Introduction to Particulate Matter
Pages in this wiki:
- Visual monitoring
- Filter-based monitoring
- Optical monitoring
- Passive monitoring
Particulate Matter (PM) is airborne dust and particle pollution that settles onto surfaces and into lungs. As a regulated pollutant PM is shorthand for inhalable and respirable particulate matter, or particulate matter that can stick in the lungs.
Based on size alone, small airborne particles can become lodged in the lungs or even enter the bloodstream. Some non-toxic materials, such as silica, can be carcinogenic.
Historically, most dust was naturally occurring, but at present natural sources of particles such as wind erosion, volcanoes, pollen, and forest fires have been overtaken by human-generated particles from combustion, roads, agriculture, construction, and mining (citation:EPA/600/R-95/115).
Monitoring sources of particle pollution and [advocating for their reduction] (/wiki/frac-sand-action-oriented-resources) can have positive public health impacts. According to the CDC, a 10% reduction in fine particles could prevent 13,000 deaths annually in the US.
Airborne particles we can see
The smallest particles we can see with a naked eye are visible only because they diffract light to make a haze, usually with a reddish-purple tint. We cannot see haze particles directly, however, haze can be monitored as a proxy for small particles. Mold spores, lint, and household dust particles can be seen individually only when reflecting light, as in the rays coming through window into a dark room. Particles of fine sand and soil that are visible can get airborne for short periods of time. Fog are small raindrops falling slowly, and are just barely visible. Of visible particles, only haze-sized particles pose a significant health risk, see Respirable Particles below.
Dust, droplets, & particle size
Almost all airborne particles are either dust: solid particles broken from larger solids, or droplets: liquid particles which grow as they condense gases out of the air. A third category of nano-sized particles, ultrafines, are short-lived emissions from combustion.
These three modes, ultrafines, droplets, and dust, are each clustered around a specific size range, such that the sizes of particles in the air are not evenly distributed. Ultrafines are short-lived, forming the center of droplets quickly. Large dust particles are also short lived, settling out. In the middle are mature droplets and fine dust that make up both the bulk of long-lived atmospheric particles and the most worrisome particles because of their respirability.
While some dust comes from biological sources (skin, bacteria, mold, pollen), most comes from dirt and rocks crushed small enough to get airborne. Only dust less than 10μm can stay airborne for days, and less than 5μm dust can travel for years. Larger dust settles out (called sedimentation), while smaller dust is removed by being washed away in rain or by running into objects (impaction).
Droplets are formed as gases cool and condense. Atmospheric droplets condense from combustion gases, especially industrial and transportation emissions, and also water. Atmospheric water dominates the droplet formation process.
Cooling gases quickly condense into droplets in what is called the ‘accumulation mode’ of droplets. Accumulating droplets are sometimes called ‘cloud scavenging’ for the way they grow by collecting gases and mixing with other droplets.
Droplets gain and lose water as the humidity changes. condensing water often brings multiple droplets together, and this ‘wetting’ and ‘drying’ of droplets can aid in droplet accumulation.
Droplets’ Beginnings: Ultrafine nulceotoids
While dust can only be ground to about .5μm and most dust particles are much bigger. Smaller solid particles can be formed under intense heat and pressure, such as in a fire or engine. These ultrafine, or nanoparticles, are less than 0.1μm and last only as long as their rapidly dissipating energy can keep them from bonding. With only a dozen to a few hundred molecules making up each ultrafine particle, ultrafines’ behavior is poorly understood. Ultrafines, especially elemental carbon nanoparticles from transportation and diesel, are a growing field of study.
As ultrafine particles lose energy, cooling gases condense around them, ‘nucleating’ (forming the center, or nucleus, of) a new droplet. Often the gases condensing onto ultrafines are in the same emissions stream from combustion, including sulfates, nitrates, and V.O.C.s. The droplets formed around ultrafines may also nucleate other droplets, especially ‘wet’ droplets of water.
The body removes objects from the lungs in two ways, by coughing (“expectorating”), or by absorption and removal by the blood stream. In order to enter the bloodstream, particles must pass the last branching passageways in the lungs: the terminal bronchioles. Particles above the terminal bronchioles are the “thoracic fraction” (thoracic means in the chest), and below the terminal bronchioles particles are considered respired particles. Respired particles may, however, still be removed by coughing.
The most particles in the respiratory system average around 2.5μm, while most in the thoracic fraction are are around 10μm. The fate of short-lived ultrafine particles in the lungs is still being studied.
Particulate Matter is one of six ‘criteria pollutants’ determining National Ambient Air Quality Standards (NAAQS All of the EPA’s technology-based particle regulations share features in common with the PM10 standard, and a deep look at the PM10 standard is illustrative.
PM10 is the US EPA’s first attempt to capture a standardized indicator of respirable particles. “PM10” stands for Particulate Matter less than or equal to 10μm. Established in 1987, PM10 is now a global benchmark. PM10 is a technology-based standard-- all PM10 tools and measurements are related back to the original reference machine, filter-based PM Monitors. Similar measurement tools with a tight correlation with this original Federal Reference Methods (FRM) now share the FRM designation. Tools that use different processes and have a less tight correlation are designated Federal Equivalent Methods (FEM).
The goal of the FRM is to generate a 24 hour average of the mass of respirable particles in the air. It does this by pumping a precise volume of air inside, selecting the particles smaller than 10μm, and depositing them on a filter. The particles are selected for size with a device called an impactor. The function of an FRM impactor is written into the regulation and legally defines what is and isn’t PM10.
An impactor sorts particles by momentum. A plate interrupts the air’s linear flow. Light particles stay in the air stream and pass around the plate. More massive particles can’t make the turn and hit the plate. This selection isn’t a hard cutoff:
50% of particles below 10μm are passed by the impactor, and 50% above. The distribution is not even, and the rate at which the impactor cuts off particles above 10μm is called the ‘sharpness’ of the cutoff.
Other categories of regulation include PM2.5 and PM10-2.5, read more in PM Monitoring Regulations.