Physical and chemical characteristics of aerosols

Different measures are commonly used to describe the aerosol concentration: these are number, area volume and mass concentrations (Table 9.3).
Table 9.3: Commonly used measures of aerosol concentration
Table 9.4: Number concentration and mass concentration of aerosol particles at different locations (source: Heintzenber, 1994).
In the lower troposphere, the total particle number concentration typically varies in the range of about 10²–10⁵ cm^–3, and typical mass concentration varies between 1 and 100 µg m^–3 (Table 9.4). Aerosol concentrations in the free troposphere are typically 1–2 orders of magnitude lower than in the atmospheric boundary layer.
PM_x (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.

Aerosol particles in the atmosphere have widely variable shapes. Their dimensions are usually characterized by a particle diameter, which span over four orders of magnitude, from a few nanometers to around 100 μm. Particle size is one of the most important parameters to describe the behaviour of aerosols, affecting both their lifetime, physical and chemical properties. Distribution of aerosol particles is generally defined by their number, surface or volume (Figure 9.6).


Based on particle distributions, different groups of atmospheric particles can be separated:
Aerosol particles below 0.1 µm in diameter constitute the nucleation (or Aitken) mode. The smallest range of these particles (with particle diameter is lower than 0.01 µm) sometimes called ultrafine mode. These particles are produced by homogeneous and heterogeneous nucleation processes (Figure 9.7). They can form during natural gas-to particle condensation (Figure 9.8) or during condensation of hot vapour in combustion processes. Due to their rapid coagulation (Figure 9.9) or random impaction onto surfaces, the lifetime of these small particles is very short (order of minutes to hours).






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 atmospheric aerosol has a very complex and variable chemical composition. Due to the various sources and transformations, each particle has individual composition. Atmospheric aerosols are generally composed of variable amounts of sulphate, nitrate, ammonium, sea salt, crustal elements and carbonaceous compounds (elemental and organic carbon) and other organic materials. Fine particles predominantly contain sulphate, nitrate, ammonium, elemental and organic carbon and certain trace metals (e.g. lead, cadmium, nickel, copper etc.). The primary components of coarse particle fraction are dust, crustal elements, nitrate, sodium, chloride and biogenic organic particles (e.g. pollen, spores, plant fragments etc.).
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.

Atmospheric particles can also be categorized by their water solubility, that is, how well they dissolve in water. In continental regions, about 80% of smaller particles are water-soluble. However, some types of coarse particles are also water soluble, like sea salt particles over oceans. Most water-soluble aerosol components are hygroscopic and they can absorb water. If aerosol particles consisting of water-soluble material, the uptake of atmospheric water vapour can results an aqueous solution droplet. During this process, the size of particle increases by hygroscopic growth, even when relative humidity is lower than 100%. Highly soluble particles are for example ammonium sulphate, ammonium nitrate and sodium chloride. These particles are efficient cloud condensation nuclei (CCN).
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).

The lifetime of atmospheric aerosol particles depends on their properties (size, chemical composition, etc.) and on altitude range, too (Figure 9.13). In the atmospheric boundary layer (lower troposphere), the residence time of aerosol particles is usually less than a week, often on the order of a day, depending on aerosol properties and meteorological conditions. In the free troposphere, the typical particle lifetime is 3–10 days on average. During this time, particle can easily be transported to a long distance. Therefore, there is a large variability in particle concentration, reflecting the geographical distribution of sources and sinks. The stratosphere also contains aerosol particles, which have much longer lifetime (up to 1 year), than in the tropospheric particles, due to the lack of precipitation.
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):