At these temperatures, the concentration of NAT
particles is often at best only 0.001 of the aerosol
population [64–66] indicating clearly a nucleation
barrier which is not well understood. Laboratory
investigations suggest that LTA must be significantly supercooled for NAT nucleation. Arctic
PSC observations in the early 2000s suggest the
importance of a small population of NAT particles
and that such a population forms at temperatures
just a few degrees below the NAT point in both
leewaves [64] and synoptic situations [66]. These
latter observations are the clearest evidence yet
indicating that NAT forms readily on a small
fraction (10
À4
–10
À5 ) of SA through heterogeneous nucleation.
Water Uptake by Aerosols
One of the most important gases in the atmosphere is water vapor as it not only contributes
predominantly to the natural greenhouse effect,
resulting in a livable global average temperature,
in its condensed liquid and ice phases, it forms
aerosol haze and cloud layers which contribute to
approximately 50% of the planetary albedo.
Many aerosol types have an affinity to water
and, under the right humidity conditions, they
can become solution droplets, larger than the
dry aerosol size, by uptake of water vapor from
the air around it. Aerosol humidification has
implications for optical scattering, condensation
processes, and chemical interactions on and
within aerosol particles. Liquid water clouds are
extensive in the atmosphere, but as was shown
above, pure water droplets cannot form under
atmospheric conditions. Condensation nuclei
are required for the formation of cloud droplets.
Cloud droplets are aerosol particles that have
rapidly grown to 10–100 times the size of the
initial nucleus, corresponding to more than 1,000
times increase in mass, almost solely through
water uptake.
Aerosol Hygroscopicity
A large fraction of the atmospheric aerosol population is water soluble or partially water soluble.
The soluble aerosols are typically salts such as
sea-salt or ammonium sulfate which readily dissociates in water to form an electrolyte solution
droplet. In subsaturated air (relative humidity < 100%), these solution droplets can exist
due to the lowering of the equilibrium vapor pressure above the droplet surface by the presence of
the dissolved ions (known as the Roault Effect).
As a result, soluble aerosol particles, depending
on the chemical composition, readily form solution drops with a water fraction dependent on
relative humidity. This means that the “wet” aerosol equilibrium size responds to, and is dependent
on, ambient relative humidity. The “wet” aerosol
size will grow with increasing humidity and
shrink with reducing humidity. Since the optical
properties (e.g., scattering) of aerosols is size
dependent (and proportional to crosssectional
area), relative humidity can significantly influence
the hazing, or optical extinction effect of aerosol
layers. A measure of this effect is the hygroscopic
growth factor, normally defined as the change in
aerosol diameter from a low relative humidity
(e.g., 40%) corresponding to a dehydrated drysize diameter to a high relative humidity
(normally 90%) with significant hydration. Salts
like ammonium sulfate and sea-salt have a growth
factor of 1.8 and 2.2, respectively, over this
humidity range, while organic aerosol possesses
a growth factor typically from 1 to 1.4 depending
on the degree of solubility.
Cloud Droplet Activation
In supersaturated environment, a subset of the
aerosol can grow to a critical size which becomes
metastable, and with an infinitesimal further
increase in humidity, the droplet will become
unstable and spontaneously grow to sizes of the
order of 10 mm and become a cloud droplet. This
process is called cloud droplet activation. The
description of the equilibrium size of a droplet
with water saturation ratio, founded on the early
work of Kőhler [67], is now well established and
can be readily derived from the Clausius–
Clapeyron equation modified to give a general
equilibrium relation between an aqueous salt solution droplet and water vapor (known as the Kőhler
curve):
260
Aerosol in Global Atmosphere
particles is often at best only 0.001 of the aerosol
population [64–66] indicating clearly a nucleation
barrier which is not well understood. Laboratory
investigations suggest that LTA must be significantly supercooled for NAT nucleation. Arctic
PSC observations in the early 2000s suggest the
importance of a small population of NAT particles
and that such a population forms at temperatures
just a few degrees below the NAT point in both
leewaves [64] and synoptic situations [66]. These
latter observations are the clearest evidence yet
indicating that NAT forms readily on a small
fraction (10
À4
–10
À5 ) of SA through heterogeneous nucleation.
Water Uptake by Aerosols
One of the most important gases in the atmosphere is water vapor as it not only contributes
predominantly to the natural greenhouse effect,
resulting in a livable global average temperature,
in its condensed liquid and ice phases, it forms
aerosol haze and cloud layers which contribute to
approximately 50% of the planetary albedo.
Many aerosol types have an affinity to water
and, under the right humidity conditions, they
can become solution droplets, larger than the
dry aerosol size, by uptake of water vapor from
the air around it. Aerosol humidification has
implications for optical scattering, condensation
processes, and chemical interactions on and
within aerosol particles. Liquid water clouds are
extensive in the atmosphere, but as was shown
above, pure water droplets cannot form under
atmospheric conditions. Condensation nuclei
are required for the formation of cloud droplets.
Cloud droplets are aerosol particles that have
rapidly grown to 10–100 times the size of the
initial nucleus, corresponding to more than 1,000
times increase in mass, almost solely through
water uptake.
Aerosol Hygroscopicity
A large fraction of the atmospheric aerosol population is water soluble or partially water soluble.
The soluble aerosols are typically salts such as
sea-salt or ammonium sulfate which readily dissociates in water to form an electrolyte solution
droplet. In subsaturated air (relative humidity < 100%), these solution droplets can exist
due to the lowering of the equilibrium vapor pressure above the droplet surface by the presence of
the dissolved ions (known as the Roault Effect).
As a result, soluble aerosol particles, depending
on the chemical composition, readily form solution drops with a water fraction dependent on
relative humidity. This means that the “wet” aerosol equilibrium size responds to, and is dependent
on, ambient relative humidity. The “wet” aerosol
size will grow with increasing humidity and
shrink with reducing humidity. Since the optical
properties (e.g., scattering) of aerosols is size
dependent (and proportional to crosssectional
area), relative humidity can significantly influence
the hazing, or optical extinction effect of aerosol
layers. A measure of this effect is the hygroscopic
growth factor, normally defined as the change in
aerosol diameter from a low relative humidity
(e.g., 40%) corresponding to a dehydrated drysize diameter to a high relative humidity
(normally 90%) with significant hydration. Salts
like ammonium sulfate and sea-salt have a growth
factor of 1.8 and 2.2, respectively, over this
humidity range, while organic aerosol possesses
a growth factor typically from 1 to 1.4 depending
on the degree of solubility.
Cloud Droplet Activation
In supersaturated environment, a subset of the
aerosol can grow to a critical size which becomes
metastable, and with an infinitesimal further
increase in humidity, the droplet will become
unstable and spontaneously grow to sizes of the
order of 10 mm and become a cloud droplet. This
process is called cloud droplet activation. The
description of the equilibrium size of a droplet
with water saturation ratio, founded on the early
work of Kőhler [67], is now well established and
can be readily derived from the Clausius–
Clapeyron equation modified to give a general
equilibrium relation between an aqueous salt solution droplet and water vapor (known as the Kőhler
curve):
260
Aerosol in Global Atmosphere