Choosing a method for Particulate Matter Monitoring
There are many different ways to go about monitoring for particulate matter air pollution. This page will try to break down the benefits and drawbacks of each type of monitoring to help you navigate the method(s) that best suit your interest best. This page reviews types of monitoring practices, and a couple examples of monitoring tools along with each. It is not an exhaustive list, but some of the more well known or documented resources.
Types of monitoring methods:
- Visual monitoring with your eyes
- Filter-based monitoring with lab analysis
- Optical monitoring with sensors
- Passive monitoring with other sample collection tools
How it’s done:
The smallest particles we can see with a naked eye are visible only because they diffract light to make a haze. A visible emission is any visible airborne particle resulting from a chemical or physical process. Visible emissions often contain respirable particles, and can be measured by their effects on the opacity (or lack of transparency) of the air. Opacity is expressed as the percentage of light that is scattered or blocked by emissions, such that an observer's view through the emission is obscured. Examples of pollutants that can have opaque emissions include combustion products and dust.
The EPA offers several methods for doing visual particulate matter monitoring, some requiring certifications, and some not. You can read more about Visual Monitoring and this method here.
|It doesn't require expensive equipment (unless you are certifying your camera for photo documenting the pollution), so there are fewer to go wrong when you go to the field to collect this type of data.||Several methods require certifications which can be expensive and require regular classes to recertify|
|You can do this type of monitoring without being certified, however the certified methods for doing this carry more legal weight.||Will not be useful for capturing PM pollution where only the small particles are the concern (silica and otherwise).|
|Estimates can be made for the distribution of the particle size from visual emissions||Requires that someone be there to observe the pollution event|
|This method is only useful when larger particles are in the sample.|
|This type of monitoring can not be done in the dark (at night)|
|It provides no ability to ascertain the chemical composition of what is being emitted, and is not useful for most vapor emissions.|
Some methods for visual monitoring include the EPA method 9 (requiring certification) and method 22 (no certification required). Certifications can be obtained through certified companies. Trainings are sometimes referred to as “smoke school,” Opacity Certification, Opacity Training, Visible Emissions Training, or VEO Training.
What does it look like?
A person who is monitoring using these methods will use their eyes to observe the pollution. Observations usually take about 6 minutes and require the observer to record at 15-second intervals. Here is a write up of an activity on how you would use EPA’s Method 9 monitoring
When might this be useful?
Visual monitoring methods can be useful if the particulate matter pollution source you are concerned with often has a visual component you can see from nearby, for example, you often see haze or clouds of smoke coming off a smoke stack. It’s also useful because the some methods with certification (such as EPA method 9) are federally recognized and accepted methods for monitoring, where as many of the other methods for monitoring PM aren’t. Here is a write up of the utility of EPA Method 9
- Here’s an example of a community group using this method to hold a mining company accountable: https://www.gvnews.com/news/county-notifies-freeport-of-another-violation-related-to-mine-dust/article_3e92ad7e-4202-11ea-9fc8-9306f5e6b551.html
Filter Based Monitoring
How it’s done:
Collecting particulate matter (PM) on a filter for analysis is the best understood method of measuring particle pollution. It is also the most expensive method. Filter-based PM monitors form the basis of US-based PM regulations, and filter based samplers are written directly into the US Code of Federal Regulations, however these monitors can cost between $20,000-60,000. There are other filter based monitors which are less expensive, but the degree to which their data is accepted by regulatory agencies varies from state to state.
This monitoring method uses a tool which consists of three components, an impactor, a filter, and a pump. The pump draws a specific volume of air into the filter at a consistent speed, the impactor sorts particles by size so only the desired size of airborne particles are collected, and the filter collects the sorted particles. After gathering the sample, the filter is weighed to conclude the precise amount of particulate matter that came in through the air sample. Using this method with lab analysis, it is also possible to determine what types of particulate matter pollution is present in the sample - for example, if there are heavy metals present. It is important to note this method of monitoring requires lab analysis which can be expensive, and the more you want to know about the sample, the more expensive the lab testing is.
|Most recognized method for collecting data on particulate matter pollution||This is the most expensive method for monitoring for PM|
|Allows for the sample to be tested for chemical composition||Most tools that use this method require people to calibrate the tool in the field and requires a good bit of setup and maintenance.|
|Some tools require that you physically change out the sample every 24hours|
|Requires lab analysis|
|Having one sample is often not enough, community groups who have used this method for monitoring often struggle with showing they have enough data -- enough monitors and continuous sampling|
- EPA Federal Reference Methods (FRMs) for PM10 and PM2.5 are filter-based. They draw one cubic meter of air per hour through their filters. Read more in PM Monitoring Regulations. FRMs cost between $20-60,000.
- The MiniVol costs around $2000, and is a lower volume (mini Volume) and more portable filter-based system manufactured by Airmetrics. While not a federal reference method, several states recognize data from the MiniVol. It comes with interchangeable impactors for PM2.5 and PM10, and can be used to produce 24-hour averages by gravimetric methods. Public Lab has two MiniVols available for loan. Learn More about the MiniVol and how to deploy it
- A combined optical/filter based tool: Dusttrak and Dusttrak II by TSI, and the PDR-1500 by Thermofisher are filter-based systems with the addition of an optical PM monitoring system.
What does it look like: A person who is doing this type of monitoring will need to deploy the tool(s) in a safe location, to the specifications required. Most tools need to be field calibrated, and need their filters changed out every 24hours. Some tools have the ability to take multiple samples in a row in which case you wouldn't need to go to the tool to change out the filters as often. The filters then need to be shipped to a lab for analysis. You can read about an example of deploying a filter based tool here
When might it be useful:
This monitoring method might be useful if you have a good bit of money to spend on your project and you need to know about the nature of the particulate matter pollution (chemical composition). When using the FRM tools, this data can be submitted directly to the EPA and state environmental agencies.
- Global Community Monitor had limited success in using the MiniVol. Read more about Global Community Monitor's experience
(Photo of a Public Lab Simple Air Sensor)
How it’s done:
Optical PM monitoring of Particulate Matter (PM) (or sensor based monitoring) is an attempt to measure the diameter of particles in flight. Optical monitors are a low-cost way to get real-time data about particles in the air.
In this method, a sensor and a beam of light sit at an angle to each other. As a particle passes in front of the light, some light is reflected towards the sensor. This is then registered as a piece of particulate matter. If the air is moving at a consistent speed, the length of this pulse of reflected light can be used to estimate the particle’s size. This can be used to estimate the average particle density and airborne concentrations of PM can be extrapolated.
|Less expensive||Is not a federally recognized method for environmental monitoring, although more state environmental agencies are starting to share this data publicly, and some tools that use this method are recognized by OSHA|
|Can capture data in real time||Does not give data on the chemical composition of the PM|
|Some tools can send data to the internet||Subject to weathering and damage|
|Can sometimes tell you a little about particle size||It can be hard to understand how the sensor produces data|
|Easier to set up outside (less restrictions and specifications for deploying it than the filter based monitoring methods)||Can be affected by factors such as humidity, temperature, particle composition|
- The Purple Air tool is a small device that can be set up outside in a variety of locations. The tool uses two sensors to measure particulate matter in the air. Some versions of the Purple Air can connect to wifi and send data directly to the internet, other version rely on an internal SD card which require an individual to download the data intermittently. The tool requires power to run in the field, and internet if you’re directly uploading data, but otherwise can be left alone to collect data without much maintenance. PurpleAir maintains a map of every user's data from around the world. Read more about the Purple Air here.
- The Simple Air Sensor uses the same sensor as the Purple Air, but is programmed to display a realtime colored light readout. It’s helpful for demonstrating what particulate matter is and how these sensors work.
What does it look like:
With optical or sensor based PM monitoring, a sensor is often deployed in the field (or indoors), and left to collect data. Depending on how data storage and transfer is set up in the device, collecting data can look different. It can display numerical data in real time (such as the Carnegie Mellon Spec), display color data in real time (such as the simple air sensor), upload data to the internet (such as the wifi enabled Purple Air), or store data internally for download.
When might it be useful:
Optical Monitoring might be useful if you need to collect a lot of data and/or collect data in real time. If you’re looking for a tool that can helpful show data visually in new ways, sensor data has the most options in terms of conveying results. It’s easily shown on a map or chart, it can be made to show colors or images correlating to levels of particulate matter in the air.
- EPA Evaluation of Field Deployed Low Cost PM Sensors
- SouthCoast AQMD's Air Quality Sensor Performance Evaluation Center
- Thoughts on low cost air sensors
Passive Particle Monitoring
(Photo of a DustScan directional monitor)
How it’s done:
Passive particle monitoring is done by capturing particles that collect on a surface over time. There are several methods to collecting particles in this way, most of them require that a sticky tape or pad be placed outside for several days to collect particles. The data it produces is visual, what can be seen on the sample.
|Low cost||Does not give real time data | Does not give real time data|
|Can be setup without electricity or internet||Particle speciation is limited by method and cost|
|Does not require an observer||No way to extrapolate to airborne concentrations|
|Actual particles are collected||Not an officially recognized method|
|Does not provide data about PM that can’t be seen with the naked eye, unless paired with magnification or other equipment|
- SEM Stub passive particle monitors (needs further development) uses a monitor that allows passive air to travel through it and for particles to drop out onto a filter.
- Sticky Pad monitoring exposes contact adhesive tape face up to the air for a specific period of time to allow particles to attach to the pad.
What does it look like:
Passive particle monitoring requires that monitors be filed deployed for a specific period of time, then analysed. Some monitors can be placed on the side of buildings or other surfaces outside, others require free standing space. Visual data can then be shared on the nature of the particulate material left on the pad after it has been deployed.
When might it be useful:
This type of monitoring could be useful in publicly sharing visual information on the nature of particulate matter pollution. If the pollution is such that a film develops on surfaces, this type of monitoring can be helpful in documenting that, capturing the pollution itself and displaying it for others to understand.