supersaturation is not always decreased since the
surfactant may not dissociate into as many ions
and while the Kelvin term may be reduced, so may
the Raoult effect to an even greater amount. This
effect for an ammonium sulfate nuclei (which
dissociates into three ions) enriched with 30%
mass fraction by the surfactant stearic acid
(single ion) is that the critical supersaturation is
actually increased. Supersaturations in natural
clouds are of the order of 0.1–0.5% for stratiform
clouds and below 1% for most convective clouds.
This provides a lower limit to the size of particles
that can act as cloud nuclei, namely, 30–50 nm,
but more commonly, aerosols larger than 100 nm
provide the majority of cloud nuclei. Nitric acid
can also influence cloud nucleus activation in that
it can temporarily partition into the aerosol phase
as a nucleus takes up water vapor in the rising
humidity field below cloud base. The uptake of
nitric acid into the solution droplet increases the
solute mass and, as a result, lowers the critical
supersaturation required for the nucleus.
Ice nuclei are also important cloud nuclei but
produce cloud particles in a very different manner
and are comprised of very different chemical composition. Typically, they comprise non-watersoluble matter such as mineral dust and certain
primary biogenic aerosol particles. Ice particles
are formed either by heterogeneous nucleation in
a supersaturated (with respect to ice) environment
or homogeneous freezing of supercooled liquid
droplets [74]. Far less is known about the nature
of ice nuclei than what is known about cloud
condensation nuclei.
Aerosols, Global Radiative Budget,
Precipitation, and Climate
Aerosols affect the global radiative budget, and
hence the global climate, directly through the
scattering and/or absorption of incoming solar
radiation (and for some aerosol species generally
of supermicron size, outgoing infrared radiation)
and indirectly through the modification of cloud
microphysics and radiative properties. The former
effect is called the direct aerosol radiative forcing
effect and the latter, the indirect radiative forcing
effect where radiative forcing is defined as “The
radiative forcing of the surface-troposphere system due to the perturbation in or the introduction
of an agent (say, a change in greenhouse gas
concentrations) is the change in net (down minus
up) irradiance (solar plus longwave in Wm
–2 ) at
the tropopause after allowing for stratospheric
temperatures to readjust to radiative equilibrium,
but with surface and tropospheric temperatures
and state held fixed at the unperturbed values.”
In simple terms, it is “the rate of energy change per
unit area of the globe as measured at the top of the
atmosphere” and often it refers to the change since
preindustrial conditions (year 1750). Radiative
forcing affects the climate, while alterations in
aerosol availability and radiative forcing can
affect precipitation.
Direct Effect
Rayleigh scattering describes the irradiance
scattered by a sphere as small compared to the
incident wavelength and proportional to 1/l
4 ,
while Rayleigh absorption is proportional to 1/l
4
[75]. When particles are comparable to the incident wavelength, Mie scattering must be invoked
where scattering is strongly dependent on particle
size, shape, composition (via refractive index),
and orientation relative to the incident radiation.
For particles large relative to the wavelength, the
extinction efficiency converges at 2 (the extinction paradox where twice the energy is removed
compared to that derived using geometrical
optics). For particles with a complex refractive
index, some fraction of the incident radiation is
absorbed. The extinction efficiency is defined as
the sum of the scattering and absorption efficiency
and the single scattering albedo (o) is the ratio of
the scattering coefficient to the extinction coefficient. One parameter used to quantify scattering
extinction is the mass scattering and mass absorption efficiency, which quantifies scattering and
extinction in terms of the aerosol mass distribution function. Both the mass scattering and mass
absorption efficiencies as a function of particle
size exhibit peak scattering and absorption efficiencies at sizes between 0.1 and 1 mm for typical
atmospheric aerosol chemical compositions
pointing to this size range as being the most
Aerosol in Global Atmosphere
263
surfactant may not dissociate into as many ions
and while the Kelvin term may be reduced, so may
the Raoult effect to an even greater amount. This
effect for an ammonium sulfate nuclei (which
dissociates into three ions) enriched with 30%
mass fraction by the surfactant stearic acid
(single ion) is that the critical supersaturation is
actually increased. Supersaturations in natural
clouds are of the order of 0.1–0.5% for stratiform
clouds and below 1% for most convective clouds.
This provides a lower limit to the size of particles
that can act as cloud nuclei, namely, 30–50 nm,
but more commonly, aerosols larger than 100 nm
provide the majority of cloud nuclei. Nitric acid
can also influence cloud nucleus activation in that
it can temporarily partition into the aerosol phase
as a nucleus takes up water vapor in the rising
humidity field below cloud base. The uptake of
nitric acid into the solution droplet increases the
solute mass and, as a result, lowers the critical
supersaturation required for the nucleus.
Ice nuclei are also important cloud nuclei but
produce cloud particles in a very different manner
and are comprised of very different chemical composition. Typically, they comprise non-watersoluble matter such as mineral dust and certain
primary biogenic aerosol particles. Ice particles
are formed either by heterogeneous nucleation in
a supersaturated (with respect to ice) environment
or homogeneous freezing of supercooled liquid
droplets [74]. Far less is known about the nature
of ice nuclei than what is known about cloud
condensation nuclei.
Aerosols, Global Radiative Budget,
Precipitation, and Climate
Aerosols affect the global radiative budget, and
hence the global climate, directly through the
scattering and/or absorption of incoming solar
radiation (and for some aerosol species generally
of supermicron size, outgoing infrared radiation)
and indirectly through the modification of cloud
microphysics and radiative properties. The former
effect is called the direct aerosol radiative forcing
effect and the latter, the indirect radiative forcing
effect where radiative forcing is defined as “The
radiative forcing of the surface-troposphere system due to the perturbation in or the introduction
of an agent (say, a change in greenhouse gas
concentrations) is the change in net (down minus
up) irradiance (solar plus longwave in Wm
–2 ) at
the tropopause after allowing for stratospheric
temperatures to readjust to radiative equilibrium,
but with surface and tropospheric temperatures
and state held fixed at the unperturbed values.”
In simple terms, it is “the rate of energy change per
unit area of the globe as measured at the top of the
atmosphere” and often it refers to the change since
preindustrial conditions (year 1750). Radiative
forcing affects the climate, while alterations in
aerosol availability and radiative forcing can
affect precipitation.
Direct Effect
Rayleigh scattering describes the irradiance
scattered by a sphere as small compared to the
incident wavelength and proportional to 1/l
4 ,
while Rayleigh absorption is proportional to 1/l
4
[75]. When particles are comparable to the incident wavelength, Mie scattering must be invoked
where scattering is strongly dependent on particle
size, shape, composition (via refractive index),
and orientation relative to the incident radiation.
For particles large relative to the wavelength, the
extinction efficiency converges at 2 (the extinction paradox where twice the energy is removed
compared to that derived using geometrical
optics). For particles with a complex refractive
index, some fraction of the incident radiation is
absorbed. The extinction efficiency is defined as
the sum of the scattering and absorption efficiency
and the single scattering albedo (o) is the ratio of
the scattering coefficient to the extinction coefficient. One parameter used to quantify scattering
extinction is the mass scattering and mass absorption efficiency, which quantifies scattering and
extinction in terms of the aerosol mass distribution function. Both the mass scattering and mass
absorption efficiencies as a function of particle
size exhibit peak scattering and absorption efficiencies at sizes between 0.1 and 1 mm for typical
atmospheric aerosol chemical compositions
pointing to this size range as being the most
Aerosol in Global Atmosphere
263