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Air Sampling

Cascade impactors are used to sample particles in specific size fractions for subsequent analysis.

Collecting a representative air sample of aerosol from the ambient environment – and changing it as little as possible while preparing it to be measured by sizing instrumentation – can be challenging. Diffusion losses must be minimized, and the aerosol must be dried sufficiently prior to measurement. In addition, some locations will need to dilute the aerosol prior to measurement; dilution accuracy is an important consideration in such cases.

Sampling Aerosols

All of these considerations are addressed in TSI’s Sampling System for Atmospheric Particles 3750200. The system includes a PM10 sampling head that can sample aerosols under a wide variety of wind conditions (omnidirectional). An optional PM2.5 cyclone (included with purchase) removes particles 2.5 micrometers and larger from the sampling flow. A Nafion drier conditions the air sample to have a relative humidity < 40%, preventing condensation and hygroscopic growth, while a flow splitter ensures that the sample is split in a representative fashion among instruments.

In ultrafine particle monitoring, both EN 16976* and CEN/TS 17434:2020 specify how the particles must be sampled and conditioned prior to measurement.

Chemical Analysis of Particles

Collecting particles on a filter is a common practice for conducting gravimetric and chemical analysis. This air sampling method, however, typically does not provide size-resolved chemical composition. Cascade impactors are then used to sample particles in specific size fractions for analysis, e.g. for carbon (total, elemental, organic, water-soluble), organic compounds or inorganic ions.

High-flow impactors are ideal for air sampling at low concentrations, for obtaining samples in short sampling intervals, or for collecting more mass per stage as compared with more conventional (i.e., medium flow rate) impactors. More size fractions are accessible using medium flow impactors. Uniform particle deposition for better quantitative analysis is achieved using impactors with rotating stages (i.e. MOUDI™ II).

 

*EN 16946 is formerly known as the Technical Specification CEN/TS 16976, and established by the European Committee for Standardization (CEN)