What is the difference between particulate matter mass concentration and number concentration?
PM2.5 can be expressed two ways — by mass or by particle count. This article explains the difference and why mass concentration is the figure you'll usually work with.
The Node-S uses an optical particle counter (OPC) that provides both number and mass concentrations for particulate matter (PM1, PM2.5, and PM10). The OPC directly measures the number of particles in the sample, then converts that to a mass concentration by approximating the size of the particles.
PM2.5 mass concentration
This is the total weight of particles 2.5 micrometers in diameter or smaller in a given volume of air. It's typically measured in micrograms per cubic meter (µg/m³) or milligrams per cubic meter (mg/m³). It tells you the overall mass of particles, regardless of their individual size or number.
PM2.5 number concentration
This counts the actual number of particles 2.5 micrometers or smaller in a given volume of air. It's typically measured in particles per cubic centimeter (particles/cm³) or particles per cubic meter (particles/m³). It tells you how many particles are present, regardless of their individual mass.
Why Clarity reports mass concentration
There's growing evidence that number concentration matters for health, but mass concentration is most often the parameter of interest, because:
- Many countries and international organizations set air quality standards and guidelines based on PM2.5 mass concentration.
- PM2.5 mass concentration is a key input to the Air Quality Index (AQI), the standardized metric used to communicate air quality to the public.
What's next
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