Commonly expressed in several ways: mass concentration (usually as \(\unicode[times]{x3BC}\text{g}\ \text{m}^{-3}\)) 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 \(100\ \unicode[Times]{x3BC}\text{m}\); the particles making the highest contribution to the total @N04262@ are in the size range below \(0.1\ \unicode[times]{x3BC}\text{m}\), those contributing most to the total surface area are in the \(0.1\) to \(1.0\ \unicode[times]{x3BC}\text{m}\) range, while those with the highest contribution to the volume or mass of the @A00176@ come from both the \(0.1\) to \(1.0\ \unicode[times]{x3BC}\text{m}\) and \(1.0\) to \(100\ \unicode[Times]{x3BC}\text{m}\) ranges.