Hi. My name is Chinomso Ibe. I am a graduate research assistant at the University of Minnesota.
This observational activity is titled "Instrument Response as a Function of Particle Size".
I developed this activity along with Pete Raynor from the University of Minnesota
and with support from TSI Inc., makers of precision measurement instruments.
The objective of this activity is to predict the responses of various direct-reading instruments
to two different particle sources with respect to the size-related property of the particles.
Finer particles will be generated by the combustion of an incense stick,
and coarse particles will be generated by pouring a bag of cat litter into a container.
The first direct reading instrument is a DustTrak Aerosol Monitor which can measure mass concentration
of particle sizes ranging from 0.1 to 15 micrometers in diameter. Mass concentration is
the mass of particles in a unit volume of aerosol.
The second instrument is an AeroTrak Nanoparticle Aerosol Monitor
which can measure the surface area concentration of particles 10 to 1,000 nanometers in diameter.
The surface area concentration is related to the roughness or porosity of the particle exterior.
The third instrument is a Condensation Particle Counter or CPC which measures number concentration in particle sizes that
range from 10 nanometers to greater than 1 micrometer.
Number concentration is the number of particles in a unit volume of aerosol.
The Aerodynamic Particle Sizer or APS is also included in this activity,
and it measures particle size distributions from 0.5 to 20 micrometers in diameter.
Particle size distribution describes the related amount of particles present in aerosols according to size.
The NanoScan SMPS or Scanning Mobility Particle Sizer also measures particle size distribution like the APS.
It measures particle sizes from 10 to 420 nanometers in diameter.
In other words, each instrument is sensitive to particle sizes.
It is important to note that all the direct-reading instruments used for this activity measure particle properties in real time or near real time.
Let’s begin this activity by observing the DustTrak’s response to the general background mass concentration of two sample areas.
Now, we’ll introduce coarse or larger particles by pouring cat litter into a container,
and fine or smaller particles by lighting an incense stick with a lighter.
Appropriate rubber or plastic tubings are used here to direct the particles to the instruments.
How does the DustTrak respond to the two different particles generated?
Which particle source will show a significant difference in mass concentration?
It’s the cat litter. The DustTrak shows a significant increase in mass concentration with the cat litter
compared to that of the incense stick. This is because the mass of smaller or nanoparticles
like that found in the incense stick are often insignificant compared to larger ones.
Therefore, for smaller or nanoscale particles, mass concentration changes
may not be detected well by a direct-reading instrument like the DustTrak.
Now, let’s continue the exercise with the AeroTrak aerosol monitor.
Here’s the background surface area concentration of the two sample areas.
In comparing the large particles generated by the cat litter and the
small particles generated by the incense stick,
which particles will show a significant difference in surface area concentration when compared to its background concentrations?
Both particle sources show significant differences compared to their background concentrations,
although that of the cat litter is more significant.
This is because surface area concentrations can be influenced by both large and small particles
and therefore, can be measured by the Aerotrak monitor.
The third direct reading instrument to observe is the Condensation Particle Counter or CPC,
which measures number concentrations. Here are the background number concentrations of the two sample areas,
and here are the particles generated by the cat litter and incense stick in the sample areas.
What is the CPC’s response to the particle sources' number concentration?
There is a significant increase in number concentration from the incense stick. The CPC often reflects the presence of
smaller or nanoparticles, being that these particles can be numerous in the air compared to larger ones.
Next is the aerodynamic particle sizer or APS,
which displays the size distribution of particles in a volume of air.
Here are the background concentrations.
Which particle source will show a wide range of particle sizes on the APS?
It is the cat litter! The cat litter has a wide range of particle sizes, many of which are larger particles above 1 micrometer in diameter.
On the other hand, the incense stick has a small range of particle sizes and most of them are below 1 micrometer in diameter.
In other words, the APS is not useful for detecting the size distribution of smaller or nanoscale particles.
The last direct reading instrument to observe for this activity is the NanoScan SMPS.
Like the APS, the SMPS also displays the size distribution of particles in an aerosol.
So here are the background concentrations of the sample areas
Next, introduce the particle sources before running the SMPS.
What particle source will show a wide particle size distribution?
And which particle source generates the most number of particles?
It’s the incense stick! The incense stick has a wider particle size distribution and shows the most number of particles detected.
The SMPS is therefore useful in detecting the size distribution of smaller or nanoscale particles
instead of large particles that can be found in cat litter.
This concludes the activity on instrument response as a function of particle size. Thank you for watching.