The last category that I will discuss are size- resolved, direct-reading instruments. These are the "top shelf" of aerosol measurements.
Most are quite expensive and large, restricting their use to specialty area sampling only.
Optical particle counters provide number concentration of particles by size, typically from 300 nanometers to 20 micrometers.
Here, I show the optical configuration of the Grimm Particle Dust Monitor 1108.
The light from a laser diode is shaped with lenses to illuminate a sensing volume.
A detector measures the intensity of the light scattered by each particle that enters the sensing volume.
The intensity of scattered light is used to classify the particle by size with larger particles scattering more light than smaller ones.
An aerodynamic particle sizer uses the principle of inertia to size particles. An aerosol is rapidly accelerated through two laser beams.
A photomultiplier tube is used to measure the light scattered by the particle as it passes through the beams creating two light pulses.
The time that it takes to traverse the two beams is called the time-of-flight.
Small particles, roughly less than 500 nanometers, accelerate rapidly to the same speed as the air in the nozzle.
The inertial properties of larger particles causes them to increasingly deviate from the airflow
velocity, and having longer and longer time-of- flights. In this way, particles can be counted and sized at the same time.
The scanning mobility particle sizer uses electrical forces to size submicrometer aerosols.
Aerosol is first neutralized and then passed through the outside wall of a column in an electrostatic size classifier.
A clean sheath air is passed over a negatively- charged rod. The negative-charged rod attracts positively-charged particles.
The particles of one size pass through a slit in the lower section of the collection rod and are
counted with a condensation particle counter, or CPC.
This device can be used to measure particle number concentration by size from 3 nanometers to 800 nanometers.
Sizers provide a wealth of data. These data shown here are from a study of nanoparticles in the city of Pittsburgh.
In the top plot, a color scale is used to indicate particle number concentration from low values (blue) to high values (red).
Particle size is shown on the y-axis by time of day on the x-axis.
Each slice of the data represents a size distribution measured at a particular time.
Early in the morning, for instance about 3 AM, there was a background level of particles in the air
but in the afternoon the particle size distribution had changed dramatically.
These data can help understand sources of aerosols in various airsheds or occupational settings.
Lastly, I will cover the top of the top shelf of aerosol instruments: aerosol time-of-flight mass spectrometry.
First, a particle is sized by measuring its time- of-flight.
Then, it is vaporized into elemental fragments that are measured in a mass spectrometer.
The mass spectra of these fragments can be used to determine the composition of the particle.
As you can imagine, this device costs a lot, about $300,000 to $500,000,
and it generates a ridiculous amount of data that need to be analyzed.
Although impractical for most occupational hygiene, it represents a combination of techniques that we have seen before and
perhaps with a lot of miniaturization could be the Star Trek tricorder that we are looking for.