Particulate matter is a mixture of solid particles and liquid droplets suspended in outdoor air. PM10 and PM2.5 are operationally defined fractions selected according to aerodynamic behaviour rather than two single particle sizes. Their measurement depends on the inlet, sampler, flow control and detection principle, so results from different instruments are not automatically interchangeable even when they carry the same fraction label.
Outdoor particulate matter is not one chemical substance. It can contain mineral material, sea salt, soot, metals, organic compounds, sulphates, nitrates and water, with the composition changing by location, season, weather and source activity. Some particles are emitted directly, while others form in the atmosphere when gases react or condense.
Particles also vary in shape, density and surface properties. A compact mineral grain and a low-density agglomerate may have the same physical width but behave differently in moving air. This is why particulate fractions are not normally defined by geometric diameter alone.
The PM10 and PM2.5 labels describe groups of particles collected according to a specified size-selective convention. They should not be interpreted as perfectly sharp sieves that accept every particle below one diameter and reject every particle above it. The transition is gradual and is governed by the performance of the sampling inlet.
Aerodynamic diameter expresses how a particle behaves in air compared with a spherical particle of standard density. It brings particle size, density and shape into one practical measure for sampling. Two particles with different physical dimensions may therefore have a similar aerodynamic diameter if their settling and transport behaviour is similar.
PM10 refers to the particulate fraction sampled through an inlet designed around an aerodynamic diameter of ten micrometres. PM2.5 refers to the finer fraction sampled through an inlet designed around an aerodynamic diameter of two and a half micrometres. These are sampling conventions, not statements that every collected particle is smaller than the number in the fraction name.
PM2.5 forms part of PM10 when both fractions are measured correctly under comparable conditions, but the difference between them is not simply a calculation made without uncertainty. The coarse portion between the two fractions may be estimated by subtraction, yet the result inherits the uncertainty of both measurements and can be distorted when instruments, averaging periods or locations differ.
A size-selective inlet separates particles according to aerodynamic behaviour before they reach the filter or detector. Common arrangements use impactors, cyclones or specially shaped inlet heads. Air is drawn through the device at a controlled flow, causing particles with greater inertia to follow a different path from particles that remain carried by the airstream.
An impactor directs the air towards a surface. Particles with sufficient inertia leave the turning airflow and strike that surface, while smaller particles continue towards the collection stage. A cyclone creates a rotating flow so that particles with greater inertia move towards the wall and are removed, while the selected fraction continues to the outlet.
Flow rate is part of the inlet design. If the actual flow differs from the specified flow, the separation characteristics change and the sampler may no longer represent the intended fraction. Inlet orientation, blockage, deposits, damage, poor sealing and unsuitable installation can also affect performance. Routine inspection and verified flow are therefore essential parts of particulate measurement.
Gravimetric measurement determines particulate mass by collecting the selected fraction on a filter and weighing the filter before and after sampling. The difference in mass is divided by the volume of air drawn through the sampler to produce a mass concentration for the sampling period. The method is conceptually direct, but reliable results depend on careful control of many small influences.
Filters are normally conditioned under controlled temperature and humidity before weighing because filter material and collected particles can gain or lose water. Clean handling, antistatic precautions, suitable balances, field blanks and documented transport reduce contamination and weighing error. The sampled air volume must also be calculated from verified flow and elapsed time.
A gravimetric result represents the average mass collected over the full sampling period. It does not show when within that period concentrations rose or fell. The filter may, however, be retained for chemical analysis where the study requires information about composition, provided that the filter type, loading and analytical method are compatible.
Optical instruments infer particle concentration from the way particles scatter light. A sample passes through an illuminated sensing region, and the instrument converts the detected scattering signal into a reported number concentration, size distribution or estimated mass concentration. Optical measurement can provide rapid or continuous data and can reveal changes that a period-average filter result cannot resolve.
The conversion from scattered light to mass is model-dependent. Light scattering varies with particle size, shape, refractive index, colour and water content. An instrument calibrated with one test aerosol may respond differently to mineral dust, sea salt, combustion particles or humid atmospheric aerosol.
Some optical instruments estimate several particulate fractions from the same sensing system. The outputs may appear precise, but they remain dependent on the instrument's internal size classification and mass-conversion assumptions. A site-specific correction may improve agreement with a reference gravimetric method, although the correction should be derived transparently and should not be assumed to remain valid when the aerosol mixture or humidity changes.
Gravimetric and optical instruments do not measure exactly the same property. Gravimetry measures collected mass after size selection, while an optical instrument measures a light-scattering response and converts it into an estimated concentration. Agreement is therefore influenced by both the inlet and the detection principle.
Humidity is a common cause of divergence. Hygroscopic particles can absorb water and grow, increasing light scattering even when the underlying dry particle mass has not increased to the same degree. Heated or dried optical systems may reduce this effect, but treatment of the sample can also alter volatile material.
Differences may also arise from flow errors, inlet losses, filter handling, overloading, optical fouling, changing particle composition and the time basis of the comparison. A short optical peak cannot be compared directly with a filter average covering a much longer period. Meaningful comparison requires collocation, matched averaging periods, synchronised clocks and documented quality checks.
The two fractions can behave differently in the atmosphere and may reflect different combinations of sources. Coarser particles are often associated with mechanically generated or resuspended material, while finer particles may include combustion-related material and particles formed through atmospheric reactions. These associations are not exclusive, so source conclusions require supporting evidence.
Particle size influences settling, transport distance and removal by rainfall or surface deposition. Coarser particles generally settle more readily, while finer particles can remain suspended for longer and travel farther. Wind, turbulence, atmospheric stability and humidity modify these patterns.
Reporting only total particulate mass can hide a change in the size distribution. A location may show similar PM10 mass on two occasions while the proportion represented by PM2.5 differs. Measuring both fractions can therefore provide information that neither fraction supplies alone, provided that the methods and data quality are suitable.
A particulate report should identify the fraction, inlet, measurement principle, flow basis, averaging period, instrument model, calibration or verification approach and treatment of invalid data. The terms PM10 and PM2.5 are not sufficient method descriptions by themselves.
Results from gravimetric and optical instruments should be labelled according to how they were obtained. An optically estimated mass concentration should not be presented as though it were a directly weighed mass without explanation. Any correction factor, humidity adjustment or algorithmic processing should be stated.
The applicable UAE ambient-air criteria and their statistical interpretation are covered separately on the ambient-air standards page. For this page's technical purpose, the central principle is that a particulate result has meaning only when the sampled fraction, inlet performance, measurement principle and averaging period are understood together.
No. They are operationally defined particulate fractions collected according to size-selective sampling conventions. Each fraction contains a distribution of particles rather than particles of one exact diameter.
Conceptually, PM2.5 is contained within PM10 when both fractions are measured correctly and under comparable conditions. In practice, subtraction and comparison are affected by sampler and measurement uncertainty.
Aerodynamic diameter describes how a particle behaves in air compared with a standard-density sphere. It accounts for the combined effects of physical size, shape and density.
Gravimetry directly determines the collected mass on a filter. Optical instruments infer concentration from light scattering, which allows rapid data but introduces dependence on particle properties and calibration assumptions.
Some particles absorb water and grow as humidity rises. The larger droplets or particles scatter more light, so an optical instrument may report an increased estimated mass even when the change in dry particle mass is smaller.
Only with caution. The instruments should use comparable size selection, averaging periods, locations and quality controls. Collocation against a suitable reference method provides a stronger basis for comparison.