Commonly expressed in several ways: mass concentration (usually as \(\unicode[times]{x3BC}\rm{g}\ \rm{m}^{-3}\)) or number concentration (number of particles \(\pu{cm-3}\)); 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 \(100\ \unicode[Times]{x3BC}\rm{m}\); the particles making the highest contribution to the total number density are in the size range below \(0.1\ \unicode[times]{x3BC}\rm{m}\), those contributing most to the total surface area are in the \(0.1\) to \(1.0\ \unicode[times]{x3BC}\rm{m}\) range, while those with the highest contribution to the volume or mass of the aerosol come from both the \(0.1\) to \(1.0\ \unicode[times]{x3BC}\rm{m}\) and \(1.0\) to \(100\ \unicode[Times]{x3BC}\rm{m}\) ranges.