Particulate Matter Measurement: Techniques And Tools

Particulate matter (PM) is a mix of solid and liquid particles suspended in the air. It can include everything from dust to toxic chemical compounds, and its sources are primarily human activities such as industrial pollution, agriculture, and combustion of fuel. As PM is often microscopic and odourless, it can be challenging to detect and measure. However, it is essential to monitor it due to its potential impacts on human health and the environment. Various methods and instruments are available for measuring PM, including gravimetric and real-time devices, indoor air quality monitors, and low-cost sensors. These tools help assess particle mass, particle number, and particle size distribution to ensure clean and safe air quality.

Indoor air quality monitoring

Methods of Measuring Particulate Matter

There are several methods to measure particulate matter, including particle mass, particle number, and particle size distribution metrics. Particle mass is measured using gravimetric methods, tapered element oscillating microbalances, and beta attenuation instruments. Particle number concentration is measured by condensation particle counters, optical particle counters, and diffusion chargers.

Indoor Air Quality Monitors

Indoor air quality monitors, such as Pressac's IoT sensors, are an effective way to measure particulate matter within a workspace or household. These sensors can measure particles as small as PM1, providing valuable data and enabling trends in particle concentration to be tracked over time. This data can then be used to adjust ventilation or replace air filters when necessary.

Low-Cost Sensors

The emergence of low-cost sensors has the potential to revolutionize indoor air quality monitoring. These sensors can be placed in different rooms to assess the relative importance of indoor and outdoor pollution sources on indoor air quality. While these sensors may not always be highly accurate, they can provide valuable information about indoor air quality, allowing individuals to make informed lifestyle decisions to reduce their exposure to particulate matter.

Outdoor Air Quality Apps

In addition to indoor sensors, outdoor air quality apps, such as the EPA's AirNow mobile app, can be a useful tool for monitoring air quality in your area. These apps often provide broad coverage and clear information about the air quality in your specific location, helping you anticipate potential issues and take necessary precautions, such as venting your home or using air purifiers.

Particle mass, number, size distribution

Particle mass, number, and size distribution are key factors in determining air quality and the safety of the air we breathe.

Particle Mass

Particle mass concentration is a measure of the mass of particles in a given volume of air. This is often used as an indicator of air quality, with higher particle mass concentrations indicating poorer air quality. For example, the mass concentration of particles in industrial regions and near-traffic city centers is typically much higher than in rural areas.

Particle Number

Particle number concentration, as the name suggests, is a measure of the number of particles in a given volume of air. This is also an important indicator of air quality, and in some cases, it is considered even more relevant than particle mass concentration. This is because smaller particles can be more toxic and more easily inhaled, and their health effects are often more closely related to their number than their mass.

Particle Size Distribution

Particle size distribution describes the range of particle sizes present in a sample. It is typically represented by a histogram, where the width of each bar corresponds to a size range, and the height of the bar indicates the quantity of particles within that size range. This distribution can also be represented by certain values, such as d10, d50, and d90, which indicate the percentage of the sample that is smaller than a given size. For example, d50, also known as the median, means that 50% of the particles are smaller than a certain size.

Different methods and technologies can be used to determine particle size distribution, and the choice depends on the specific application. The mean particle size can also be calculated by multiplying the quantity in each measurement class by the mean size and then summing the values.

The particle size distribution can be further characterized by the standard deviation or span value, which indicates the width of the distribution. The mode size refers to the x-value where the density distribution reaches a maximum.

Gravimetric, optical, microbalance, electrical charge

Gravimetric determination of particulate matter involves the use of filters to collect and weigh the particles. This method is commonly employed in emissions testing to assess particulate matter in exhaust gases, evaporative emissions, the atmosphere, or ambient air. The weight of particulate matter collected on filters can be extremely low, requiring highly accurate weighing equipment such as ultra-microbalances. These balances are designed to measure very small samples with a high degree of precision. To ensure accurate results, it is crucial to maintain a stable ambient temperature and humidity during the weighing process, as microbalances are highly sensitive to environmental disturbances.

Optical methods, such as the use of optical particle counters (OPCs), offer another approach to measuring particulate matter. OPCs employ a light source, typically a laser diode, to illuminate particles, and a photodetector to measure the scattered light. By calibrating OPCs against reference-grade monitors, their accuracy can be enhanced. Optical methods are widely used by regulatory bodies due to their high data accuracy, although they may incur higher operational costs.

Microbalance instruments are also crucial in measuring particulate matter. These highly sensitive instruments can weigh extremely small samples accurately. Microbalances are commonly used in emissions testing to weigh particulate matter collected on filters. To ensure accurate measurements, it is essential to minimise environmental disturbances such as air currents, temperature fluctuations, and vibrations. Proper handling of weighed substances and personal protection equipment are also important considerations when using microbalances, especially when dealing with toxic or hazardous materials.

Electrical charge plays a role in ensuring accurate measurements of particulate matter. For example, passing filters over an electrode of an AntiStatic Kit can neutralise any electrostatic charges and ensure the true particulate matter result is obtained. This helps to eliminate potential errors caused by static electricity.

Continuous monitoring

Indoor Air Quality Sensors:

Indoor air quality sensors, such as Pressac's IoT sensors, provide continuous measurements of particle concentrations. These sensors can detect different PM levels, including PM1, PM2.5, PM4, and PM10, along with other parameters like VOC levels, CO2, humidity, and temperature. This data helps track trends and take appropriate actions, such as increasing ventilation or replacing air filters when PM levels exceed certain thresholds.

Federal Reference Methods (FRMs) and Federal Equivalent Methods (FEMs):

FRMs and FEMs are reference-grade monitors that produce highly accurate data for particulate matter measurements. These methods are often used as a standard for calibrating other instruments, such as optical particle counters (OPCs). However, they come with high operational costs and lower spatial and temporal resolution.

Optical Instruments and Optical Particle Counters (OPCs):

OPCs are popular optical instruments used for continuous monitoring of particulate matter. They use a light source, such as a laser diode, to illuminate particles and a photodetector to measure the scattered light. By calibrating OPCs against FRM and FEM instruments, their accuracy can be enhanced.

Particle Counters and Nephelometers:

Particle counters, like purple air sensors, use optical systems to count particles within a sample volume and extrapolate the data to a standard volume. Nephelometers are often co-located with filter-based monitors to build appropriate correlation relationships for specific PM sources in a region. However, these relationships may need adjustment when PM emission source profiles change significantly.

Filter-based Technology:

This method involves pulling a known volume of air over a set period and measuring the mass of the filter before and after. This approach allows for the calculation of mass per volume and is considered the most accurate way to measure particulate matter. It is the federal reference method for determining attainment of federal air quality PM standards.

By employing these continuous monitoring techniques, organisations can ensure optimal indoor air quality, meet health standards, and create a safe and healthy environment for occupants and employees.

Low-cost sensors

The low cost and compact size of light-scattering-based PM sensors provide an opportunity for improved spatiotemporally resolved PM measurements. However, these sensors have limitations and need to be characterized under realistic conditions. For example, low-cost optical particle sensors have the potential to supplement existing PM monitoring systems and to provide high spatial and temporal resolutions, but they have often shown questionable performance under various ambient conditions. Temperature, relative humidity, and particle composition have been identified as factors that directly affect the performance of low-cost PM sensors.

To address these challenges, several scientific articles have evaluated the properties of low-cost sensors. For instance, a study by Sayahi et al. evaluated the Plantower particulate matter sensors in the field, and the raw data collected by the sensors had a high correlation of R2 > 0.858 in the winter season. Another study by Omidvarborna et al. designed an environmental pollution chamber to test LCPMS performance under controlled temperature and relative humidity.

Despite the limitations and challenges, low-cost sensors have been widely adopted by concerned citizens to monitor PM exposure. For example, the PurpleAir PA-II outdoor air quality monitor, which utilizes two low-cost PM sensors, is frequently deployed near air quality monitoring sites of the Environmental Protection Agency (EPA). The PurpleAir is a small, inexpensive device for monitoring atmospheric concentrations of particulate matter. While state-of-the-art monitors approved by the EPA can cost $10,000 or more, the PurpleAir costs around $260.

Frequently asked questions