THE IMPORTANCE OF BEING SMALL*

August 3, 2018 § Leave a comment

In matters concerning airborne particulates, size certainly matters.  But in this realm smaller is more powerful, not necessarily in a good way.  In fact, decidedly not in a good way when it comes to toxicity.  In every step along the way from inception of the particle nucleus in the combustion process, to growth and transport, and finally to action on human organs, size plays a critical part.  And the smallest particles, less than 0.1 µm in size, are the most effective in each of those reactions.  The somewhat larger particles, up to an order of magnitude larger, are no slouches either, when it comes to toxicity to humans.  But the nanoparticles are the star act in a dark play.

Over 80% of anthropogenic airborne particulate matter (PM) results from either the production or use of energy.  The three principal players are coal combustion, automobiles and wood burning for cooking and heat.  The mechanism of PM production is common to all three.  It begins with the formation of a nucleus, followed by growth and completed by reactions with atmospheric constituents such as ozone, and mediated by photochemical action.

Particulate matter can range in size from a few nanometers to a hundred micrometers.  That covers four orders of magnitude.  The range is another few orders of magnitude greater when one considers surface area of the particles, instead of diameter.  Surface area is the key parameter in the chemical reactions affecting particles.  In chemical processes, for example, the best catalysts have high surface area to volume ratios.  Not surprisingly, size does matter in the impact on human endeavor.  Except, in the PM world, the littlest guys kick serious sand into the eyes of the large ones.

Diesel exhaust particles and mass distribution

Source: https://www.dieselnet.com/tech/dpm_size.php retrieved July 31, 2018

In considering the impact of ultrafine particles, one first needs to examine their prevalence.  The figure shows a typical distribution of particles in diesel exhaust.  On a mass basis, the majority are in the fine range (0.1 < x < 2.5 µm).  But, on a particle count basis, the majority are in the ultrafine range (x < 0.1 µm).  The mass associated with the bulk of the particles is a small fraction of the total mass emitted.  This is understandable, because it takes thousands of ultrafine particles to equal the mass of a single fine particle.  But, it calls into question regulations using mass as the variable.

This might appear to be splitting the proverbial hair.  But recognize that amelioration schemes rely on the regulatory variable.  With mass as the yardstick, a low mass count could simply be the result of removal of the coarse and fine particles.  This would leave the bulk of the particle count still in the medium, and if they are in fact the true bad actors, not much will have been achieved with the filtering.  All the foregoing argument applies only to health effects.  Visibility and climate change related parameters, such as radiative forcing, may be affected by the larger size PM.  Certainly, the recently reported reduction in solar panel efficiency by PM, will be more impacted by the larger particles, because the ultrafine PM is less likely to settle on to the panels.

Since the mass related regulation may continue to have utility, an additional parameter may be prudent to consider.  Two candidates are surface area and particle count.  Surface area to volume ratio increases dramatically with reduced size.  In a sense, just particle count by size will do the job.  But, in health-related outcomes, both size and surface area are separately in play.  Size determines the degree to which the particles enter and impact various organ systems.  Surface area likely mostly impacts from the standpoint that high surface area particles are scavengers for toxic substances such as volatile organic chemicals.  They are also more likely to have highly toxic reactive oxygen species on their surface.  From a public understanding and acceptance standpoint, the particle count may be simpler to communicate.  Multiple measuring devices are currently available to perform the count.

In summary, ultrafine particles play exceptional roles in the health impact of particulate matter.  Regulatory focus ought to shift to address this fact, to better inform intervention schemes.

Vikram Rao

*with apologies to Oscar Wilde

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