Commonly expressed in several ways: mass concentration (usually as $\text{μ}\text{g}{\text{m}}^{-3}$) or number concentration (number of particles ${\text{cm}}^{-3}$); modern instrumentation allows measurement of the number of particles as a function of size as well as the total number present in a given air volume. For atmospheric aerosols, this is a complex distribution for which diameters range from below $0.01$ to above $100\text{μ}\text{m}$; the particles making the highest contribution to the total number density are in the size range below $0.1\text{μ}\text{m}$, those contributing most to the total surface area are in the $0.1$ to $1.0\text{μ}\text{m}$ range, while those with the highest contribution to the volume or mass of the aerosol come from both the $0.1$ to $1.0\text{μ}\text{m}$ and $1.0$ to $100\text{μ}\text{m}$ ranges.