ISO 16000-42:2023

Indoor air - Part 42: Measurement of the particle number concentration by condensation particle counters

This document specifies the measurement methods and strategies for determining the total number of airborne particles per unit volume of air indoor, using a condensation particle counter (CPC) for particles approximately between 10 nm to 3 µm. NOTE As the particle number concentration is usually dominated by the ultrafine particle (UFP) fraction, the obtained result can be used as an approximation of the UFP concentration. Quality assurance, determination of the measurement uncertainty and minimal reporting information are also discussed in this document. This document is applicable to indoor environments as specified in ISO 16000-1. This document does not address the determination of bioaerosols or the chemical characterization of particles. Nevertheless, some bioaerosols can be detected by the CPC and then contribute to the measured count of particles.

Air intérieur — Partie 42: Mesurage de la concentration en nombre de particules au moyen de compteurs de particules à condensation

General Information

• Status: Published
• Publication Date: 14-Aug-2023
• ICS: 13.040.20 - Ambient atmospheres
• Technical Committee: ISO/TC 146/SC 6 - Indoor air
• Drafting Committee: ISO/TC 146/SC 6/WG 21 - Strategies for the measurement of airborne particles
• Current Stage: 6060 - International Standard published
• Start Date: 15-Aug-2023
• Due Date: 08-May-2022
• Completion Date: 15-Aug-2023

Overview

ISO 16000-42:2023 - "Indoor air - Part 42: Measurement of the particle number concentration by condensation particle counters" provides standardized methods and strategies for measuring the particle number concentration in indoor environments using a condensation particle counter (CPC). The standard covers particles roughly from 10 nm to 3 µm, and notes that measured counts are often dominated by the ultrafine particle (UFP) fraction (≤100 nm), so CPC results can approximate UFP concentrations. It also sets out quality assurance, uncertainty evaluation and minimal reporting requirements for indoor particle number measurements. Chemical characterization and targeted bioaerosol analysis are outside the scope (though CPCs can register some bioaerosols).

Key technical topics and requirements

• Measurement principle: use of CPC technology (working fluid, optical detection after enlargement) to obtain total particle number concentration.
• Size range and detection: guidance on minimal detection size, detection efficiency and interpretation of CPC results for particles ~10 nm to 3 µm.
• Instrument requirements: minimal CPC performance, nominal and factory-certified flow rates, and instrument settings checks.
• Sampling and strategy: practical sampling recommendations for indoor environments (placement, averaging, temporal resolution).
• Measurement strategies: procedures for estimating average room concentration, assessing resting vs active states, source investigation, infiltration from outdoors or adjoining rooms, measurement in vehicle cabins, and evaluating mitigation success.
• Quality assurance & uncertainty: instrument performance checks (zero/leak tests), periodic calibration, and methods to estimate measurement uncertainty.
• Reporting & interpretation: minimal information to include in reports and contextual checklist items to aid interpretation.
• Informative annexes: real-world concentration examples, particle size distribution methods (DMAS), water-based CPC considerations, and a measurement checklist.

Practical applications and who uses this standard

• Indoor air quality (IAQ) consultants and field technicians conducting particle number monitoring.
• Environmental and occupational health laboratories seeking reproducible CPC-based measurements.
• Building engineers and HVAC professionals assessing ventilation effectiveness, infiltration, or remediation success.
• Researchers studying UFP exposures, source emissions (cooking, smoking, combustion, equipment), and indoor particle dynamics.
• Regulators and policymakers developing indoor air assessment protocols or guidance.

Use cases include baseline IAQ surveys, source identification, evaluating control/mitigation measures (filtration, ventilation), and exposure assessment in homes, offices, schools and vehicle cabins.

Related standards

• ISO 16000-1 - General aspects of sampling strategy (indoor air)
• ISO 16000-34 - Strategies for measurement of airborne particles
• ISO 27891 - Calibration of condensation particle counters
• CEN/TS 16976 - Ambient air particle number concentration methods

Keywords: ISO 16000-42:2023, indoor air, particle number concentration, condensation particle counter, CPC, ultrafine particles, UFP, measurement strategy, quality assurance, indoor environments.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 16000-42
First edition
2023-08
Indoor air —
Part 42:
Measurement of the particle number
concentration by condensation
particle counters
Air intérieur —
Partie 42: Mesurage de la concentration en nombre de particules au
moyen de compteurs de particules à condensation
Reference number
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
ii
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 3
5 Sources of airborne particles . 4
5.1 General . 4
5.2 Combustion of organic material . 5
5.3 Smoking . 5
5.4 Cooking . 5
5.5 Particle formation — Formation of secondary organic aerosol . 5
5.6 Outdoor air . 5
5.7 Other sources . 5
6 Dynamics of ultrafine particles indoors . 6
6.1 General . 6
6.2 Infiltration and exfiltration . 7
6.3 Deposition . 7
6.4 Particle formation, phase transition and coagulation . 7
7 Principle of measurement .8
7.1 General . 8
7.2 Working fluid . 8
7.3 Minimal detection size . 10
7.3.1 General . 10
7.3.2 Optical detection after enlargement . 10
7.3.3 Particle size distribution. 11
7.4 CPC minimal requirement . 11
7.5 General sampling requirements and recommendations .13
8 Measurement strategy .13
8.1 General .13
8.2 Average room concentration . 14
8.2.1 General . 14
8.2.2 Resting state without activity . 15
8.2.3 Resting state with equipment activity . 15
8.2.4 Active state .15
8.3 Source investigation/identification . 15
8.4 Infiltration from outdoor or connecting rooms . 16
8.5 Measurement in vehicle cabins . 17
8.6 Success of control and mitigation measures . 17
9 Quality assurance and uncertainty evaluation .17
9.1 General . 17
9.2 Instrument parameters . 18
9.3 CPC’s settings check . 18
9.4 Performance check, zero check or leak check . 18
9.5 Uncertainty . 19
10 Evaluation and reporting of the results .19
Annex A (informative) Examples of particle number concentrations encountered during
room user activities .21
iii
Annex B (informative) Determination of the particle number size distribution of indoor
aerosol using a differential mobility aerosol spectrometer .22
Annex C (informative) Water-CPCs .25
Annex D (informative) Checklist to collect information useful for interpreting indoor
measurement of particle number concentration .27
Bibliography .31
iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 6,
Indoor air.
A list of all parts in the ISO 16000 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
Introduction
People spend most of their day indoors where they are exposed to various sources of particles. Such
particles can be dust particles, particles from combustion processes such as candles, cooking and
fireplaces. Particles can also be emitted by do-it-yourself activities and the operation of electrical
equipment such as printers. Classical building envelope materials are not efficient to prevent particle
transport between indoor and outdoor environments. Sources of outdoor particles are various
and include traffic and other combustion processes, and industrial and agricultural activities. Air
exchanges are driven by natural infiltration and ventilation, but also mechanical ventilation present in
the building.
All this can result in highly variable levels of indoor particles concentration that are not easily
ascertained or assessed in terms of their impacts on health.
Epidemiological studies have shown that ultrafine particles (UFP) can have a negative impact on
[1]
peoples' health. Due to their very small size they can indeed penetrate deeply into the human body.
Particle measurement instrumentation allows determining either the total particle number
concentration or the particle number size distribution. This document describes the general strategies
for the measurement of indoor sub-micron particles with the focus on determining the total number
concentration.
This document was prepared in response to the need for improved comparability of methods for
particle measurement.
vi
INTERNATIONAL STANDARD ISO 16000-42:2023(E)
Indoor air —
Part 42:
Measurement of the particle number concentration by
condensation particle counters
1 Scope
This document specifies the measurement methods and strategies for determining the total number
of airborne particles per unit volume of air indoor, using a condensation particle counter (CPC) for
particles approximately between 10 nm to 3 µm.
NOTE As the particle number concentration is usually dominated by the ultrafine particle (UFP) fraction,
the obtained result can be used as an approximation of the UFP concentration.
Quality assurance, determination of the measurement uncertainty and minimal reporting information
are also discussed in this document.
This document is applicable to indoor environments as specified in ISO 16000-1.
This document does not address the determination of bioaerosols or the chemical characterization
of particles. Nevertheless, some bioaerosols can be detected by the CPC and then contribute to the
measured count of particles.
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 16000-1, Indoor air — Part 1: General aspects of sampling strategy
ISO 16000-34, Indoor air — Part 34: Strategies for the measurement of airborne particles
ISO 27891, Aerosol particle number concentration — Calibration of condensation particle counters
CEN/TS 16976, Ambient air — Determination of the particle number concentration of atmospheric aerosol
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
aerosol
multi-phase system of solid and/or liquid particles (3.2) suspended in a gas, ranging in particle size
from 0,001 µm to 100 µm
[SOURCE: CEN/TS 16976:2016, 3.2]
3.2
particle
piece of matter with a defined physical boundary
Note 1 to entry: The phase of a particle can be solid, liquid or between solid and liquid, and a mixture of any of the
phases.
[SOURCE: ISO 27891:2015, 3.23]
3.3
fine particle
particle that is less than a few micrometers in diameter
3.4
ultrafine particle
UFP
particle (3.2) with a diameter of 100 nm or less
[SOURCE: ISO 16000-34:2018, 3.8]
3.5
particle number concentration
number of particles (3.2) related to the unit volume of indoor air
[SOURCE: ISO 27891:2015, 3.25, modified — Note 1 to entry and the symbol C have been deleted.]
3.6
detection efficiency
ratio of the concentration reported by an instrument to the actual concentration at the inlet of the
instrument
[SOURCE: ISO 27891:2015, 3.11, modified — the symbol η has been deleted.]
3.7
D
x
particle diameter for which a detection efficiency of the percentage of x is obtained when the CPC result
is compared to the reference concentration
Note 1 to entry: This detection efficiency is a function of the CPC itself, but depends also to some extent on
particle type.
Note 2 to entry: For the purpose of this document, silver particles and test conditions described in ISO 27891 are
considered.
3.8
nominal flow rate
volumetric flow rate indicated on the instrument specification sheet by the manufacturer
Note 1 to entry: The nominal flow rate is that flow rate, which a specific CPC model is designed for by the
manufacturer. The real flow rate of individual instruments can differ from the nominal flow due to manufacturing
tolerances.
[SOURCE: CEN/TS 16976:2016, 3.7]
3.9
factory-certified flow rate
volumetric flow rate of an individual instrument at the time of factory calibration, measured at its inlet
under the actual air conditions, and documented on a check out certificate
[SOURCE: CEN/TS 16976:2016, 3.6]
3.10
actual flow rate
volumetric flow rate of an individual instrument, measured at its inlet under the actual air conditions
Note 1 to entry: It is recommended that the actual flow rate be measured in regular intervals during operation.
[SOURCE: CEN/TS 16976:2016, 3.1]
3.11
calculation flow rate
flow rate which directly relates count rate and particle number concentration
Note 1 to entry: This flow rate is used for instrument internal calculation of the particle number concentration. It
depends on the instrument type and can be nominal, factory-certified or actual inlet flow rate. It can also include
a calibration factor unless the total inlet flow is analysed.
[SOURCE: CEN/TS 16976:2016, 3.3]
3.12
calibration
operation that, under specified conditions, in a first step, establishes a relation between the quantity
values with measurement uncertainties provided by measurement standards and corresponding
indications with associated measurement uncertainties and, in a second step, uses this information to
establish a relation for obtaining a measurement result from an indication
[SOURCE: JCGM 200:2012, 2.39, modified — the notes have been deleted.]
3.13
uncertainty
parameter, associated with the result of a measurement, that characterizes the
dispersion of the values that can reasonably be attributed to the measurand
[SOURCE: JCGM 100:2008, 2.2.3, modified — the notes have been deleted.]
3.14
parallel measurement
measurement from a measuring system that takes samples from the same air over the same time period
[SOURCE: ISO 16000-37:2019, 3.13]
3.15
coincidence error
error that occurs with counting measuring methods when two or more particles are counted
simultaneously as a single particle
Note 1 to entry: Coincidence error is related to particle number concentration, flow velocity through the sensing
zone and the size of the sensing zone.
[SOURCE: CEN/TS 16976:2016, 3.4]
4 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
CPC condensation particle counter
DEG diethylene glycol
DEMC differential electrical mobility classifier
DMAS differential mobility aerosol spectrometer
MPSS mobility particle size spectrometer
QA quality assurance
QC quality control
SES size enhancer stage
SMPS scanning mobility particle sizer
SOA secondary organic aerosol
UFP ultrafine particle
5 Sources of airborne particles
5.1 General
Figure 1 shows the size range of airborne particles associated with different sources. These particles
can be generated by activities or without activities or be transported by air movement. These sources
[2],[3]
are different in their time of action and in the number and type of particles generated .
Figure 1 — Usual size range generated by usual indoor sources of airborne particles
Size range is different from one source to the other. Sources that can influence the indoor UFP
concentration are briefly listed below. Nevertheless, the total number is always clearly driven by the
smallest particles. Neglecting particles bigger than 1 µm will thus have an impact on total number
which is much lower than the total uncertainty of the method.
5.2 Combustion of organic material
[28]
Each combustion of organic material releases particles of different sizes, the majority of which are
particles in the size range <1 µm, primarily in the UFP range <0,1 µm. Research has shown that when
candles are burned, the level of UFP emissions depends on the candle material, but also the burn-up
5 3
process and that high particle number concentrations of more than 10 particles/cm can occur. Fine
and ultrafine particles can also be released into the room air during the operation of fireplaces and
stoves for example.
5.3 Smoking
Smoking is a significant anthropogenic and time-varying source of UFP indoors. Particle number
5 3
concentrations >10 particles/cm are easily produced, depending on the scenario under investigation.
[4],[5]
The use of electronic cigarettes (e-cigarettes) also leads to an increase in indoor air concentration
[6]
of UFPs and PM2,5 .
5.4 Cooking
Cooking activities of various kinds (e.g. baking, frying, deep-frying and toasting) can lead to very
5 3
high increases in ultrafine particle number concentrations (>10 particles/cm ). However, this can
vary greatly depending on the type of activity, energy input, food, ventilation conditions and room
[7],[8]
geometry .
5.5 Particle formation — Formation of secondary organic aerosol
Chemical reactions of the gas and aerosol phase can be responsible for the formation of new SOA
and for the modification of existing particles in indoor environments. SOA are formed mainly in the
presence of unsaturated compounds (e.g. monoterpenes) and ozone, nitrogen oxides and/or hydroxyl
[9]
radicals . User behaviour is also of great importance; for example, the use of chemical cleaning agents
[10]
can produce significant amount of SOA particles .
5.6 Outdoor air
UFPs also enter the interior from outside, in particular through infiltration and ventilation processes.
Typical external air sources are emissions from road traffic, combustion processes of all kinds and
industrial emissions. Photochemically induced secondary formation can also be a relevant source in the
outside air. During prolonged ventilation, the indoor and outdoor concentrations are usually equalized.
After ventilation, the concentration changes again due to prevailing sources, sinks and dynamics.
5.7 Other sources
The operation of office equipment with laser printing functions (printers, copiers, multifunction
devices) releases particles with diameters down to about 300 nm. Commercial and private 3D
printers (e.g. fused filament fabrication printers), which are becoming increasingly popular, process
plastic filaments into 3D objects in typical periods of up to several hours. Fine and ultrafine particles
are emitted into the environment. Devices in the lower and middle price categories are usually not
[11]
equipped with filters. UFP and fine particles of various types, quantities and size distributions are
also produced by processing spray paints and vanishes as well as by material processing, for example,
grinding, sawing or drilling during renovation work and do it yourself activities. Cleaning activities,
in particular vacuuming, can also lead to an increased release of UFP when using equipment without
effective filtration. Such emissions are situation specific and depend to a large extent on the materials,
products and equipment used as well as on the scope and frequency of the activity.
6 Dynamics of ultrafine particles indoors
6.1 General
In addition to the source emission of ultra-fine particles described in Clause 5, there are various dynamic
processes which can affect the measurement result (see Figure 2). Particle number concentrations and
particle size distributions indoors can indeed be subject to high spatial and temporal variability.
Responsible for this are:
— the number of possible emission sources, their spatial arrangement and time-dependent emission
patterns;
— the contribution of the particles penetrating from the outside and associated influencing factors,
such as environmental conditions (outdoor air quality, meteorology) and building conditions
(ventilation conditions, ventilation systems with and without filtering, construction, tightness
location of the object, floor);
— particle transport mechanisms (aerosol dilution, sedimentation, resuspension, thermophoresis and
diffusion);
— the laminar or turbulent air movement and air mixing in the room;
— temperature and humidity;
— conversion by chemical (oxidation) and physical processes (coagulation, evaporation, re-
condensation, gas-particle partitioning).
The processes depend on the concentration, size distribution and chemical composition of the emitted
[12]
primary particles. Compared to coarser particles, UFPs sometimes behave more like gas molecules;
they follow the air flow in the room and are distributed primarily by diffusion processes. In contrast
to coarser particles, sedimentation and resuspension are practically irrelevant for UFP. The speed and
extent of coagulation effects are strongly dependent on the initial concentration, size and width of the
size distribution of the primary particles. In spatially limited areas of very high number concentration
(downstream from a source), for example, coagulation of primary particles can occur much faster than
after homogeneous distribution of the particles over the entire spatial volume with a correspondingly
−3
smaller number concentration. At a concentration below approximately 10 000 cm , coagulation
[13]
effects in the ultrafine particle fraction typically occur only after a few hours .
These aspects should be considered when planning the measurement strategy and also when evaluating
the measurements, for example, by measuring and analysing the time response of an aerosol or different
aerosol size fractions over a longer period of time. In general, it should be considered that aerosol size
fractions can also be present outside the measuring range of the instruments used.
Key
1 exfiltration
2 phase transition
3 deposition
4 coagulation
5 formation
6 infiltration
Figure 2 — Dynamic processes influencing indoor particle pollution according to Reference [2]
6.2 Infiltration and exfiltration
Since a building envelope is never completely sealed, particles from the outside air always enter the
interior (infiltration equals to the input) and vice versa (exfiltration equals to the output). This inflow
and outflow play a major role in the particulate pollution of the interior and is naturally increased
when active ventilation systems are in operation. The infiltration of particles from the outside air can
be described by a size-dependent penetration factor, which is called structural property of a room or
[14]
building .
6.3 Deposition
The main mechanism for the precipitation or deposition of UFP on surfaces is transport by diffusion.
[15]
In purely laminar flow, deposition on surfaces is relatively slow. In practice, however, air flow and
rough surfaces cause turbulence and a significant increase in deposition rate compared to the laminar
case. Furthermore, the deposition rate increases with the decreasing particle diameter.
6.4 Particle formation, phase transition and coagulation
In the interior, particles can be newly formed by chemical and physical processes. Nucleation,
condensation and coagulation can play a role. Conversely, particles can also be converted back into
gaseous components. Coagulation is a process that is particularly effective in high particle number
concentrations and turbulent flow conditions. Therefore, number and size distribution can differ
depending on the distance to a particle source.
Resuspension of UFP is not often observed. When UFPs impact a surface (e.g. wall, furniture), they are
generally permanently linked to this surface.
7 Principle of measurement
7.1 General
In a CPC, particles are grown by vaporizing a working fluid that supersaturates and condenses onto
[16]
the primary particles to form bigger droplets which are then detected using scattered light. This
process is necessary since UFPs are too small to be directly detected by an optical instrument.
There are several different methods to achieve supersaturation needed to initiate condensational
growth:
— laminar flow and diffusional heat transfer;
— turbulent mixing of sample air with particle free gas flow saturated with the working fluid;
— adiabatic expansion of the sample air and working fluid vapour mixture.
7.2 Working fluid
To initiate the condensation growth of particles, a minimum saturation ratio with respect to a
condensable vapour shall be present. The most common CPC is the laminar flow design using alcohol
(see Figure 3), as saturation can be easily obtained with this type of working fluid.
N-butanol CPC shall be the reference method for the determination of the particle number concentration
of atmospheric aerosol in ambient air in Europe, as specified in CEN/TS 16976:2016, and is commonly
used worldwide for this type of measurement. Using similar instrument indoors and outdoors will lead
to a better comparison between measurement results, but indoor toxicity aspect should also be taken
into account and n-butanol is considered as possibly harmful when inhaled over long periods. N-butanol
CPCs have also a very strong smell.
Isopropanol CPC is an alternative as it provides similar results with fewer nuisances indoors. Any alcohol
CPCs can be used, provided measures are taken to filter the exhaust and avoid olfactory nuisances (i.e.
using a carbon trap or another emission removal system). Alcohol released by the instrument can also
interfere with volatile organic compounds (VOCs) and particular care should thus be taken in case of
parallel measurement.
Key
1 aerosol inlet
2 vapour substance reservoir
3 heated saturator
4 UFP
5 thermoelectric cooling and heating device
6 condenser
7 droplet (not true to scale)
8 light source
9 illumination optics
10 receiving optics
11 photodetector
12 aerosol outlet
SOURCE Reproduced from ISO 27891.
Figure 3 — Example of a CPC design
CPCs that use water as working fluid are also regularly used in indoor settings to avoid odour and
[17]
toxicity problems. Strongly hydrophobic particles can possibly not be counted correctly and
particular care should thus be applied to ensure correct results when using water CPCs (see Annex C).
In a mixing type CPC, the supersaturation is achieved by adiabatically mixing the sample air with
heated particle-free air saturated with the working fluid. The grown droplets are counted individually
using an optical particle counter. By changing the mixing ratio between sample and vapour laden clean
air, the supersaturation and thus the smallest detectable particle size can be changed easily. The user
should select the right parameter to leisure with a particle diameter at 50 % of the normalized detection
[18]
efficiency, D , of 10 nm (or 7 nm, see 7.2) .
In an expansion type CPC, the supersaturation is achieved through adiabatic expansion of a gas mixture
containing both aerosol sample and the working fluid vapour. Expansion triggers particle growth
through condensation. In a typical form of expansion CPCs, the optical detection is done by measuring
the scattered light of a population of growing droplets. Through Mie-theory, the particle number
concentration can be calculated. It is not recommended to use this type of CPC as the uncertainty of
[19]
measurement is higher than with other techniques .
7.3 Minimal detection size
7.3.1 General
The number concentration of enlarged droplets is equal to the number concentration of the primary
particles (condensation nuclei) with a size larger than the Kelvin diameter determined by the
supersaturation achieved in the instrument. Minimal detection size is thus an intrinsic characteristic
of the instrument and it can deeply influence the final counting result. No comparison between
instruments with different D is possible without additional experimental effort. The lower detection
limit of a CPC is typically controlled by the difference between saturator and condenser, and carefully
maintained at a constant level.
CPCs with a D of 10 nm have been selected as reference for this document.
Using a CPC with a D = 7 nm instead of 10 nm is allowed.
NOTE 1 National regulations can apply.
NOTE 2 Many CPCs can detect particles down to 5 nm. Alcohol CPCs optimized for the small size range (UF-
CPCs) use the same particle counting technique as conventional CPCs but are further optimized to enlarge very
small particles more effectively and reduce losses, which allow detection down to 3 nm. To extend the detectable
size range of an alcohol CPC further, a SES is used upstream. By using DEG, a working fluid with low vapour
[20]
pressure and high surface tension, even extremely small nuclei down to 1 nm can be enlarged and counted.
In the latest design of water CPCs, an additional conditioning section in which the aerosol is brought to 100 %
relative humidity at ambient temperature is added before entering the heating section in order to better specify
the state of the aerosol and improve the counting efficiency for different aerosol materials down to about 1 nm.
[20]
Increasing the sensitivity down to smaller sizes can provide insight into particle formation processes, but
also increases the cost and the maintenance effort. Losses are also much higher and makes data interpretation
sometimes complex. Taking all aspects into account, 10 nm is a good consensus. This value is also expected to be
adopted in the future for ambient air monitoring and car exhaust regulations.
NOTE 3 In the automotive sector, a D of 23 nm is used as of the date of publication of this document, but more
and more epidemiologic studies push to decrease this value to 10 nm in future regulations as a large fraction of
the engine emission is for the moment neglected. This will also provide a better estimate of the real impact that
the automotive sector has on ambient air quality.
7.3.2 Optical detection after enlargement
7.3.2.1 General
The droplets produced by the condensation process are then transported through a light beam. The
light scattered by the droplets is collected by a receiving optic under a defined solid angle (receiver
aperture) and guided onto a detector (e.g. photodiode).
7.3.2.2 Single count detection
At low concentrations, the droplets cross the light beam one after another, thus producing single
electrical pulses at the detector output. From the count rate of these pulses and the calculation flow
rate, the total number concentration of particles per unit of volume can be determined.
7.3.2.3 Coincidence correction
There is always a finite probability that two or more particles will transit a CPC’s optical detection
system in a given time interval. The resulting electrical pulses can become indistinguishable from
one another, depending on their magnitudes and shapes, the time difference(s) between them, and the
signal processing method and speed for individual pulses. This occurrence is known as a coincidence
event. As the concentration of particles entering the CPC increases, the probability of a coincidence
event increases, and the coincidence error leads to a systematic underestimation of the actual particle
number concentration.
Most manufacturers specify concentration ranges for which coincidence events can be neglected
4 3 5 3
(usually up to 10 particles/cm to 10 particles/cm ). Modern CPCs have electronic circuitry capable of
compensating it automatically by built in algorithms (so called dead-time correction algorithms; “single
count”, “single count mode with continuous lifetime coincidence correction”, “correctional mode” and
other synonyms all have essentially the same meaning) to a certain limit (usually up to 10 particles/
3 6 3
cm to 10 particles/cm ).
7.3.2.4 Photometric mode
For higher particle number concentrations, single pulses are no longer used to determine the particle
number concentration, but the light scattered by the whole population of particles present in the
sensing volume is used as an analogue signal (photometric method).
Since in the ideal case droplet growth due to condensation yields the same size independently of the size
of the condensation nuclei and since the optical properties of the droplets are determined essentially by
the condensing material, there is, in principle, a linear relationship between this photometer signal and
the particle number concentration.
Determining this relationship requires a strict calibration which is quite difficult to obtain in the
field. Furthermore, very high particle number concentrations lead also to a depletion of the vapour
concentration by the condensation process. This leads not only to nonlinearity of the calibration curve
but also influences the lower detection limit of the CPC. Also, contamination of the optical surfaces will
change the measured photometric signal.
To avoid higher concentration counting, dilution systems can be used upstream of the CPC. Dilution is
another factor to control in the field and also increases measurement uncertainty.
Due to these effects, the uncertainty of the number concentration measured using photometric mode is
higher compared to concentration measured using single particle counting mode.
7.3.3 Particle size distribution
As the final droplet size is almost independent of the initial diameter of the particle, a CPC does not
provide information about the initial size of the particles.
If size information is required, a DEMC should be used. The resulting combination with a CPC is known
as DMAS. Further information is given in Annex B.
Although the information about the size is important as this parameter often drives the toxicity of the
particles, the scope of this document has been limited to the determination of the total particle number
concentration in the size range of the CPC.
7.4 CPC minimal requirement
CPC minimal performance criteria are listed in Table 1. All criteria refer to the counting mode of the
CPC, including counting with coincidence correction, after any pre-determined calibration factors have
been applied.
The CPC’s counting efficiency shall be traceably calibrated by a reputable calibration laboratory and
based on an internationally accepted and accessible procedure. Silver particles shall be used to run
these tests. Follow ISO 27891.
For other criteria, it is recommended to follow the test procedures specified in CEN/TS 16976.
Table 1 — Main CPC performance criteria
No. Performance characteristic Criteria
Actual flow rate ≤5 % of the difference to the nominal flow rate
≤2 % of the difference to the factory-certified flow rate
–3
Number concentration measurement range ≤100 cm (based on at least 1 500 particle counts)
–3
Lower limit ≥10 000 cm (including coincidence correction)
Upper limit At least 3 orders of magnitude
Dynamic range If the measured concentration reaches more than 95 %
of the upper limit specified by the manufacturer, the
measurement result should be rejected.
Number concentration detection limit Smaller than lower limit of number concentration meas-
urement range
Concentration response
4 Slope 1 ± 0,05
Linearity All residuals <4 % of the measured value
Detection efficiency at low particle size D = 10 nm ± 1,0 nm
(or 7 nm ± 1,0 nm – see 7.3.1)
Diameter for which the normalized efficiency reaches
90 %, D D < 20 nm
90: 90
Detection efficiency at intermediate particle >95 % at 40 nm ± 10 nm
sizes
7 Upper particle size detection limit >90 % detection efficiency at 1 000 nm ± 100 nm
–1
8 Zero count rate <1 min
Response time t < 5 s
rise
t < 5 s
fall
tt−
rise fall
< 10 % or < 0,5 s
t
rise
where
t is the time taken by the CPC signal to reach 95 %
rise
of the final concentration, when a change in the
concentration applies to the CPC from a value
–3
below 100 cm to a constant concentration above
–3
5 000 cm (usually near zero using an absolute
filter);
t is the time taken by the CPC signal to decrease by
fall
95 %, when a change in the concentration applies
–3
to the CPC from a value above 5 000 cm to a
–3
concentration below 100 cm (usually near zero
using an absolute filter).
10 Dependence of flow rate on supply voltage ≤5 %
Maximum uncertainty of temperature and T ≤ 3 K
pressure sensors
P ≤ 1 kPa
12 Effect of failure of main voltage Instrument parameters shall be secured against loss.
The instrument shall enable the following parameters to be recorded in 1 min time intervals:
— date, start time and end time of each reported concentration;
— calculation flow rate;
–3
— raw concentration (count rate divided by the ca
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