Editing Area density (section)

===Atmospheric physics===
It is a quantity commonly retrieved by [[remote sensing]] instruments, for instance the [[Total Ozone Mapping Spectrometer]] (TOMS) which retrieves ozone columns around the globe.  Columns are also returned by the [[differential absorption spectroscopy|differential optical absorption spectroscopy (DOAS)]] method<ref name="Sinreich_etal2008">
{{cite news
 |author1=R. Sinreich |author2=U. Frieß |author3=T. Wagner |author4=S. Yilmaz |author5=U. Platt | title=Retrieval of Aerosol Distributions by Multi-Axis Differential Absorption Spectroscopy (MAX-DOAS)
 | journal=Nucleation and Atmospheric Aerosols
 | year=2008
 | pages= 1145–1149
 | doi=10.1007/978-1-4020-6475-3_227
|isbn=978-1-4020-6474-6 }}</ref> and are a common retrieval product from [[nadir]]-looking [[microwave]] [[radiometers]].<ref name="Melseimer_Heygster2008">
{{cite journal
 |author1=C. Melsheimer |author2=G. Heygster | year=2008
 | title=Improved retrieval of total water vapor over polar regions from [[Advanced Microwave Sounding Unit|AMSU]]-B microwave radiometer data
 | journal=IEEE Trans. Geosci. Remote Sens.
 | volume=46
 | number=8
 | pages=2307–2322
 | doi=10.1109/TGRS.2008.918013
|bibcode = 2008ITGRS..46.2307M |s2cid=20910677 }}</ref><ref name="Melseimer_etal2009">
{{cite news
 |author1=C. Melsheimer |author2=G. Heygster |author3=N. Mathew |author4=L. Toudal Pedersen | year=2009
 | title=Retrieval of Sea Ice Emissivity and Integrated Retrieval of Surface and Atmospheric Parameters over the Arctic from [[AMSR]]-E data| journal=Journal of the Remote Sensing Society of Japan | volume=29
 | number=1
 | pages= 236–241
}}</ref>

A closely related concept is that of ice or [[liquid water path]], which specifies the volume per unit area or depth instead of mass per unit area, thus the two are related:
<math display=block>P = \frac{\sigma}{\rho_0}.</math>

Another closely related concept is [[optical depth]].