Commonly expressed in several ways: mass concentration
(usually as

)
or number concentration (number of particles

);
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

to above

;
the particles making the highest contribution to the total
number density are in the size range below

,
those contributing most to the total surface area are in the

to

range, while those with the highest contribution to the volume or mass of the
aerosol come from both the

to

and

to

ranges.
Source:
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2181