Condensation particle counter

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A condensation particle counter (CPC) counts tiny aerosol particles by making them grow into visible droplets inside a supersaturated vapor.

Key points:
- It can detect particles as small as about 2 nanometers.
- The particle is enlarged so it can be counted by light, which many detectors use.

How it works:
- Air with particles enters a chamber where vapor is added and kept at high enough supersaturation so each particle acts as a nucleus and grows into a droplet.
- The droplets are then detected by measuring light scattered from them.

Nucleation methods:
- The most common and efficient method is cooling by thermal diffusion.
- The working liquid is usually n-butanol, though water is used in newer systems.

How the vapor is produced:
- The liquid is in contact with a heated porous block, which humidifies the air.
- The air then cools in the chamber, causing nucleation and droplet growth.
- The degree of supersaturation is controlled by the temperature difference; larger differences allow smaller particles to grow.

Flow arrangement:
- The gas flow can be split so most goes through a filtering sheath flow, while the particle-containing portion goes into the growth region.

Why this helps counting:
- The grown droplets can reach about 10–12 micrometers, making them easier to detect with standard optical methods.
- Supersaturation must be enough to grow real droplets but not so high that droplets form without particles (false counts). This balance is usually kept below around 300% supersaturation.
- In addition, droplets can form on solid surfaces at lower supersaturation, helping ensure reliable growth.