EN 14625:2005

SLOVENSKI STANDARD
01-september-2005
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Ambient air quality - Standard method for the measurement of the concentration of
ozone by ultraviolet photometry
Luftqualität - Messverfahren zur Bestimmung der Konzentration von Ozon mit Ultraviolett
-Photometrie
Qualité de l'air ambiant - Méthode normalisée pour le mesurage de la concentration en
ozone par photométrie U.V.
Ta slovenski standard je istoveten z: EN 14625:2005
ICS:
13.040.20 Kakovost okoljskega zraka Ambient atmospheres
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 14625
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2005
ICS 13.040.20
English version
Ambient air quality - Standard method for the measurement of
the concentration of ozone by ultraviolet photometry
Qualité de l'air ambiant - Méthode normalisée de mesurage Luftqualität - Messverfahren zur Bestimmung von Ozon in
de la concentration d'ozone par photométrie UV Luft mit dem UV-Verfahren
This European Standard was approved by CEN on 10 December 2004.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Central Secretariat or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 14625:2005: E
worldwide for CEN national Members.

Contents
Page
Foreword.4
1 Scope .5
2 Normative references .5
3 Terms and definitions .5
4 Symbols and abbreviated terms .9
5 Principle.12
5.1 General.12
5.2 Measuring principle.12
5.3 Type approval test .13
6 Sampling equipment .13
6.1 General.13
6.2 Sampling location.13
6.3 Sampling inlet and sampling line.14
6.4 Particulate filter.14
6.5 Control and regulation of sample flow rate .15
6.6 Sampling pump for the manifold.15
7 Analyser equipment .15
7.1 General.15
7.2 Interferences .15
7.3 Ultraviolet absorption cell.15
7.4 Ultraviolet source lamp .16
7.5 UV detector.16
7.6 Ozone-specific scrubber.16
7.7 Switching valve.16
7.8 Temperature indicator.16
7.9 Pressure indicator .16
7.10 Flow rate indicator.17
7.11 Sampling pump for the analyser.17
7.12 Residence time in the sampling system and inside the analyser .17
7.13 Internal ozone span source .17
8 Type approval of ultraviolet photometric ozone analysers.17
8.1 General.17
8.2 Relevant performance characteristics and performance criteria .17
8.3 Design changes .20
8.4 Procedures for determination of the performance characteristics during the laboratory test .20
8.5 Determination of the performance characteristics during the field test.30
8.6 Expanded uncertainty calculation for type approval.34
9 Field operation and ongoing quality control .34
9.1 General.34
9.2 Suitability evaluation.34
9.3 Initial installation.35
9.4 Ongoing quality control .36
9.5 Calibration of the analyser.37
9.6 Checks .38
9.7 Maintenance .41
9.8 Data handling and data reports.41
10 Expression of results .42
11 Test reports and documentation.42
11.1 Type approval tests.42
11.2 Field operation .42
Annex A (normative) Correction for ambient nitric oxide .44
Annex B (normative) Calculation of lack of fit.45
Annex C (informative) Sampling equipment.47
Annex D (informative) Sampling on micro scale.49
Annex E (informative) Ultra violet photometric analyser .50
Annex F (informative) Manifold testing equipment.51
Annex G (normative) Type approval.52
Annex H (normative) Calculation of uncertainty in field operation at the hourly alert threshold value.74
Bibliography.86

Foreword
This document (EN 14625:2005) has been prepared by Technical Committee CEN/TC 264 “Air quality”, the
secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an identical text or
by endorsement, at the latest by September 2005, and conflicting national standards shall be withdrawn at the
latest by September 2005.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark,
Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
1 Scope
This document specifies a continuous measurement method for the determination of the concentration of ozone
present in ambient air based on the ultraviolet photometric measuring principle. This document describes the
performance characteristics, and sets the relevant minimum criteria required to select an appropriate ultraviolet
photometric ozone analyser by means of type approval tests. It also includes the evaluation of the suitability of an
anayser for use in a specific fixed site so as to meet the Directives data quality requirements and requirements
during sampling, calibration and quality assurance.
The method is applicable to the determination of the mass concentration of ozone present in ambient air in the
3 3
range from 0 µg/m to 500 µg/m . This concentration range represents the certification range for the type approval
test.
3 3
NOTE 1 0 µg/m to 500 µg/m of O corresponds to 0 nmol/mol to 250 nmol/mol of O .
3 3
The method covers the determination of ambient air concentrations of ozone in zones classified as rural areas,
urban and urban-background areas.
NOTE 2 Other ranges may be used for measurement systems applied at rural locations monitoring Ecosystems.
The results are expressed in µg/m (at 20 °C and 101,3 kPa).
When the standard is used for other purposes than the EU-directive, the range and uncertainty requirements need
not apply.
2 Normative references
The following referenced documents are indispensable for the application 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.
ENV 13005, Guide to the expression of uncertainty in measurement
EN ISO 14956:2002, Air quality — Evaluation of the suitability of a measurement procedure by comparison with a
required measurement uncertainty (ISO 14956:2002)
ISO 13964:1998, Air quality — Determination of ozone in ambient air — Ultraviolet photometric method
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
3.1
ambient air
outdoor air in the troposphere excluding workplace air
3.2
sample gas temperature
temperature at the sampling inlet outside the monitoring station

1)
As stated in the relevant EU-legislation.
3.3
availability of the analyser
fraction of the total time period for which usually valid measuring data of the ambient air concentration is available
from an analyser
3.4
calibration
comparison of theanalyser response to a known gas concentration with a known uncertainty
3.5
certification range
concentration range for which the analyser is type approved
3.6
combined standard uncertainty
calculation result of combining the uncertainties determined from all performance characteristics specified in this
document according to the prescribed procedures given in this documentd
3.7
coverage factor
numerical factor used as a multiplier of the combined standard uncertainty in order to obtain an expanded
uncertainty
3.8
designated body
body which has been designated for a specific task (type approval tests and/or QA/QC activities in the field) by the
competent authority in the Member States
NOTE It is recommended that the designated body is accredited for the specific task according to EN ISO 17025.
3.9
expanded uncertainty
quantity defining an interval about the result of a measurement that may be expected to encompass a large fraction
of the distribution of values that could reasonably be attributed to the measurand
NOTE for the purpose of this document the expanded uncertainty is the combined standard uncertainty multiplied by a
coverage factor k = 2 resulting in an interval with a level of confidence of 95 %
3.10
fall time
difference between the response time (fall) and the lag time (fall)
3.11
independent measurement
individual measurement that is not influenced by a previous individual measurement by separating two individual
measurements by at least four response times
3.12
individual measurement
measurement averaged over a time period equal to the response time of the analyser
3.13
influence quantity
quantity that is not the measurand but that affects the result of the measurement (VIM 2.7), either an interferent
influence quantity (i.e. the concentration of a substance in the air under investigation that is not the measurand), or
an external influence quantity (i.e. a quantity that is not the measurand nor the concentration of a substance in the
air mass under investigation)
NOTE Examples are:
 presence of interfering gases in the flue gas matrix (interferent influence quantity);
 temperature of the surrounding air (external influence quantity);
 atmospheric pressure (external influence quantity);
 pressure of the gas sample (external influence quantity).
3.14
interference
response of the analyser to interferents
3.15
interferent
component of the air sample, excluding the measured constituent, that effects the output signal
3.16
initial ozone concentration
ozone concentration in the ambient air just before entrance into the sampling inlet
3.17
lag time
time interval from the instant at which a step change of sample concentration occurs at the inlet of the analyser to
the instant at which the output reading reaches a level corresponding to 10 % of the stable output reading
3.18
lag time (fall)
lag time for a negative step change
3.19
lag time (rise)
lag time for a positive step change
3.20
limit value
level fixed on the basis of scientific knowledge, with the aim of avoiding, preventing or reducing harmful effects on
human health and/or the environment as a whole, to be attained within a given period and not to be exceeded once
attained
3.21
lack of fit
maximum deviation of the average of a series of measurements at the same concentration from the linear
regression line
3.22
long-term drift
difference between zero or span readings over a determined period of time (e.g. period of unattended operation)
3.23
monitoring station
enclosure located in the field in which an ozone analyser has been installed in such a way that its performance and
operation comply with the prescribed requirements
3.24
parallel measurement
measurements from different analysers, sampling from one and the same sampling manifold, starting at the same
time and ending at the same time
3.25
performance characteristic
one of the parameters assigned to equipment in order to define its performance
3.26
performance criterion
limiting quantitative numerical value assigned to a performance characteristic, to which conformance is tested
3.27
period of unattended operation
time period over which the drift is within the performance criterion for long-term drift
3.28
repeatability (of results of measurement)
closeness of the agreement between the results of successive individual measurements of the same measurand carried out
under the same conditions of measurement [1]
NOTE These conditions are called laboratory repeatability conditions and include:
-the same measurement procedure;
-the same observer,
-the same analyser, used under the same conditions;
-at the same location;
-repetition over a short period of time.
3.29
reproducibility under field conditions
closeness of the agreement between the results of simultaneous measurements with two analysers in ambient air
carried out under the same conditions of measurement
NOTE 1 These conditions are called field reproducibility conditions and include:
the same measurement procedure;
two identical analysers, used under the same conditions;
at the same monitoring station;
the period of unattended operation.
NOTE 2 In this document the reproducibility under field conditions is expressed as a value with a level of confidence of 95 %
3.30
residence time inside the analyser
time period for the sampled air to be transported from the inlet of the analyser to the outlet of the absorption cell
3.31
residence time in the sampling system
time period for the sampled air to be transported from the sampling inlet (of the sampling system) to the inlet of the
analyser
3.32
response time
time interval from the instant at which a step change of sample concentration occurs at the inlet of the analyser to
the instant at which the output reading reaches a level corresponding to 90 % of the stable in output reading

3.33
response time (fall)
response time at a negative step change
NOTE Response time (fall) is the sum of the lag time (fall) and the fall time.
3.34
response time (rise)
response time at a positive step change
NOTE Response time (rise) is the sum of the lag time (rise) and the rise time.
3.35
rise time
difference between the response time (rise) and the lag time (rise)

3.36
sampled air
ambient air that has been sampled through the sampling inlet and sampling system
3.37
sampling inlet
entrance to the sampling system where ambient air is collected from the atmosphere
3.38
short-term drift
difference between zero or span readings at the beginning and end of a 12 h period
3.39
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation
[ENV 13005]
3.40
surrounding temperature
temperature of the air directly surrounding the analyser (temperature inside the monitoring station or laboratory)
3.41
type approval
decision taken by a designated body that the pattern of an analyser conforms to the requirements as laid down in
this document
3.42
type approval test
examination of two or more analysers of the same pattern which are submitted by a manufacturer to a designated
body; including the tests necessary for approval of the pattern
3.43
uncertainty(of measurement)
parameter associated with the result of a measurement that characterises the dispersion of the values that could
be attributed to the measureand
4 Symbols and abbreviated terms
For the purposes of this document, the following symbols and abbreviated terms apply.
A availability of the analyser
a
Av average concentration of the measurand during the field test
b sensitivity coefficient of the analyser to sample gas pressure change expressed as a percentage of the
gp
measured value, obtained during the laboratory type approval test
b sensitivity coefficient of the analyser to sample gas temperature change
gt
b sensitivity coefficient of the analyser to surrounding air temperature change
st
b sensitivity coefficient of the analyser to electrical voltage change
V
C O concentration of the applied gas
C average concentration of the measurements at sampling gas pressure P
P1 1
C average concentration of the measurements at sampling gas pressure P
P2 2
C average concentration of the measurements at span level at the beginning of the drift period
s,1
C average concentration of the measurements at span level at the end of the drift period
s,2
c test gas concentration
t
C average concentration of the measurements at sample gas temperature T
T1 1
C average concentration of the measurements at sample gas temperature T
T2 2
C average concentration reading of the measurements at voltage V
V1 1
C average concentration reading of the measurements at voltage V
V2 2
C average concentration of the measurements at zero at the beginning of the drift period
z,1
C average concentration of the measurements at zero at the end of the drift period
z,2
av
average of at least four independent measurements during the constant concentration period (t )
c
C
const
av
average of at least four independent measurements during the variable concentration period (t )
v
C
var
average difference of parallel measurements
d
f
d the ith difference in a parallel measurement
f,i
D long-term drift at span concentration c
l,s t
D long-term drift at zero
l,z
D short-term drift at span level
s,s
D short-term drift at zero
s,z
D difference sample/calibration port
sc
E sample system collection efficiency
ss
F response factor in concentration units per voltage output of the analyser
r
P sampling gas pressure P
1 1
P sampling gas pressure P
2 2
R mean analyser response to the test gas directly sampled by the analyser
d
r reproducibility under field conditions
f
r absolute reproducibility in the field
f,abs
R mean analyser response to the test gas via the sample manifold
m
s repeatability standard deviation at zero
r,z
s repeatability standard deviation at concentration c
r,ct t
s reproducibility standard deviation under field conditions
r,f
s repeatability standard deviation
l
T surrounding air temperature
T sample gas temperature T
1 1
T sample gas temperature T
2 2
t relative difference between response time (rise) and response time fall
d
t response time (fall)
f
t surrounding air temperature at the laboratory
l
t two-sided Students t-factor at a confidence level of 0,05, with n-1 degrees of freedom
n-1, 0,05
t response time (rise)
r
t time period of the field test minus the time for calibration, conditioning and maintenance
t
t total time period with validated measuring data
u
t whole number of t and t pairs
V O3 zero
V minimum voltage V (V) specified by the manufacturer
1 min
V maximum voltage V (V) specified by the manufacturer
2 max
average of measurements
x
x first average of the measurements at T just after calibration
1 l
x second average of the measurements at T just before calibration
2 l
x average of the measurements at zero
z
x average of the measurements at concentration c
ct t
X averaging effect
av
x average of the measurements using the calibration port
c
X influence quantity of H O with concentration 19 mmol/mol
H2O,z,ct 2
x the ith measurement
i
influence quantity of the interferent at concentration c
X t
int,ct
X influence quantity of the interferent at zero
int,z
X lack of fit (largest residual from the linear regression function)
l
x average of the measurements using the sample port
s
X difference between the readings of the recent zero check and the most recent calibration
s
x average of the measurements at T or T
T min max
X influence quantity of benzene with concentration 1 µmol/mol
benx,z,ct
X difference between the readings of two consecutive zero checks
z
(x ) the ith measurement result of analyser 1
1,f i
(x ) the ith measurement result of analyser 2 at the same time as the measurement of analyser 1
2,f i
Z reading of the first zero check
Z reading of the second zero check
∆P measured pressure drop induced by the manifold pump
m
∆R change in the analyser’s response due to the influence of the pressure drop induced by the manifold
a
pump, expressed as a percentage
5 Principle
5.1 General
This document describes the method for measurement of the concentration of ozone in ambient air by means of
ultraviolet photometry. The requirements, the specific components, the ultraviolet photometric analyser and its
sampling system are described. For the analyser a number of performance characteristics with associated
minimum performance criteria are given. The actual values of these performance characteristics for a specific type
of analyser have to be determined in a so-called type approval test for which procedures have been described. The
type approval test comprises a laboratory and a field test. The selection of a suitable analyser for a specific
measuring task in the field is based on the calculation of the combined expanded uncertainty of the measuring
method. In this combined expanded uncertainty calculation the actual values of the various performance
characteristics of a type approved analyser and the site-specific conditions at the monitoring station are taken into
account. The expanded uncertainty of the method shall meet the requirements of the (EU) legislation. For ongoing
measurements in the field, requirements and recommendations for quality assurance and quality control are given
(see 9.4).
5.2 Measuring principle
Sampled air is drawn continuously through an optical absorption cell where it is irradiated by monochromatic
radiation, centred on 253,7 nm, from a stabilised low-pressure mercury (Hg) discharge lamp. The UV radiation,
which passes through the absorption cell, is measured by a sensitive photodiode or photomultiplier detector and
converted to a measurable electrical signal. Absorption of this radiation by the sampled air within the absorption
cell is a measure of the ozone concentration in the ambient air.
Two different systems for the measurement of the ultraviolet absorption by ozone are usually employed.
NOTE In one system the ultraviolet absorption by ozone is determined by means of the difference in ultraviolet absorption
between a sample cell and a reference cell (dual-cell type).
In the other system only a single sample cell is employed. The ultraviolet absorption of ozone is determined by
alternately supplying sampled air containing ozone to the absorption cell and ozone-free sampled air. Ozone-free
sampled air is obtained by passing the sampled air through an ozone catalytic converter in which the ozone is
destroyed.
Most modern commercial ozone analysers measure the temperature and pressure of the sampled air in the
absorption cell. Using these data an internal microprocessor automatically calculates the measured ozone
concentration relative to some chosen reference conditions. For analysers without this automated pressure and
temperature compensation, the concentrations need to be corrected manually to the chosen reference conditions.
The concentration of ozone is measured in volume/volume units (if the analyser is calibrated using a
volume/volume standard). The final results for reporting are expressed in µg/m using standard conversion factors
(see Clause 10).
5.3 Type approval test
The type approval test is based on the evaluation of performance characteristics determined under a prescribed
series of tests. In this document test procedures are described for the determination of the actual values of the
performance characteristics for at least two analysers in a laboratory and two analysers in the field. These tests
shall be performed by a designated body. The type approval of an analyser is based on the calculation of the
combined expanded uncertainty in the measurement result derived from the numerical values of the tested
performance characteristics and compared to a prescribed maximum uncertainty.
The type approval of an analyser and subsequent OA and QC procedures provide evidence that the defined
requirements concerning data quality laid out in relevant EU directives can be satisfied. Appropriate experimental
evidence shall be provided by:
 type approval tests performed under conditions of intended use of the specified method of measurement,
and
 calculation of expanded uncertainty of results of measurement by reference to ENV 13005.
Field operation and quality control:
Prior to the installation and operation of a type approved analyser at a monitoring station, an expanded uncertainty
calculation shall be performed with the actual values of the performance characteristics, obtained during the type
approval tests, and the site-specific conditions at that monitoring station. This calculation shall be used to
demonstrate the suitability of a type-approved analyser under the actual conditions present at that specific
monitoring station.
After the installation of the approved analyser at the monitoring station its correct functioning shall be tested.
Requirements for quality assurance and quality control are given for the operation and maintenance of the
sampling system, as well as for the analyser, to ensure that the uncertainty of subsequent measurement results
obtained in the field is not compromised.
6 Sampling equipment
6.1 General
Depending on the installation of the ultraviolet photometric analyser at a monitoring station, a single sampling inlet
for the analyser may be chosen. Alternatively sampling can take place from a common sampling inlet with a
sampling manifold to which other analysers and equipment may be attached. Conditions and layout of the sampling
equipment will contribute to the combined expanded uncertainty of the measurement; to minimise this contribution
to the combined expanded uncertainty, performance criteria for the sampling equipment are given in the following
subclauses.
NOTE In Annex C different arrangements of the sampling equipment are schematically presented.
6.2 Sampling location
The location where the ambient air is sampled and analysed is not specified, as this depends on the measurement
requirements (such as measurements in e.g. a rural area or urban-background area).
NOTE For guidance on sampling points on a micro-scale, see Annex D.
6.3 Sampling inlet and sampling line
A specific method for sampling or sampling equipment is not described. It is acceptable that the equipment fulfils
the following minimum requirements.
The sampling inlet shall be constructed in such a way that ingress of rainwater into the sampling line (or system) is
prevented. The sampling line shall be as short as practicable to minimise the residence time.
The material of the sampling inlet as well as the sampling line (or system) can influence the composition of the
sample. In practice, the best materials such as polytetrafluoroethylene (PTFE), perfluoro-ethylene-propylene (FEP),
PFA or borosilicate glass shall be used. Copper or copper-based alloys, shall not be used. The influence of the
sampling system on the measured concentrations of ozone due to losses shall be less than 2 %.
NOTE The sample line may be moderately heated to avoid any condensation. Condensation may occur in the case of high
ambient temperature and/or humidity.
As NO is generally present in the sampled air, a change in the initial ozone concentration will occur due to the
reaction of ozone and NO. Therefore the sampling line shall be as short as practicable to minimise the residence
time. Calculations have shown that if the residence time is less than 0,5 s then the decay in the initial ozone
concentration will be less than 1 % for most ambient ozone and nitric oxide concentrations encountered. In order to
avoid a significant change in the concentration of ozone, the total residence time (the sum of the residence time in
the sampling system and the residence time inside the analyser) shall be less than 5 s. The effect of residence time
on the decay of the initial ozone concentration shall be calculated according to Annex A.
When the calculated loss of initial ozone is more than 5 % and less than 10 %, then corrections shall be made to
the measured ozone concentration. When the calculated loss of initial ozone is more than 10 %, measures are to
be taken to reduce the residence time (e.g. by shortening of the sampling line or increasing the flow rate through
the sampling line).
Depending on the placement of the particulate filter in the sampling system (see 6.4), the sampling line can be
contaminated by deposition of dust. This will induce losses of ozone in the sampling line(s). The sampling system
shall be cleaned (as stated in 9.4.2) with a frequency which is dependent on the site-specific conditions.
The influence of the pressure drop over the sampling line including the filter shall be such that it causes less than
1 % of the signal output of the analyser.
The sampling line manifold and filter shall be conditioned (at initial installation and after each cleaning) to avoid
temporary decrease in the measured ozone concentrations. Both sampling line and filter shall be conditioned with
ambient air for a period of at least 30 min at the nominal sample flow rate. The concentrations measured during
these periods shall not be included in the calculation of data capture, hourly and annual averages.
These conditioning periods shall not be included in the calculation of the availability of the analyser during the type
approval test (8.5.6) and during field operation (11.2.3). Conditioning may also be done before installing in the
laboratory.
6.4 Particulate filter
A particulate filter may be placed in this sampling line. The particulate filter shall retain all particles likely to alter the
performance of the analyser. It shall be made of PTFE.
NOTE 1 A pore size of the filter of 5 µm usually fulfils this requirement.
NOTE 2 Suitable materials for the filter housing are for example PTFE, PFA, FEP and borosilicate glass.
Aparticulate filter shall be changed periodically depending on the dust loading at the sampling site (as indicated in
8.6.2). The filter housing shall be cleaned at least every six months. Overloading of thisparticulate filter may cause
loss of ozone by adsorption on the particulate matter and can increase the pressure drop in the sampling line.
6.5 Control and regulation of sample flow rate
The sample flow rate into the analyser shall be maintained within specifications of the manufacturer of the analyser.
NOTE Flow rate into the UV photometric analyser is usually controlled by means of restrictors.
6.6 Sampling pump for the manifold
When a sampling manifold is used, a pump (or fan) is necessary for sampling ambient air and suction of the
sampled air through the sampling manifold. The inlet of the sampling pump (or fan) for the sampling manifold shall
be located at the end of the sampling manifold (see Annex C). The sampling pump or fan shall have sufficient
rating to ensure that all analysers connected to the manifold are supplied with the required amount of air and to
ensure that the residence time of a sample from inlet until entering the analyser is less than 5 s. To verify
functioning of this pump, it is recommended to install a flow alarm system. An example of a sampling manifold is
given in Annex C.
The influence of the pressure drop induced by the manifold sampling pump on the measured concentration shall be
< 1 %.
7 Analyser equipment
7.1 General
In Annex E a schematic diagrams is given of one type of ultraviolet photometric ozone analysers.
The ultraviolet photometric analyser consists of the principal components that are described in 7.3 to 7.11.
7.2 Interferences
The UV photometric method is not subject to interference from any of the common gaseous air pollutants in
ambient air at concentrations less than 100 nmol/mol. However, there are reported interferences of about
1 nmol/mol in equivalent ozone for a concentration of 125 nmol/mol (v) of sulphur dioxide and 1 nmol/mol in
equivalent ozone for a concentration of 500 nmol/mol of nitrogen dioxide.
NOTE Water vapour normally present in the sampled air is the major interferent. From an intercomparison (1994/1995) at
ERLAP [17] H O interferences (with 85 % RH at 22 °C) were:
 maximum 5,4 % (12 nmol/mol) and minimum 2 % (4,4 nmol/mol) at a level of 220 nmol/mol O and
 maximum 27,4 (9,6 nmol/mol) and minimum 9,7 % (3,4 nmol/mol) at a level of 35 nmol/mol O .
3 3
NOTE 2 In general it is not expected that Hg concentrations in ambient air (2 ng/m to 4 ng/m ) have an interference on the
measurement of ozone. However, care and precautions should be taken when elevated Hg concentrations in ambient air are
expected. Interferences of Hg have been reported at a level of 10 ng/m of Hg resulting in a response of 1 nmol/mol of ozone.
Additionally, there are some reported interferents for analysers using a manganese dioxide ozone scrubber; these
are tabulated in Annex A of ISO 13964:1998.
7.3 Ultraviolet absorption cell
The ultraviolet absorption cell shall be constructed of materials inert to ozone, such as fluorocarbon polymer,
borosilicate glass, fused silica or fluorocarbon-coated metal. The cell assembly shall be mechanically stable so that
vibration or change in the surrounding temperature does not affect the optical alignment. Provisions shall be
available for measuring the temperature and pressure of the gas in the ultraviolet absorption cell.
7.4 Ultraviolet source lamp
The UV lamp (e.g. a low-pressure mercury vapour discharge lamp) shall emit monochromatic UV radiation centred
at 253,7 nm. The UV lamp shall be electronically stabilized to ensure a constant UV radiance; any variation in UV
radiance during the measurement cycles will result in a measurement bias or noise.
When employing a mercury vapour discharge lamp the radiation at 185 nm (which photolyses oxygen to produce
ozone) shall be eliminated by means of a high-silica glass enclosure or shield.
NOTE Spectral data show that ambient water vapour should not interfere in the UV wavelength region of interest here
(about 200 nm to 300 nm). However, it has been observed in some analysers of one manufacturer that the quartz window of the
UV source lamp was covered with a significant number of microscopic scratches. This resulted in a measurable variation in light
scattering at the internal window surface due to changing humidity in the sample air. Replacement of such faulty windows
eliminated this interference (see [5]).
7.5 UV detector
The optical system shall be designed in such a way that nearly all radiation sensed by the detector is centred at
253,7 nm. The response of this sensor and its associated electronics shall be sufficiently stable so that the
analyser meets the required performance criteria.
NOTE Vacuum photodiodes with caesium telluride sensitisation meet this requirement at 253,7 nm and have a negligible
sensitivity at other mercury emission wavelengths. Other detectors, such as photomultiplier tubes also satisfy the purpose.
7.6 Ozone-specific scrubber
The ozone-specific scrubber shall contain a material that selectively catalyses the destruction of ozone in the
sampled air.
NOTE 1 Manganese dioxide on a metal substrate has been found to do this satisfactorily. Even though this particular
compound does not remove any of the other more common pollutant it has been found to partly remove some organic
compounds which may be present in the ambient air. Annex A of ISO 13964:1998 lists such interfering chemical compounds.
NOTE 2 A significant decrease in response to ambient ozone may be an indication of scrubber failure. Normally,
manufacturers give an average lifetime of such scrubbers; however, the actual lifetime will depend on the sampling location. For
example: high pollutant concentrations at an urban-background station may deactivate the scrubber prematurely.
7.7 Switching valve
A switching valve is used to direct the sampled air alternately either through or around the ozone-specific scrubber.
It shall be made of a material that is i
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