Physical and chemical characteristics of aerosolsThe environmental impacts of atmospheric particles depend on their physical, chemical properties, lifetimes and abundances. The concentration, size distribution and composition of atmospheric aerosol particles are highly variable both temporally and spatially.
Table 9.3: Commonly used measures of aerosol concentration
|Area concentration||µm2 cm–3||4 r2 π N|
|Volume concentration||µm3 cm–3||4/3 r3 π N|
|Mass concentration||µg m–3||4/3 r3 π N ρ|
(cm –3 )
( µg m –3 )
|Urban||105 – 107||101 –104|
|Rural||103 – 104||101 – 102|
|Remote maritime||102||101 – 102|
|Polar||100 – 103||10–1 – 101|
PMx (particulate matter with diameter smaller than x µm) is another often-used measure to describe the aerosol mass concentration. PM (2.5) and PM (10) are routinely monitored values.
Based on particle distributions, different groups of atmospheric particles can be separated:
- nucleation (Aitken) mode (particle diameter < 0.1 µm),
- accumulation mode (particle diameter: 0.1 µm > d > 1 µm),
- coarse mode (particle diameter d > 1 µm).
Larger aerosol particles in the size range 0.1 to 1 µm in diameter can accumulate in the atmosphere because their removal mechanisms are least efficient. Their lifetime in the atmosphere is 7–10 days and during this period they can transported to a long distance from their sources. Particles belonging to this accumulation mode are formed mainly by coagulation (Figure 9.9) of smaller particles or condensation of vapours onto existing particles, and during these mechanisms they growth into this size range. At the same time, they can also be emitted to the atmosphere from different sources, mainly from incomplete combustion. Accumulation particles removed from the atmosphere mainly by wet deposition.
The Coarse mode contains particles with diameter larger than 1.0 μm. These particles mostly emitted to the atmosphere during mechanical processes from both natural and anthropogenic sources (e.g. sea-salt particles from ocean surface, soil and mineral dust, biological materials). Due to their relatively large mass, they have short atmospheric lifetimes because of their rapid sedimentation.
The distribution of atmospheric aerosol particles can be seen if Figure 9.6. Small particles in nucleation mode constitute the majority of atmospheric particles by number. However due to their small sizes, their contribution to the total mass of aerosols are very small (around a few percent). The Accumulation mode particles have the greatest surface area. The mass or volume concentration is dominated by the aerosols in coarse and accumulation modes. Size, area and volume distributions of aerosol particles show characteristic pattern at different locations (e.g. urban, rural, remote continental or marine regions).
The main precursors of sulphate component () in the troposphere are sulphur dioxide (SO2) emitted from anthropogenic sources and volcanoes, and dimethyl sulphide (DMS) from biogenic sources, especially from marine planktons. In the stratosphere, sulphate aerosols mostly converted from carbonyl sulphide (COS).
Nitrate () is formed mainly from the oxidation of atmospheric nitrogen dioxide (NO2). Ammonium salts are also common components of atmospheric aerosols. They are formed during the reactions between ammonia (NH3) and various acids, like sulphuric (H2SO4) and nitric acids (HNO3). When atmospheric ammonia neutralises these acids, ammonium sulphate ((NH4)2SO4), and ammonium nitrate (NH4NO3) particles are formed. Main source of chloride (Cl–) is sea spray, but ammonium chloride (NH4Cl) particles form also during the reaction between ammonia and hydrochloric acid (HCl).
Carbonaceous materials constitute a large but highly variable fraction of the atmospheric aerosol. The carbonaceous fraction of the aerosols consists of both elemental carbon (EC) or black carbon (BC) and organic carbon (OC). The ratio of elemental to total carbon (EC+OC) is strongly depends on the sources. The main sources of elemental carbon particles are biomass and fossil fuel burning. Particles containing organic carbon can emitted directly to the atmosphere also from biomass burning or combustion processes and by secondary organic aerosol (SOA) formation during the atmospheric oxidation of biogenic or anthropogenic volatile organic compounds (VOC).
Typical chemical composition of aerosols can vary at different locations, times, weather conditions and particle size fractions. Figure 9.10, Figure 9.11 and Figure 9.12 show the relative abundance of different chemical components of fine particles in different locations, in urban area, in rural region and in a remote site, respectively.
Several particles are poorly soluble or insoluble in water. Insoluble aerosols for example particles derived from soil dust or volcanoes (e.g. metal oxides, silicates, clay minerals).
Smaller particles are efficiently removed by coagulation with other particles. Therefore, their lifetime is very short (in a range of ten minutes to day). Similarly, the large particles spend only a short time in the atmosphere due to the sedimentation. Particles in the accumulation mode have the longest lifetime (7–10 days on average), as in this range, both the Brownian diffusion and sedimentation are less important. These particles removed from the atmosphere predominantly by wet deposition.
Table 9.5 summarises the sources and formations as well as the main physical and chemical properties of different size aerosol particles.
Table 9.5: Main properties of different aerosol particles (Adapted from Wilson and Suh, 1997):
|Nucleation mode||Accumulation mode||Coarse mode|
|(Fine particles)||(Coarse particles)|
|Size:||d < 0.1 µm||0.1 µm > d > 1 µm||d > 0.1 µm|
Gas-to Particle conversion
Gas-to Particle conversion
Evaporation of droplet
|Mechanical disruption of surface
Suspension[a] of dust
Evaporation of ocean spray
Low-volatility organic compounds
Trace metals (Pb, Cd, V, Ni, Cu, Zn, Fe, etc.)
Biogenic organic particles
|Solubility||Largely soluble, hygroscopic||Largely soluble, hygroscopic||Largely insoluble, non-hygroscopic|
|Travel distance||<a few 10 of km||a few 100 to 1000 of km||<a few 10 of km
|Typical atmospheric lifetime||Minutes to hours||Days to weeks||Minutes to days|
|Sinks||Growth into accumulation mode,
wet and dry deposition
dry deposition (Brownian diffusion, turbulence)
dry deposition (sedimentation, turbulence)
[a] Suspension: (in atmospheric chemistry :) a dispersion of fine solid or liquid particles in the atmosphere. Dust is an example of atmospheric suspension.