Aerosols and particulate measurement

An aerosol is solid or liquid particles suspended in air, and almost everything about one follows from its size: particles above a few micrometres settle out within minutes, those below a micrometre stay airborne for days and are carried by air currents and Brownian motion rather than gravity, and only the small ones reach deep into the lungs — which is why air quality limits are written for PM10 and PM2.5 rather than for mass alone. Measuring them means choosing which physical effect stands in for size: weighing what a filter collects, counting scattered-light pulses one particle at a time, or reading an attenuation, each of which answers a different question and can disagree with the others.

Practise this concept

Nothing about an aerosol matters as much as the diameter of its particles. Settling under gravity goes as diameter squared, so a 50 micrometre grain drops out of still air in seconds, a 10 micrometre one takes minutes, and below about a micrometre gravity has effectively stopped mattering — those particles are carried by air currents, wander by Brownian motion, and leave the atmosphere by sticking to surfaces, clumping together or being rained out. That is why coarse dust settles onto a windowsill while smoke haze crosses a border.

Size decides the health effect too, and for the same mechanical reason. Particles larger than about ten micrometres are caught in the nose and throat; below that they reach the airways, and below 2.5 micrometres they penetrate to the alveoli where gas exchange happens. Air-quality regulation is written around exactly those cut-offs — PM10 and PM2.5 — and both are mass concentrations, micrograms per cubic metre, not counts. Since mass goes as diameter cubed, the number is dominated by the largest particles in each range, and an event that produces enormous numbers of ultrafine particles can barely move the PM2.5 figure.

Measuring an aerosol therefore means choosing which physical effect will stand in for size. The reference method is gravimetric: pull a known volume of air through a filter with a size-selective inlet and weigh it, which is unambiguous, traceable and gives one number every 24 hours. Optical counters instead flash each particle through a laser beam and size it from the scattered pulse, giving a live size distribution — but scattering depends on refractive index and shape, so the reading is the diameter of an equivalent calibration sphere and soot reads differently from sea salt. Other instruments measure attenuation of a beam or of beta radiation through collected material, or charge particles and measure their electrical mobility. Each answers a slightly different question, which is why a cheap sensor and a reference station can disagree without either being faulty, and why co-location and calibration are part of any serious monitoring programme.

Key ideas

Requires: Atmospheric and ocean physics, Rayleigh and Mie scattering, Measurement, precision and accuracy

Formulas

v = \frac{\rho d^{2} g}{18 \eta}

Symbol	Name	Unit
$v$	velocity	m/s (metre per second)
$rho$	density	kg/m^3 (kilogram per cubic metre)
$d$	diameter	m (metre)
$eta$	viscosity	Pa*s (pascal-second)

Common misconceptions

Reading PM2.5 as "the number of particles smaller than 2.5 micrometres". It is a mass concentration — micrograms of such particles per cubic metre of air — and mass is dominated by the largest particles in the range, since mass goes as the cube of diameter. A million ultrafine particles can weigh less than one coarse one, which is why particle counts and PM readings can move in opposite directions during the same event.
Assuming small particles settle out of the air the way large ones do, only more slowly. Settling speed falls with the square of diameter, so below about a micrometre gravity stops mattering altogether: those particles are removed by coagulation, by diffusion onto surfaces and by rain, not by falling. That is why smoke haze can persist for days and travel across a continent, while visible dust is gone from a still room within minutes.
Treating an optical particle counter as a direct measurement of size and mass. It measures scattered light, and how much a particle scatters depends on its refractive index and shape as well as its diameter — so the instrument reports the size of an equivalent calibration sphere, not the particle in front of it. Soot and sea salt of the same true diameter read differently, which is why a low-cost sensor needs local calibration against a gravimetric reference before its numbers mean anything absolute.

Resources

Articles

Part of these UT courses

This concept is mapped as covering part of the following course(s) in the University of Tartu curriculum.

Organisation of air pollution monitoringLOFY.05.032