Geoscience Reference
In-Depth Information
Various other luorine-containing gases are manufactured and used in indus-
trial processes, sometimes as substitutes for ozone-depleting CFCs and HCFCs.
Their emission rates are small, but they are extremely potent greenhouse gases.
An example is sulfur hexafluoride (SF
6
), one of the most powerful greenhouse
gases (by mass). It is very long-lived in the atmosphere and is increasing at a rate
of about 0.2 ppt (parts per trillion) per year, or about 7% per year. Sulfur hexa-
fluoride is released into the atmosphere primarily by electric power industries.
Halons, or bromofluorocarbons, are fluorocarbons that contain bromine
or iodine. A common use of halons is as fire extinguishers, and they are fairly
long-lived, powerful greenhouse gases.
AEROSOLS
Atmospheric aerosols
are suspended solid particles and liquid droplets, rang-
ing in diameter from a few nanometers (1 nm 10
−9
m) to tens of micrometers.
Atmospheric aerosols include smoke, water droplets, sulfate particles, organic
carbon, black carbon, mineral dust, nitrates, smog, and sea salt; hybrid aero-
sols form when they interact and/or clump together.
Several natural sources of aerosols significant for climate have been identi-
fied.
Volcanic aerosols
are most commonly formed when ejected sulfur dioxide
is converted into droplets of sulfuric acid.
Mineral dust aerosols
are lifted by
surface winds over deserts and dry regions, for example, during periods of
drought. The Sahara Desert is the largest global source. Forest fires emit smoke,
black carbon, and organic carbon. Plants release VOCs, primarily isoprene,
which is converted, in part, to OH in the atmosphere and also contributes
to tropospheric ozone production. Ocean microalgae release
dimethylsulfide
(
DMS
), a precursor compound for sulfate aerosols.
Anthropogenic aerosols
originate in a wide variety of human activities. The
greatest source is sulfate aerosols from fossil fuel (both coal and oil) combus-
tion. This anthropogenic source is currently thought to be greater than the natu-
ral sources of sulfate aerosols. Biomass burning, for example, to clear land is
another prominent source of anthropogenic aerosols in the form of organic car-
bon and black carbon. VOCs are emitted by thousands of manufactured prod-
ucts, including paints, furnishings, pesticides, glues, and office equipment. Some
are toxic to humans, and they are important contributors to indoor pollution.
Most aerosols in the troposphere have relatively short residence times—
under a few weeks. Removal processes include both wet and dry deposition.
Aerosols in the stratosphere have longer residence times—often a year or more.
The removal of stratospheric aerosols is related to mass exchange between the
stratosphere and troposphere and is thought to occur in intense weather sys-
tems and upper-tropospheric jet streams.
10.2 RADIATIVE EFFECTS OF
GREENHOUSE GAS INCREASES
What are the implications of the observed changes in greenhouse gases for
climate? The first step in answering that question is to understand how the
radiative fluxes in the atmosphere are affected.
