Chapter 9. Aerosol particles

Table of Contents

9.1. Sources and sinks of atmospheric aerosols
9.1.1. Sources of aerosol particles
9.1.2. Sink processes
9.2 Physical and chemical characteristics of aerosols
9.2.1. Concentrations:
9.2.2. Size distribution:
9.2.3. Chemical composition
9.2.4. Water solubility
9.2.5. Atmospheric lifetime
9.3. Effects of atmospheric aerosols
9.3.1. Direct effects: direct radiative forcing due the scattering radiation.
9.3.2. Indirect effects: indirect radiative forcing through cloud formation effects

Aerosol is a system of solid or liquid particles suspended by a mixture of gases. The term aerosols covers a wide spectrum of small particles, like sea salt particles, mineral dust, pollen, drops of sulphuric acid and many others. Aerosols have a great impact on several atmospheric phenomena, Earth’s climate and on biosphere. During their atmospheric residence, different size solid and liquid particles influence the radiation and energy budget of the Earth’s, the hydrological cycle, atmospheric circulation and the abundance of trace gases. Aerosol particles can be characterized by their concentration, size distribution, structure and chemical composition, which are highly variable both temporally and spatially.

9.1. Sources and sinks of atmospheric aerosols

After the emission of aerosol particles, they undergo various physical and chemical processes. During these processes, size, composition and structure of particles can be changed. Finally, they can be removed from the atmosphere to the surfaces by dry or wet deposition processes (Figure 9.1).

Life cycles of aerosol particles

Figure 9.1: Atmospheric cycles of aerosol particles

9.1.1. Sources of aerosol particles

Aerosols originate from a wide variety of natural and anthropogenic sources. Various aerosol particles are generated through a combination of physical, chemical and biological processes. Based on the formation processes, different source types can be distinguished. Primary particles emitted directly to the atmosphere as liquids or solids, through a wide range of processes (Bulk-to-Particle Conversion, BPC). Some particles formed by nucleation[18] and condensation[19] of precursor gases (Gas-To-Particle Conversation, GPC), and others from the reactions of dissolved substance in cloud droplets.

During Bulk-to-Particle Conversion, many different aerosol particles are generated from solid or liquid base materials. Oceans and dry continental regions are the two main natural sources of atmospheric aerosol. A large amount of water droplets and sea salts are released to the atmosphere through sea spray and air bubbles at the surface of the seas (Figure 9.2).

Droplet formation over the sea

Figure 9.2: Droplet formation during the bursting of small bubble

As a water droplet evaporates, the salt is left suspended into the atmosphere forming a maritime aerosol particle (e.g. sodium-chloride (NaCl), magnesium sulphate (MgSO4)). Other major source of primary particles is the windblown mineral dust from dry continental area, like deserts and semi arid regions. Further natural sources are volcanic ash, clay particles from soil erosion and biological materials (plant debris, pollen etc.). Additionally, different particles are formed by anthropogenic activities, like biomass burning, combustion of fossil fuel or industrial activities (Table 9.1).

Aerosol particles originated during Gas-To-Particle Conversion (e.g., sulphates, secondary organics) are not directly emitted, but are formed in the atmosphere from gaseous precursors (Table 9.2). Two basic processes can cause the formation of these secondary particles: an existing particle may grow through material condensing from gas phase, or new particle may forms through homogeneous nucleation.

Some other particles can form or transform by cloud droplets. When a cloud condensation nucleus[20] as an aerosol particle dissolves in the water, and then reacts with other substances, it can build new aerosol substance and form a new aerosol particle when the water evaporates.

Table 9.1: Primary particle emissions (Tg / year) Source: IPCC (2001)

Sources

Range

Carbonaceous aerosols

66 – 217

 Organic Matter:

 

  Biomass burning

45 – 80

  Fossil fuel

10 – 30

  Biogenic

0 – 90

 Black carbon:

 

  Biomass burning

5 – 9

  Fossil fuel

6 – 8

  Aircraft

0.006

Industrial dust

40 – 130

Sea salt

1000 – 6000

Mineral (soil) dust

1000 – 3000

Table 9.2: Annual source strength for present day emissions of aerosol precursors (Tg N, S or C /year). Source: IPCC (2001)

Sources

Range

NOx (Tg N y–1)

28 – 58

  Fossil fuel

21.0

  Aircraft

0.4 – 0.9

  Biomass burning

2 – 12

  Agricultural soil

0 – 4

  Natural soil

3 – 8

  Lightning

2 – 12

NH3 (Tg N y–1)

40 – 70

  Domestic animals

10 – 30

  Agriculture

6 – 18

  Human

1.3 – 6.9

  Biomass burning

3 – 8

  Fossil fuel and industry

0.1 – 0.5

  Natural soils

1 – 10

  Wild animals

0 – 1

  Oceans

3 – 16

SO2 (Tg S y–1)

67 – 130

  Fossil fuel and industry

60 – 100

  Aircraft

0.03 – 1.0

  Biomass burning

1 – 6

  Volcanoes

6 – 20

DMS and H2S (Tg S y–1)

12 – 42

  Oceans

13 – 36

  Land biota and soils

0.4 – 5.6

Volatile organic emissions (Tg C y–1)

100 – 560

  Anthropogenic

60 – 160

  Terpenes

40 – 400

9.1.2. Sink processes

Aerosols can be removed from the atmosphere by different ways in the function of their size and disposition. Two main types of removing processes of aerosol particles are wet and dry deposition (see e.g. Sportisse, 2007; Petroff et al., 2008). In an annual global mean, about 80–90% of aerosol particles are removed from the atmosphere by in-cloud and below-cloud scavenging (wet deposition). Remaining part of particles is removed by different ways of dry deposition.

Wet deposition processes (the main sink of atmospheric aerosol particles):

Rain-out and washout: a part of cloud droplets form precipitation which reaches Earth’s surface removing aerosols from cloud and from the column of air below the cloud.

Cloud deposition: deposition form of aerosols in high elevation ecosystems due to interception of cloud droplets by vegetation.

Dry deposition processes (less important on a global scale):

  • Turbulent diffusion: for larger particles (with a diameter larger than 1 µm) eddy diffusivity becomes important.

  • Gravitational settling (sedimentation): larger particles are influenced more by gravity and fall back to the surface. This process becomes increasingly important for particle sizes above 1 µm.

  • Impaction: if a particle cannot follow the flow streamline around an obstacle (e.g. a larger particle), small particle can hit this obstacle (Figure 9.3).

  • Interception: if an object is not directly in the path of particle moving in the gas stream (as in case of impaction), but particle approaches the edge of the obstacles, it may collected by the obstacle (Figure 9.4).

  • Brownian diffusion: randomly moving smaller particles bump each other (thermal coagulation[21]) or to a larger obstacles (Figure 9.5). This process dominates for particle sizes below 0.2 µm. Brownian diffusion coefficients increase as particle diameter decreases. Additionally, in a very thin (about 1 mm) layer over the surface, the Brownian diffusion becomes more important for larger particles too.

Impaction of an aerosol particle on an obstacle

Figure 9.3: Impaction of an aerosol particle on an obstacle (for example on a larger water droplet)

Particle interception by an obstacle

Figure 9.4: Particle interception by an obstacle (for example by a larger water droplet)

Brownian diffusion of a small aerosol particle

Figure 9.5: Brownian diffusion of a small aerosol particle



[18] Nucleation: generally defined as creation of molecular embryos or clusters prior to formation of a new phase during the transformation of vapor → liquid → solid. Nucleation can occur within the original phase (homogeneous nucleation), or on another phase, e.g. on a small particles (heterogeneous nucleation).

[19] Condensation: gas to liquid phase change.

[20] Cloud condensation nuclei (CCN): hygroscopic aerosol particles that can serve as nuclei of atmospheric cloud droplets, that is, particles on which water vapour condenses.

[21] Particle coagulation: a process, in which small particles collide with each other and coalesce completely to form a larger particle.