CHAPTER 2 • The Chemical and Physical Properties of Marine Aerosols: An Introduction
39
this very fine particle mode are highly mobile. They rapidly coagulate to form larger
particles that typically fall in the size range between 0.1 to 111m diameter (a size class
referred to as the "accumulation" mode) or they can diffuse to the surface of cloud or
fog droplets or to larger particles (e.g. sea salt, mineral dust).
Because of the various routes by which particles can be formed, the atmospheric
aerosol is extremely complex and dynamic. Some classes of aerosols can be formed
by both primary and secondary processes. For example, sulphate particles can be
emitted directly from smokestacks; but in advanced technologies, the direct emission
of particles contributes a relatively minor source because of emission controls. Furthermore, individual aerosol particles rarely exist as a pure type (e.g. a "pure" sea-salt
droplet, a specific sulphate compound). Rather, most particles are comprised of a wide
range of compounds with properties and atmospheric lifetimes that differ from those
of their individual components. As a result, atmospheric particles often display a wide
range of chemical and physical properties.
2.1.2
The Role of Clouds in the Aerosol Cycle
Clouds playa very important role in the aerosol cycle. Cloud droplets are an important patlIway for the reaction of gaseous species - for example, the oxidation of gasphase S02 to SO~- takes place largely in cloud droplets through the aqueous phase reaction with H 2 0 2 (see below). But most clouds do not produce rain. Instead they evaporate, and the cloud droplets are converted to aerosol particles with sizes that typically
fall in the range 0.1-1.0 11m diameter (i.e. in the "accumulation" mode; see Fig. 2.1).
Nonetheless, the small fraction of clouds that do precipitate play an important role in
cleansing the atmosphere. Thus, clouds are important both in the formation of aerosols and in their removal from the atmosphere; the delicate balance between these
processes has huge implications from the standpoint of climate. Consequently, cloud
processes have the highest priority in climate studies.
The role of clouds in the atmospheric aerosol cycle is paradoxical: clouds are involved in the formation and removal of aerosols from that atmosphere, and yet clouds
can not form without the presence of aerosols. At the water vapour supersaturations
typically found in the atmosphere (only a few tenths of a percent), water vapour can
only condense on hygroscopic aerosol particles (e.g. sulphate and nitrate aerosols, sea
salt droplets). Thus, clouds and aerosols are involved in a complex and tightly coupled
atmospheric cycle that results in the transformation, transport, and removal of aerosol particles (Wang and Prinn 2000). These processes are depicted schematically in
Fig. 2.2: water vapour condensation on the aerosol particle, in-cloud reactions, droplet coalescence, and the formation of the precipitation, which then falls towards the
ground. In many cases precipitation will evaporate before it reaches the Earth's surface; the particles so-produced are larger and more chemically complex than those
which entered the cloud. Also, as indicated above, the cloud could evaporate before
precipitation forms, a process that also yields a transformed aerosol ensemble. Clouds
also play an important role in atmospheric nitrogen chemistry in that lighting associated with clouds is a significant source of nitrogen oxides (Wang and Prinn 2000).
Finally, clouds can also vent unreacted gases and particles to the middle and upper
troposphere (Wang and Prinn 2000), as discussed below.
39
this very fine particle mode are highly mobile. They rapidly coagulate to form larger
particles that typically fall in the size range between 0.1 to 111m diameter (a size class
referred to as the "accumulation" mode) or they can diffuse to the surface of cloud or
fog droplets or to larger particles (e.g. sea salt, mineral dust).
Because of the various routes by which particles can be formed, the atmospheric
aerosol is extremely complex and dynamic. Some classes of aerosols can be formed
by both primary and secondary processes. For example, sulphate particles can be
emitted directly from smokestacks; but in advanced technologies, the direct emission
of particles contributes a relatively minor source because of emission controls. Furthermore, individual aerosol particles rarely exist as a pure type (e.g. a "pure" sea-salt
droplet, a specific sulphate compound). Rather, most particles are comprised of a wide
range of compounds with properties and atmospheric lifetimes that differ from those
of their individual components. As a result, atmospheric particles often display a wide
range of chemical and physical properties.
2.1.2
The Role of Clouds in the Aerosol Cycle
Clouds playa very important role in the aerosol cycle. Cloud droplets are an important patlIway for the reaction of gaseous species - for example, the oxidation of gasphase S02 to SO~- takes place largely in cloud droplets through the aqueous phase reaction with H 2 0 2 (see below). But most clouds do not produce rain. Instead they evaporate, and the cloud droplets are converted to aerosol particles with sizes that typically
fall in the range 0.1-1.0 11m diameter (i.e. in the "accumulation" mode; see Fig. 2.1).
Nonetheless, the small fraction of clouds that do precipitate play an important role in
cleansing the atmosphere. Thus, clouds are important both in the formation of aerosols and in their removal from the atmosphere; the delicate balance between these
processes has huge implications from the standpoint of climate. Consequently, cloud
processes have the highest priority in climate studies.
The role of clouds in the atmospheric aerosol cycle is paradoxical: clouds are involved in the formation and removal of aerosols from that atmosphere, and yet clouds
can not form without the presence of aerosols. At the water vapour supersaturations
typically found in the atmosphere (only a few tenths of a percent), water vapour can
only condense on hygroscopic aerosol particles (e.g. sulphate and nitrate aerosols, sea
salt droplets). Thus, clouds and aerosols are involved in a complex and tightly coupled
atmospheric cycle that results in the transformation, transport, and removal of aerosol particles (Wang and Prinn 2000). These processes are depicted schematically in
Fig. 2.2: water vapour condensation on the aerosol particle, in-cloud reactions, droplet coalescence, and the formation of the precipitation, which then falls towards the
ground. In many cases precipitation will evaporate before it reaches the Earth's surface; the particles so-produced are larger and more chemically complex than those
which entered the cloud. Also, as indicated above, the cloud could evaporate before
precipitation forms, a process that also yields a transformed aerosol ensemble. Clouds
also play an important role in atmospheric nitrogen chemistry in that lighting associated with clouds is a significant source of nitrogen oxides (Wang and Prinn 2000).
Finally, clouds can also vent unreacted gases and particles to the middle and upper
troposphere (Wang and Prinn 2000), as discussed below.