Editing Aerosol (section)

== Definitions ==
{{See also|Particulates}}
[[File:Fly Ash FHWA dot gov.jpg|thumb|[[Photomicrograph]] made with a Scanning Electron Microscope (SEM): [[Fly ash]] particles at 2,000× magnification. Most of the particles in this aerosol are nearly spherical.|alt=Fly ash particles shown at 2,000 times magnification]]
[[Image:Aerosol.png|thumb|[[Aerosol spray dispenser|Aerosol spray can]]]]
Aerosol is defined as a suspension system of solid or liquid particles in a gas. An aerosol includes both the particles and the suspending gas, which is usually air.{{sfn|Hinds|1999|p=3}} Meteorologists and climatologists often refer to them as particle matter, while the classification in sizes ranges like PM2.5 or PM10,<ref>PM2.5 refers to the mass of particles with sizes between 0 and 2.5 micrometers, and PM10 for sizes between 0 and 10 micrometers.</ref><ref name="auto">{{cite web |title=Aerosols: Tiny Particles, Big Impact |url=https://earthobservatory.nasa.gov/features/Aerosols |website=earthobservatory.nasa.gov |language=en |date=2 November 2010}}</ref> is useful in the field of atmospheric pollution as these size range play a role in ascertain the harmful effects in human health.<ref>{{Cite web |last=US EPA |first=OAR |date=2016-04-19 |title=Particulate Matter (PM) Basics |url=https://www.epa.gov/pm-pollution/particulate-matter-pm-basics |access-date=2024-11-04 |website=www.epa.gov |language=en}}</ref> [[Frederick G. Donnan]] presumably first used the term ''aerosol'' during [[World War I]] to describe an aero-[[Solution (chemistry)|solution]], clouds of microscopic particles in air. This term developed analogously to the term [[Sol (colloid)|hydrosol]], a [[colloid]] system with water as the dispersed medium.{{sfn|Hidy|1984|p=5}} ''Primary aerosols'' contain particles introduced directly into the gas; ''[[Secondary organic aerosol|secondary aerosols]]'' form through gas-to-particle conversion.{{sfn|Hinds|1999|p=8}}

Key aerosol groups include sulfates, organic carbon, black carbon, nitrates, mineral dust, and sea salt, they usually clump together to form a complex mixture.<ref name="auto"/> Various types of aerosol, classified according to physical form and how they were generated, include dust, fume, mist, smoke and fog.{{sfn|Colbeck|Lazaridis|2014|p= Ch. 1.1}}

There are several measures of aerosol concentration. [[Environmental science]] and [[environmental health]] often use the ''[[Mass concentration (chemistry)|mass concentration]]'' (''M''), defined as the mass of particulate matter per unit volume, in units such as μg/m<sup>3</sup>. Also commonly used is the ''[[Number density|number concentration]]'' (''N''), the number of particles per unit volume, in units such as number per m<sup>3</sup> or number per cm<sup>3</sup>.{{sfn|Hinds|1999|pp=10-11}}

Particle size has a major influence on particle properties, and the aerosol particle radius or diameter (''d<sub>p</sub>'') is a key property used to characterise aerosols.

Aerosols vary in their [[dispersity]]. A ''monodisperse'' aerosol, producible in the laboratory, contains particles of uniform size. Most aerosols, however, as ''polydisperse'' colloidal systems, exhibit a range of particle sizes.{{sfn|Hinds|1999|p=8}} Liquid droplets are almost always nearly spherical, but scientists use an ''equivalent diameter'' to characterize the properties of various shapes of solid particles, some very irregular. The equivalent diameter is the diameter of a spherical particle with the same value of some physical property as the irregular particle.{{sfn|Hinds|1999|p=10}} The ''equivalent volume diameter'' (''d<sub>e</sub>'') is defined as the diameter of a sphere of the same volume as that of the irregular particle.{{sfn|Hinds|1999|p=51}} Also commonly used is the [[#Aerodynamic diameter|aerodynamic diameter]],&nbsp; ''d<sub>a</sub>''.