Community Observation Networks for Air: What is $PM

by Gustavo Olivares

Gaseous pollutants have a very clear definition because they are discrete species. Ozone will always be ozone and carbon monoxide will always be the carbon monoxide. Their chemical composition is known and regardless of where they are measured, their composition doesn't change.

Electron microscope image of a $PM_{10}$ filter.
Obtained from: http://www.npl.co.uk/upload/pdf/20100608_mansa_maynard_3.pdf

Now, what we call particulate matter is a different beast altogether. First, it is not one species but a rather complicated mixture of many things, some of them with known composition (ammonium sulphate or sodium chloride) and others not that well defined like black carbon that require their own explanation. On top of that, these mixture changes both in time and space. This means that we can't define particulate matter as a species and therefore we can't really measure it so we measure its characteristics. Look at this image from a $PM_{10}$ filter. All the particles you see fall within the definition of $PM_{10}$!

It's like having a party and wanting to measure the guests. For every party the guests will be different so we can't measure guests as a separate entity but we can measure things about the guests. For example, we can count the guests, we can measure the weight of the room with and without guests and we can try to describe specific characteristics of the guests like how many of them wore hats, what's the average shoe size of the guests.

Well, that's pretty much what we do when we say we measure particulate matter. We weigh filters with and without particles, we count the particles, we measure the carbon content of the particles and we measure their average size.

One of the most used methods is to weigh the particles (actually weigh the filters where they are sampled before and after) but that also needs clarification because the particles come in all sizes and shapes. In air pollution we are mostly concerned with those particles that can penetrate to our lungs and therefore we set maximum limits and define

From: Lílian Lefol Nani Guarieiro and Aline Lefol Nani Guarieiro (2013). Vehicle Emissions: What Will Change with Use of Biofuel?, Biofuels - Economy, Environment and Sustainability, Prof. Zhen Fang (Ed.), ISBN: 978-953-51-0950-1, InTech, DOI: 10.5772/52513. Available from: http://www.intechopen.com/books/biofuels-economy-environment-and-sustainability/vehicle-emissions-what-will-change-with-use-of-biofuel-

$PM_{10}$: Mass of particles smaller than $10 \mu m$ per volume of air. Because all these particles can get past the nose.
$PM_{2.5}$: Mass of particles smaller than $2.5 \mu m$ per volume of air. Because all these particles can get past the nose and then past trachea.
$PM_1$: Mass of particles smaller than $1 \mu m$ per volume of air. Because all these particles can get past the nose, and then past the trachea and onto the alveoli
$PM_{0.1}$: Mass of particles smaller than $0.1 \mu m$ per volume of air. Because all these particles can get past the nose, and then past the trachea and onto the alveoli and into the bloodstream.

Then why people think of $PM_{10}$ as a pollutant different from $PM_{2.5}$?

In a nutshell ... because we (scientists) have been bad at communicating these subtleties to the general public and we have allowed sloppy language to misrepresent what PM and their metrics are. $PM_1$ is part of $PM_{2.5}$ which is part of $PM_{10}$.

Now, is $PM_{2.5}$ more dangerous than $PM_{10}$?
Not per-se. If we think that the particles cause more damage the further they go into our respiratory system, the particles that cause the most impact are the smallest ones and those are measured as part of $PM_{10}$ and $PM_{2.5}$. What is clear is that the metric $PM_{2.5}$ has fewer "larger" particles compared to the smaller ones and therefore it is expected to correlate better with health effects.

So ... going back to the beginning. "What is $PM_{10}$?" ... it's one of many ways of describing the particles that cause adverse effects on the population and it is not a separate species.