Particulate matter sampler

A particulate matter sampler is an instrument for measuring particulates (solids and liquids) suspended in air. Particulate matter (PM) contains a mixture of solid particles and liquid droplets. It can include both inorganic materials (e.g. carbon, metals and other chemicals from dust, smoke, soot, sea salt, or sulfates) and organic materials (e.g. bioaerosols from microbial, fungal, animal, or plant sources). Air quality is monitored worldwide at regulatory stations. Scientists, doctors, industries, and the public are interested in using low-cost particulate matter sensors to detect air quality locally and in real time. Some particulate matter samplers can continuously measure and report properties of particulates, while others collect samples for later analysis in a laboratory. Different types of PM samplers can measure particulate matter in terms of particle size, particle mass, particle number, particle size distribution, and particle surface area. Inertial separators may be used to eliminate particles outside of the size range that is desired to be measured.

Measurement types

Airborne particulate matter (PM) is a complex mixture that can include small liquid droplets, dry solids, and liquid coatings around solid centers. Particles vary widely in size, shape and chemical composition. Each source has its own particular emission profile or signature in terms of particle size and chemical composition. The distribution of particles of different sizes within an air sample is important to understanding how particles will behave in the atmosphere and in the respiratory system.

For air quality regulation, measurements are often defined in terms of the maximum diameter of the particles being measured. Coarse particulates with a diameter of 10 microns or less (PM10) are of concern because they can be breathed into the lungs and cause harm. Fine particulate matter is smaller, with a diameter of 2.5 micrometers or less (PM2.5), and can penetrate farther into the lungs. The term ultrafine (UF) is sometimes used for the smallest particulates, less than 0.1 micrometers. The terms PM10 and PM2.5 define common cutoffs for particle size which are used when taking measurements. The numerical values reported for each category give the amount of particles in the size range being measured. Exposure standards for PM10 and PM 2.5 have been set by various countries. However, medical research suggests that no level of particulate exposure is safe. Particulate mass concentration (PMC) is a key air quality indicator used to measure pollution levels and assess potential health risks. Particulate mass concentration is the total mass of particles present in a given volume of air. Such measurements typically describe particulate matter in terms of units of mass per volume, e.g. micrograms per cubic meter (μg/m3). PMC is generally determined by filtering, sampling, and weighing particles collected from air. Filters may be designed to detect a particular size range (e.g. PM10 or PM2.5) or type of particulate. Measurements can be made using techniques such as gravimetric analysis, beta attenuation monitoring, and tapered element oscillating microbalances. Particulate mass concentration is often used in compliance monitoring for quality assurance, data collection for trend analysis, and high spatial resolution measurements. However, PMC methods may tend to ignore ultrafine particles, because total mass can be dominated by a small number of large particles. In number-based measurements, the number of individual particles is counted, independent of their size or mass. Particle number concentration can be measured using condensation particle counters, light scattering methods (e.g. optical particle counters), and instruments based on diffusion chargers. Number concentration measurements can reveal important information about specific types of particles and their distribution over time. Number-based measurements may be particularly useful for identifying ultrafine particles. However, care must be taken to ensure that methods chosen are not biased towards detection of larger particles. Diffusion chargers can be used to measure the active surface area of particles. Particle size distributions can be measured by using cascade impactors (e.g. gravimetric impactors), scanning mobility particle sizers, aerodynamic particle sizers, fast mobility particle sizers, and electrical low pressure impactors.

Instrument types

Particulate mass concentration (PMC)

Gravimetric mass determination Gravimetric mass determination of particulate mass concentration is a two-stage process of collection and analysis. Sample collection uses standardized equipment to draw a known volume of air through a filter. The filter is weighed on an analytical balance before and after sampling. Collection may occur over a fixed time period such as 24 hours. The filter is brought back to a laboratory for analysis to determine the concentration of particulate material that it contains. The difference in weight divided by the volume of air pulled through the filter gives the mass concentration of the particulate. For gravimetric air quality analysis, water that is combined with a contaminant is considered part of particulate matter. However, the definition of particulate matter does not include uncombined water, such as pure steam or water vapor. Uncombined water in a particulate sample should be removed, either by heating the sample to evaporate the water before the sample is weighed, or by placing the sample in a low humidity environment before weighing. In the USA, the U.S. Environmental Protection Agency (EPA) sets standards for filter-based Federal Reference Methods (FRM) and continuous Federal Equivalency Methods (FEM) for air monitoring. Gravimetric filter-based PM2.5 FRM measurements are considered the gold standard for accuracy of measurement.

Beta Attenuation Monitor

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