CHAPTER 2 • The Chemical and Physical Properties of Marine Aerosols: An Introduction
37
• Mineral dust: Mineral dust is a major aerosol species over many ocean regions. It is
an important contributor to deep-sea sediments. The iron associated with dust is believed to be a limiting micronutrient over many remote ocean regions; as such, the dust
cycle could be a major factor in the global oceanic carbon cycle. Other elements associated with dust could play an important role in the biogeochemistry of the oceans.
This paper can only serve as a brief overview of the field of aerosol chemistry. For
those interested in further study, Hobbs (2000) and Jacob (1999) present excellent and
concise introductions to the field of atmospheric chemistry. Graedel and Crutzen (1993)
offer a broad and easily readable review of the fundamentals of atmospheric chemistry in climate processes, while Hobbs (1993) provides a good introduction to aerosolcloud-climate interactions; Charlson and Heintzenberg (1995) present broad coverage
of the subject. Broader and more detailed coverage of atmospheric chemistry and aerosols can be found in the excellent texts by Finlayson-Pitts and Pitts (2000) and by
Seinfeld and Pandis (1998).
2.1.1
Physical Characteristics of Aerosols
An aerosol is defined as a dispersion of solid or liquid particles in a gaseous phase -
in this case, the atmosphere. The size of the particles can range from macromolecules
to over 100 !lm diameter. In practice most of the aerosol mass that is normally of interest over the oceans is found in the size range under 10-20 !lm diameter. Larger particles have high settling velocities in the atmosphere, and consequently they are not
carried far from their sources. (For example, a 20 !lm diameter particle with unit density has a settling velocity at sea level of 1.2 cm s -\, about 1 km per day.) While aerosols are usually thought of as a solid or liquid particle, in reality, in many regions most
particles are complex mixtures of soluble and insoluble species, and depending on the
relative humidity, they could consist of both solid and liquid phases.
Aerosols can be produced directly ("primary" aerosols) or as a product of the reactions of gases in the atmosphere ("secondary" aerosols). The mode of production
affects the size distribution of the resulting aerosol. Figure 2.1 shows a schematic of
the distribution of particle surface area of an idealized ensemble of aerosol particles
formed by a variety of processes. (Seinfeld and Pandis 1998, Fig. 2.15, p. 101). Examples
of primary aerosols are sea salt, mineral dust, soot emitted from smokestacks, diesel
exhaust, and particles shed by plants (e.g. plant waxes, fibres). Physical processes
(grinding of rocks, bursting bubbles) normally produce relatively large particles; as a
result, the size distribution of primary particles places most of the mass above 1 !lm
diameter as shown in Fig. 2.1; these are normally referred to as "coarse" particles. For
example, the mass median diameter (MMD) of sea salt particles over the ocean is generally in the range of about 5-10 !lm diameter or greater (depending on wind conditions); nonetheless, a significant and important fraction of the sea salt mass lies below l!lm (O'Dowd et al.1997). The MMD of mineral dust particles over the source regions can be extremely large: many lO's of micro metres, but over the oceans it is typically only several!lm (Duce 1995).
37
• Mineral dust: Mineral dust is a major aerosol species over many ocean regions. It is
an important contributor to deep-sea sediments. The iron associated with dust is believed to be a limiting micronutrient over many remote ocean regions; as such, the dust
cycle could be a major factor in the global oceanic carbon cycle. Other elements associated with dust could play an important role in the biogeochemistry of the oceans.
This paper can only serve as a brief overview of the field of aerosol chemistry. For
those interested in further study, Hobbs (2000) and Jacob (1999) present excellent and
concise introductions to the field of atmospheric chemistry. Graedel and Crutzen (1993)
offer a broad and easily readable review of the fundamentals of atmospheric chemistry in climate processes, while Hobbs (1993) provides a good introduction to aerosolcloud-climate interactions; Charlson and Heintzenberg (1995) present broad coverage
of the subject. Broader and more detailed coverage of atmospheric chemistry and aerosols can be found in the excellent texts by Finlayson-Pitts and Pitts (2000) and by
Seinfeld and Pandis (1998).
2.1.1
Physical Characteristics of Aerosols
An aerosol is defined as a dispersion of solid or liquid particles in a gaseous phase -
in this case, the atmosphere. The size of the particles can range from macromolecules
to over 100 !lm diameter. In practice most of the aerosol mass that is normally of interest over the oceans is found in the size range under 10-20 !lm diameter. Larger particles have high settling velocities in the atmosphere, and consequently they are not
carried far from their sources. (For example, a 20 !lm diameter particle with unit density has a settling velocity at sea level of 1.2 cm s -\, about 1 km per day.) While aerosols are usually thought of as a solid or liquid particle, in reality, in many regions most
particles are complex mixtures of soluble and insoluble species, and depending on the
relative humidity, they could consist of both solid and liquid phases.
Aerosols can be produced directly ("primary" aerosols) or as a product of the reactions of gases in the atmosphere ("secondary" aerosols). The mode of production
affects the size distribution of the resulting aerosol. Figure 2.1 shows a schematic of
the distribution of particle surface area of an idealized ensemble of aerosol particles
formed by a variety of processes. (Seinfeld and Pandis 1998, Fig. 2.15, p. 101). Examples
of primary aerosols are sea salt, mineral dust, soot emitted from smokestacks, diesel
exhaust, and particles shed by plants (e.g. plant waxes, fibres). Physical processes
(grinding of rocks, bursting bubbles) normally produce relatively large particles; as a
result, the size distribution of primary particles places most of the mass above 1 !lm
diameter as shown in Fig. 2.1; these are normally referred to as "coarse" particles. For
example, the mass median diameter (MMD) of sea salt particles over the ocean is generally in the range of about 5-10 !lm diameter or greater (depending on wind conditions); nonetheless, a significant and important fraction of the sea salt mass lies below l!lm (O'Dowd et al.1997). The MMD of mineral dust particles over the source regions can be extremely large: many lO's of micro metres, but over the oceans it is typically only several!lm (Duce 1995).