About PM mass
monitoring methods
There are several
methods for measuring the concentration of airborne PM mass. Methods have been
developed over decades, and they have been improved in their sophistication.
The basic technique is to pull a known volume of air through a filter, and
weigh the filter under controlled conditions before and after the sampling. An
important part of such type of PM instrument is the air/particle intake device,
which separates the particles that should be measured (i.e. particles of
diameter less than 10 μm, or particles less than
2.5 um) from the larger ones. Different types and makes of inlet separators
have different separation performance.
This method is the
basis for the Reference method for PM10 measurement described in the
Directive, which actually specifies the actual instruments (products) which are
accepted. The reason for the need to specify the instrument makes in the
reference method is that the combination of air intake and filtering unit
design defines the PM sample which is actually collected for weighing, and for
various instrument designs/makes, the PM sample will differ. Thus, to accept
all instrument makes of a certain type as reference method without testing,
would introduce an uncontrollable variation results from reference PM
instruments. The pragmatic solution was to select a certain small number of
instruments as reference samplers. Other filtering type methods can also be
used, but the user has to show that they are equivalent to the reference
instruments, within certain prescribed level of accuracy.
Automatic instruments
which are now available measure PM concentrations continuously, providing
hourly values, while filtering methods generally provide 24-hour averages.
These are used extensively in monitoring networks in Europe.
Through experience and research is it clear that such instruments most often
give significantly lower concentrations than filtering methods. Also the
automatic methods are based on collection of the particles on filters, and
various sensing techniques determine indirectly the mass of the PM. The filters
are kept at elevated temperatures in these instruments to avoid problems
related to humidity, and this elevated temperature results in loss of
semi-volatile PM mass (mostly ammonium nitrate and organic compounds, of which
the nitrate is most often the dominating one in terms of mass losses).
Automatic monitors also have to be tested for equivalency with the reference
method, to be accepted for monitoring under the AQ Directives. The most often
used automatic monitoring methods are the beta attenuation method and the TEOM
method. These and other methods are described in the 1st PM10
Position Paper (see web link section below).
Methods in relation to
the assessment and information requirements in the 1st AQ Directive
The AQ Limit values for
PM10 relates to daily (24-hour) averages and annual averages. Such
statistics are also required from the PM2.5 measurements that the
Member States have to set up. To assess these averages by measurements, filter
methods giving 24-hour averages have sufficiently good time resolution.
However, if there is a need to assess which sources give
the main contributions to the measured values (This is needed when action plans
for pollution reduction must be worked out), a better time resolution is
generally needed, such as hourly data. Also, the Directive requires that
information about PM10 concentrations shall be made available to the
public “as soon as possible”, especially when limit values are exceeded. This
requires the use of automatic methods, since with gravimetric filter methods
data are available, at the earliest, several days after the sampling.
Reference methods
PM10
The following types of instruments, utilising a plane filter of certain
specifications as the particle collection medium, can be assessed as being
reference instruments for PM10 measurements in Europe:
ˇ
medium-volume filter samplers;
ˇ
high-volume filter samplers;
ˇ
very-high-volume filter samplers (called
WRAC-type samplers).
They all have to be equipped with suitable air intakes that separate
the PM10 fraction from the larger particles, with effectiveness
according to set specifications.
This is described in CEN Standard EN 12341:1998 – “Determination of the
PM10 fraction of suspended particulate matter – Reference method and
field test procedure to demonstrate reference equivalence of measurement
methods”.
PM 2.5
A decision on reference methods for PM2.5 determination in Europe is under development (CEN
standard prEN 14907, Under
approval, as of December 2004).
Determination of equivalence of other instruments and methods
The procedure for determining whether a candidate method is equivalent
to the PM10 reference method is described in the CEN Standard EN
12341.
The concept is that to use another instrument or method, a correction
factor (CF) must be determined, so that when the data measured by a
non-reference method is corrected by the CF, the resulting value is equivalent
with what the reference method would give. The accepted level of uncertainty in
the measurements as compared to the Reference method is prescribed in the
standard. It is necessary to show that the CF is valid for area and PM10
level and composition (city, region, country) where the monitoring in carried
out.
A report on the use of Correction Factors for PM10
determinations with various instruments in Europe has been worked out by the
European Topic Centre on AQ and Climate Change (ETC/ACC) http://air-climate.eionet.eu.int/
of the European Environment Agency (EEA): “Correction factors and PM10
measurements”, available as a final draft report (as of December 2004).
The similar procedures for determining equivalence of PM 2.5
methods is described in the CEN standard prEN 14907.
Quality control of monitoring data
Annex VIII of the 1st Daughter Directive (see web link
below) specifies the required accuracy of measurement data, as well as the
minimum data capture (parts of the year the measurement data must be available
from a monitoring station). These requirements are laid down as a guide to what
quality-control and –assurance (QA/QC) programmes that monitoring network
operators need to follow to comply with the quality objectives.
QA/QC procedures are dealt with in the Topic How to secure the quality of the monitoring data? Quality assurance (QA) and Quality Control (QC) systems and procedures..
Minimum number and location of PM10 monitoring stations in a
Member State, according to the 1st
AQ Daughter Directive
Annex VII of the 1st Daughter Directive (see web link below)
state the requirements to what type of areas (zones) should be monitored for PM10,
and the minimum number of sampling stations as a function of the population in
an agglomeration or zone. In zones where the upper assessment threshold is
exceeded and there should be more than 1 station, there should be at least one
urban background and one traffic-oriented station.
The determination of which zones shall have PM10 monitoring
stations is generally done through the activities of the “Preliminary
Assessment” that the Member State shall carry out, in accordance with the
Directive on ambient air quality assessment and management (the Framework
Directive) (96/62/EC) (see web link below).
For PM2.5, each EU Member State should operate a number of
monitoring stations, the number to be determined by each State so that PM2.5
concentrations representative for the variation of PM 2.5 within the
State can be determined.
Location of sampling points, and exposure related monitoring
Annex VI of the 1st Daughter Directive presents
considerations that shall apply regarding the selection of fixed point
measurements. The annex describes macroscale siting and microscale siting considerations, both for health- and
ecosystems-protection related stations.
Stations directed at human health protection should be located such
that they provide data on areas representing typical or maximum concentrations
that the population in the area is likely to be exposed to, directly or
indirectly. This exposure should occur for a period which is a significant part
of the averaging period of the limit value. Thus, it is prescribed that the
monitoring should be exposure related.
This means simply that health-protection directed stations should be
located where people are living or frequenting, and that the period that they
are likely to be exposed in the area is comparable to the averaging time of the
limit value. For PM10, the averaging times of the limit values are
either 24 hours, or one year. So, monitoring stations for PM10
should be positioned either in residential areas where people are potentially
exposed throughout the whole year or indeed during any 24-hour period, or they
should in “hot-spot” areas where people are likely to be exposed over a
significant part of any 24-hour period. This could be residences very close to
roads with high traffic, or in areas close to industrial sources with large PM
emissions.
This also means that areas where exposure is likely only over shorter
periods, such a one or a few hours, should not be considered for monitoring of
PM10.
Guidance on selection of monitoring method(s)
ˇ
The selection of which method(s) to use for PM monitoring, in response
to the requirements of the Directive, is influenced by, and is in practice a
trade-off, between several factors: compliance with reference method
prescription, need for fast availability of data, and costs (both for purchase,
maintenance and system operation).
ˇ
Reference method requirements: would be easiest to just use the
reference instruments.
ˇ
Need for fast information on PM levels (faster
than several days): automatic method is needed. This requires that correction
factor is determined for each area in question and
ˇ
Minimisation of total costs: need to check
prices. Automatic instruments are not always more costly than gravimetric
filter samplers. Automatic instruments require fairly costly CF determination.
Operating costs should be considered: manpower costs for field and laboratory
operations; calibration needs and laboratory resources, equipment maintenance.
|