge conditioners
p
Charge distribution
d
(nm)
-6 -5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6
1 0 0 0 0 0 0,004 8 0,999 3 0,004 5 0 0 0 0 0
2 0 0 0 0 0 0,008 3 0,974 2 0,007 5 0 0 0 0 0
5 0 0 0 0 0 0,022 5 0,969 3 0,018 9 0 0 0 0 0
10 0 0 0 0 0 0,051 4 0,912 4 0,041 1 0 0 0 0 0
20 0 0 0 0 0,000 2 0,109 6 0,793 1 0,084 6 0,000 1 0 0 0 0
50 0 0 0 0 0,011 4 0,222 9 0,581 4 0,169 6 0,006 6 0 0 0 0
100 0 0 0,000 1 0,003 7 0,056 1 0,279 3 0,425 9 0,213 8 0,031 7 0,001 7 0 0 0
200 0 0,000 5 0,005 3 0,034 0 0,121 1 0,264 1 0,299 1 0,204 3 0,071 9 0,015 3 0,001 8 0,000 1 0
500 0,006 7 0,020 7 0,050 4 0,098 0 0,149 0 0,181 6 0,181 8 0,140 3 0,089 1 0,044 0 0,017 3 0,005 4 0,001 4
1 000 0,035 7 0,058 4 0,085 4 0,111 3 0,126 1 0,138 5 0,123 5 0,103 9 0,075 4 0,050 0 0,029 3 0,015 4 0,007 2
5.4.3 Charge distribution functions for other bipolar and unipolar charge conditioners
Calculations of the respective charge distribution functions for non-radioactive bipolar and unipolar charge
conditioners are complicated and require careful experimental verification before usage in data inversion
routines for the analysis of measured data.
The theoretical concepts described in Annex A can be helpful for the experienced user to calculate a charge
distribution function for a given bipolar charge conditioner. Annex A also shows an example of the charge
distribution function for a charge conditioner based on an X-ray ionization source.
The charging of particles in unipolar charge conditioners depends on individual designs and operating
parameters. Therefore, no general approximation for the charge distribution function can be given for the
variety of unipolar charge conditioners.

6 Factors influencing the resulting charge distribution
6.1 General
The purpose of operating a charge conditioner, as described in this document, is to achieve either:
—a steady state;
— equilibrium charge distribution (bipolar charge conditioners);
—a well characterized and repeatable charge distribution (unipolar charge conditioners).
If charge conditioning is done for particle measurement purposes (e.g. for measurements with the DMAS),
any changes in the aerosol properties during the charging process must be negligible with respect to the
result of the measurement.
All charge conditioners described have individual permissible upper particle concentration limits and
particle size dependent charging efficiencies. Their respective charge distribution functions, f (d), depend
p
on ion concentrations, ion mass and ion mobility, as well as on residence time and concentration of particles.
Situational factors such as carrier gas composition, purity, humidity and temperature can also influence the
performance. Charge conditioner manufacturers should provide the respective charge distribution function
and describe the conditions under which the device performs in a predictable way and does not produce
artefacts.
Considering the influence of operating conditions, there is a major difference between bipolar and unipolar
charge conditioners.
— In bipolar charge conditioners, the resulting charge distribution depends on the ion mass distribution
and the ion mobility distribution. Since the necessary time to reach steady state conditions is very small
compared to the residence time of the particles in the charging zone, the steady state charge distribution
will be reached and remain unchanged as long as the particle concentration, in combination with the
particle size distribution, does not exceed the design-based (N ·t-product based) limit of the charge
I
conditioner.
— In unipolar charge conditioners, the achieved mean charge per particle and the charge distribution depend
on the N ·t product. Therefore, besides the ion properties (mass and mobility), the ion concentration
I
profile within the charging region as well as the aerosol flow rate through the charge conditioner must
be maintained within narrow tolerances to guarantee reproducible operation achieving the expected
charge distribution.
Clause 6 covers the influence of aerosol particle and carrier gas characteristics as well as the influence of
charge conditioner operating parameters (and others) on the resulting charge distribution and eventual
changes in aerosol properties. Sub-clauses 6.2 to 6.5 cover topics under the assumption that all other
conditions do not change.
6.2 Aerosol particle characteristics influencing the charge distribution
6.2.1 Particle size and surface area
The mean number of elementary charges per particle increases and the charge distribution widens with
increasing particle size.
Figure 5 shows a comparison
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