What are ppm and ppb?

PPM and PPB are units used in atmospheric chemistry to describe the concentration of gases.

PPM stands for parts of gas per million parts of air, and PPB is parts per billion. If that's not easy to imagine, try these examples:

Part per million (ppm):

  • 1 second in just under 2 weeks.
  • 1 litre of water in a swimming pool.

Part per billion (ppb):

  • 1 second in 32 years.
  • 1 ml of water in a swimming pool (that's about the size of a sugar cube).

Part per trillion (ppt):

  • The size of a newspaper compared to the whole of New Zealand
  • 15 cents compared with the whole New Zealand economy

The reason gases are measured at these levels is that a small amount of some gases can have a very large effect, and if you want to study how these levels are changing over time, you need make very sensitive measurements.