Geoscience Reference
In-Depth Information
the point of activation,
w
is the water density,
M
w
is the molar mass of water,
R
is
the universal gas constant, and
T
is the ambient temperature. It can be shown that
S
c
scales with
D
dry
3/2
for hygroscopic aerosol; as the soluble fraction decreases to
zero,
S
c
approaches the Kelvin limit, where it scales as
D
dry
3/2
(Pruppacher and
Klett
1997
; Petters and Kreidenweis
2007
; Khvorostyanov and Curry
2007
;Kumar
et al.
2009a
,
b
).
KT implies that dust particles devoid of any solute would require very high
ambient supersaturations (dictated by the Kelvin equation) to act as CCN (Kumar
et al.
2009a
,
b
). However, for particles containing a large fraction of insoluble
material (such as dust), physisorption of water onto the insoluble surface may
also depress water activity. In fact, when the insoluble surface contains significant
amounts of hydrophilic material (such as clays), the adsorption process may be
strong enough to yield significant CCN activity to the particle, without the need for
any soluble material at all (Henson
2007
; Sorjamaa and Laaksonen
2007
;Kumar
et al.
2009a
,
b
,
2011a
,
b
; Navea et al.
2010
;Lathemetal.
2011
). To describe
the effects of adsorption on CCN activity, Sorjamaa and Laaksonen (
2007
)and
Kumar et al. (
2009a
) suggested the combination of the Frenkel-Halsey-Hill (FHH)
adsorption isotherm together with particle curvature to express the CCN activity
of completely insoluble particles that exhibit hydrophilicity. The resulting FHH
activation theory (FHH-AT) contains two adjustable parameters,
A
FHH
and
B
FHH
,
which describe the intensity and range of adsorption, respectively. Similar to KT,
the critical supersaturation of a particle following FHH-AT with dry diameter
D
dry
is computed from the maximum of the following equation:
A
FHH
D
P
B
FHH
D
dry
2D
H
2
O
4
W
M
W
RT
W
D
P
D
S
(12.2)
The value of
B
FHH
is the key measure of the particle hydrophilicity, with lower
B
FHH
values corresponding to a more hydrophilic particle. As
B
FHH
increases,
particles become less hydrophilic and resemble to insoluble (but wettable) particles
(Kumar et al.
2011b
).ThemaximaofEq.
12.2
can then be expressed in the form
S
c
D
C
D
dry
x
, where both
C
and
x
depend on
A
FHH
and
B
FHH
(Kumar et al.
2009a
).
As the intensity of water adsorption goes to zero, it can be shown that
S
c
approaches
the Kelvin limit, where it scales as
D
dry
1
(Kumar et al.
2009a
).
Depending on the theory (KT, FHH-AT),
S
c
scales with
D
dry
x
,where
x
is
an exponent that ranges between
0.8 for FHH-AT (Kumar
et al.
2009b
). Based on the discussion of Kumar et al. (
2009b
), from laboratory
experiments of size-resolved CCN activity of particles, one can experimentally
determine the relationship of
S
c
and
D
dry
; a power law fit between the data can
then determine the “experimental” exponent,
x
exp
, S
c
1.5 for KT and
CD
x
exp
dry
(Kumar et al.
2009b
). Comparing
x
exp
against theoretical predictions of the exponent can then
be used to infer the mechanism that dominates particle-water interaction, with
x
exp
D
1.5 indicating that the solute effect dominates (hence, KT applies), while
Search WWH ::
Custom Search