Understandings of Particulate Matter

Particulate Matter, or PM, is the term for a mixture of solid particles and liquid droplets found in the air we breathe. These particles can come from natural sources like dust and wildfires, as well as human-made sources such as vehicles, industrial activities, and burning of fossil fuels. Major components are black carbon, sulfate, nitrates, ammonia, sodium chloride, mineral dust and water.

Various types of PM are named after their size and measured in micrograms per cubic meter, μg/m3. Particulate matter can be classified based on their size, ranging from coarse particles (PM10) to fine particles (PM2.5) and ultrafine particles (PM0.1). 

A synonym for particle matter commonly used when particles occur together with gases is aerosol.

There are a number of types of aerosols, commonly referred to as:

Dust – solid particles that form when a material disintegrates through a mechanical process, such as crushing, grinding and blasting.

Fumes - solids produced by reactions such as combustion (burning or reacting with oxygen in a way that gives off heat and light), sublimation (the process in which a gas turns into a solid) or distillation.

Smoke - a visible aerosol produced by some sort of oxidation process, such as burning.

Fog and mist - liquid particle aerosols produced by the breakdown of liquid or combustion of vapour.

Smog - a term that comes from a combination of smoke and fog which refers to solid and liquid particles that are created, at least partly, by photochemical reactions (i.e. reactions caused by light being absorbed).

Natural and anthropogenic PM

PM is mainly generated through two processes: natural and anthropogenic. The natural process includes events such as volcanic eruptions, erosion from sea salt, spontaneous forest fires and soil erosion. The anthropogenic, or man-made, process refers to emissions to the earth’s atmosphere that are caused directly or indirectly by human beings. These emissions mainly originate from traffic, other forms of transportation or from industrial sources including electricity generation, mining, welding and building.[1]

Natural occurring sources of PM:

• Soil dust
• Sea salt
• Volcanic activity
• Forest fires
• Plants (ex pollen & spores)

Anthropogenic (man-made) occurring sources of PM:

• Coal burning
• Transportation/Fuel combustion
• Industrial processes
• Mining, welding, building [2]

Primary and Secondary Emissions

If particles are emitted directly into the atmosphere from any source, they are classified as primary particulate matter. Mineral dust, metals, soot, salt particles, pollen, and spores are all primary aerosols.

However, secondary particulate matter can also form in the atmosphere through processes that convert gas into particles.

Secondary aerosols are formed by gases such as sulfates, nitrates, and organic compounds.The size of particles ranges from clusters of molecules measuring 0.001 micrometers (μm), to fog droplets and dust particles as large as 100 μm.

Once aerosols are present in the atmosphere they are affected by several mechanisms which change their size, number and chemical compositions, until they are ultimately removed by natural processes.[2] Aerosol particles of natural origin tend to have a larger radius than human-produced aerosols.

Size and sources

Particulate matter is categorized by size, where a number following “PM” represents particles in any size up to that number in micrometers (μm). So, PM10 describes anything under 10μm in size, including PM2.5 and PM0.1, while PM2.5 describes anything under 2.5μm in size, including PM0.1. However, when PM10 is mentioned, it is often to refer to particles close to 10 micrometers in diameter. For this reason, PM10 are often referred to as “coarse particles”, even though it also contains both fine and ultrafine particles. In most locations in Europe, 50–70% of PM10 is made up of PM2.5. [3]

PM10:

Particles less than 10 micrometers in diameter (10 000 nm). Often called coarse particles.

PM2.5:

Particles less than 2.5 micrometers in diameter (2500 nm). Often called fine particles.

PM0.1:

Particles less than 0.1 micrometers in diameter (100 nm). Called ultrafine particles.

Coarse particles account for the largest volume and mass of all air airborne particles, but are actually very few in number compared to fine and ultrafine particles. The same amount of PM will fit a higher number of fine particles than coarse particles, meaning that the particle size with the largest combined surface area are fine particles, around 700 nm (0.7 micrometer) in size.

Figures 2C-2E shows how the volume, number and weight of particles are divided on the different sizes. The surface area of particles can have a great effect on the health, as a large surface area means a large reactive area, as well as a more space to hold toxic material.

Fig 2A shows how the particle diameter differs for different types of particles, some of them having very different sources. Heavy dust is some of the biggest particles to find in air, whilst viruses, soot and smog are very fine particles ans some of the smallest.

It can be quite difficult to imagine the size of a particle in the range of a couple of nanometers in diameter. Fig 2B compares the particle sizes PM10, PM2.5 and PM0.1 with a human hair for better understanding.

Coarse Particles, PM10

As previously mentioned, PM10 is commonly referred to as “coarse particles”. These are relatively large airborne particles mostly produced by larger solid particles breaking into smaller pieces.

Examples of coarse particles include dust, pollen, spores and ash, as well as plant and insect parts.

Over continents, coarse particles are mainly produced by the wind stirring up loose soil. The potential source area for the production of coarse particles is significant, about one third of the total landmass on earth.

Regions in which dusts are a major part of the atmospheric aerosol can be found concentrated in different geographical locations where there are deserts or dry regions, such as the Sahara, the Saudi Arabian peninsula, the US Southwest and Tajikistan. [2]

Fine Particles, PM2.5

Unlike coarse PM, a much greater part of fine PM (PM 2.5) is made up of secondary particles. Secondary particles are formed in the atmosphere through the chemical reactions between emissions in gas form. As such, these particles can form at locations very far away from the sources that released the gases.

Examples include sulfates formed from sulfur dioxide emissions from power plants and industrial facilities and nitrates formed from nitrogen oxides released from power plants, mobile sources, and other combustion sources. [4]

Ultrafine Particles, PM0.1

Although there is no formal definition for ultrafine particles, or UFP, this term commonly refers to particles of 100 nanometers (nm) or less in diameter, also known as PM0.1.The biggest source of ultrafine PM is combustion.

Common sources are vehicles both on and off the road, but also include fuel combustion from stationary machines and a number of processes such as charbroiling, petroleum refining, and waste burning. [5]

Measures to Reduce Particulate Matter Pollution

There are several measures that can be taken to

reduce particulate matter pollution. Some of these measures include: 

• Use of cleaner fuels like natural gas instead of coal or diesel. 
• Use of filters and other technologies to reduce emissions from vehicles and industrial activities. 
• Implementation of strict emission standards for vehicles and industrial activities. 
• Encouraging the use of electric vehicles and promoting sustainable transportation. 
• Planting trees and creating green spaces to absorb and filter pollutants. 
• Educating the public on the health and environmental effects of particulate matter and promoting
• Individual actions to reduce personal exposure. 

Read more about health effects of Particulate Matter (PM).

Conclusion

Particulate matter is a major contributor to air

pollution and poses serious threats to human health and the environment. While particulate matter is present in the atmosphere at all times, high levels of PM

can have serious health effects, particularly on vulnerable populations like children, the elderly, and those with pre-existing respiratory and cardiovascular conditions.

Therefore, it is important to take measures to reduce particulate matter pollution, including using cleaner fuels, implementing strict emission standards, promoting sustainable transportation, creating green spaces, and educating the public on the effects of particulate matter. By taking these steps, we can create a cleaner, healthier, and more sustainable environment for ourselves and give clean air to our next generation.