Air quality expert group - Fine particulate matter (PM2.5) in ... - Defra

Chapter 2
Measuring PM2.5 and its components
2.1 Introduction
1. Most measurements of PM2.5 are made to check compliance with air quality
legislation. Measurement data are also critical for understanding the chemical
and physical processes that affect particulate matter (PM), and so they also
support the development of models, and decisions about measures to reduce
PM concentrations. This chapter discusses the measurement techniques for
monitoring PM2.5 and components of PM2.5 that are referred to in this report,
and highlights the difficulties of obtaining reliable measurements. Data quality
issues are also discussed. Techniques such as aerosol mass spectrometry, which
provide valuable information in other contexts, are not described because they
do not provide data used in this report.
2. PM2.5, along with PM10, is unusual among regulated air quality metrics in
being effectively defined by the measurement method rather than as some
unambiguous chemical or physical component of the air. This was not the
original intention, and it is, to a large extent, the consequence of the metric
featuring in legislation before a good scientific understanding of airborne
particles was available. As a better understanding has emerged, it has proved
difficult to modify the definition of PM2.5 (or PM10) accordingly, because of the
implications for the legislation.
2.1.1 Challenges with PM2.5 measurement
Measuring PM2.5 and its components
3. In principle, PM2.5 is the mass concentration of airborne particles with an
aerodynamic diameter of less than 2.5 µm, expressed in µg m -3 , where the
volume of air is its volume at ambient conditions (rather than at standardised
temperature and pressure). The size of 2.5 µm was chosen because of its
significance for the penetration of human lungs, set out by the high risk
respirable convention in the document ISO 7708:1983, Air quality – Particle
size fraction definitions for health-related sampling. For comparison, PM10
corresponds to the thoracic convention in the same document, i.e. the size of
inhaled particles that penetrate beyond the larynx.
4. There is a long history of particle mass concentration measurements based on
the removal of unwanted large particles (in this case > 2.5 µm) using a
size-selective inlet such as an impactor, followed by the weighing of the
particles that remain in the airstream. This is done by passing the airstream
through a particle filter that is weighed before and after sampling. The key
measurement issue is that the process of collecting particles onto a filter prior
to mass determination can lead to significantly different results depending on
the partial or total loss of semi-volatile particles (i.e. those that may evaporate
during collection), and because variable amounts of water can remain bound to
the particles.
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