Measuring Project Management: A Guide To Success

Particulate matter (PM) is a mixture of solid particles and liquid droplets found in the air. It is often measured and communicated as a proxy for overall air quality. There are several methods for measuring PM, including federal reference methods (FRMs), federal equivalent methods (FEMs), and low-cost sensors. FRMs are considered the gold standard for measuring PM but are typically operated by governmental organizations due to their high cost. FEMs, such as the Beta Attenuation Monitor (BAM), are more affordable and accessible but still require significant investment. Low-cost sensors, like the PurpleAir, use laser counters to detect particles and provide real-time data. The most accurate method for measuring PM is the gravimetric approach, which involves collecting particles on a filter and weighing it to determine concentration. Other techniques include sieving, optical spectroscopy, and laser scattering, which offer improved accessibility and affordability over traditional methods.

Federal reference methods (FRMs)

FRMs are used to assess implementation actions needed to attain the federal Clean Air Act's National Ambient Air Quality Standards (NAAQS). These methods are based on modern technology and have adequate instrument sensitivity to accurately measure pollutants. Potential new FRMs are rigorously tested for accuracy by EPA scientists in both laboratory and field settings.

One specific FRM for measuring PM2.5 involves a gravimetric method where particle mass concentration is determined by weighing filters before and after the sampling period. This technique is widely used in epidemiology studies and is recognised by the EPA as the primary acceptable sampling design for collecting and testing PM2.5 concentrations.

However, FRMs also have certain disadvantages. For example, the gravimetric method of measuring PM2.5 is not a real-time form of measurement and only provides a 24-hour average. To address such limitations and foster innovation, the EPA also reviews, tests, and approves other methods, known as Federal Equivalent Methods (FEMs). FEMs utilise different sampling and/or analysing technologies but are held to the same standards as FRMs when making NAAQS attainment determinations.

Optical particle counters (OPCs)

OPCs work by illuminating particles with a light source, typically a laser beam, and measuring the light scattered as a result. This is done using a photodetector, which measures the amplitude of scattered light. The amount of light scattered towards the detector increases in proportion to the size of the particle. OPCs can be used to detect particles as small as 50 nm (0.05 μm) and as large as several hundred micrometres in diameter. However, they are limited in their ability to detect particles smaller than 0.1 μm due to the inherent sensitivity limitations of photodetectors.

OPCs can be calibrated and verified against FRM and FEM instruments, such as a USEPA reference-grade monitor, to increase their accuracy. They can also be used in conjunction with condensation particle counters (CPCs) to detect ultra-small particles. CPCs use a condensation process to physically grow particles, allowing them to be detected using an OPC sample cell.

The Centre for Atmospheric Science at the University of Manchester uses several OPC instruments, including the Ultra High Sensitivity Aerosol Spectrometer (UHSAS), which can size and count particles in the 50-800 nm size range with a very high-resolution size distribution.

Beta Attenuation Monitors (BAMs)

BAMs are easy to operate and maintain, and they are also simple and not overly complex. They use beta ray attenuation to measure the mass concentration of PM in ambient air. This involves sending beta particles through a piece of filter tape that collects particulate matter in the air. The difference in the degree of attenuation between the pre-and-post-sampled filter tape is directly proportional to the mass of PM in the sampled air. This process is based on the principle that when high-energy electrons (beta rays) emanating from the radioactive decay of 14C (carbon-14) interact with nearby matter, they lose their energy and, in some cases, are absorbed by the matter.

The BAM 1020 from Met One Instruments is a continuous particulate monitor that automatically measures and records airborne particulate concentration levels (in milligrams or micrograms per cubic meters) using the industry-proven principle of beta ray attenuation. It was the first instrument to obtain U.S. EPA Federal Equivalent Method (FEM) designation for continuous PM2.5 monitoring, in addition to its longstanding EPA designation for PM10 monitoring. The BAM 1020 has also obtained the corresponding PM2.5 and PM10 certifications in the European Union.

The E-BAM from Met One Instruments is another example of a Beta Attenuation Mass Monitor. It is a portable, real-time beta gauge comparable to U.S. EPA methods for PM2.5 and PM10 particulate measurements. It is rugged, portable, battery-operated, and can be deployed in 15 minutes. It provides accurate, precise, real-time measurement of fine particulate matter.

Low-cost sensors

One example of a low-cost sensor is the PurpleAir, which uses two small, identical laser particle counters to verify that the unit is functioning properly. It counts the number of particles in several size classes and converts them to estimates of mass concentration. In addition to PM2.5 and PM10, PurpleAir also reports the concentration of particles less than 1.0 µm in diameter, called PM1.0. The PurpleAir also measures temperature and relative humidity.

Another example of a low-cost PM sensor is the GRIMM Model 180 particulate monitor, which is approved by the EPA for determining whether air quality meets national standards.

Several other low-cost PM2.5 sensors have been evaluated in laboratory and field conditions, including the Sharp GP2Y1010AU0F, Shinyei PPD42NS, Plantower PMS1003, Innociple PSM305, Nova SDS011, and Nova SDL607. These sensors performed well in laboratory conditions, with an R2 > 0.91 when the PM2.5 concentration was > 50 µg m–3. However, some limitations were noted at lower concentrations, and the Sharp and Shinyei sensors showed poor correlations (R2 < 0.1) with the DustTrak when the ambient PM2.5 concentration was < 20 µg m–3.

While low-cost sensors have the potential to revolutionise air quality monitoring, there are concerns about their accuracy and reliability, particularly regarding the influence of environmental parameters such as humidity and temperature, and their performance with different PM sources and concentrations. These sensors generally utilise light-scattering to detect particles with aerodynamic diameters between 0.3 and 10 μm, but they are only able to infer PM mass.

Gravimetric methods

One example of a gravimetric method is the use of Harvard Impactors and Personal Environmental Monitors (PEMs), which are types of portable gravimetric instruments. Another example is the use of a diffusion charger (DC) to monitor PM accumulation on the filter during sampling, which provides real-time detection alongside the gravimetric measurement.

Overall, gravimetric methods provide a flexible and accurate way to measure PM concentration and composition, making them a valuable tool for assessing air quality and emissions.

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