Aerosol in the UTLS

rangebeaverMécanique

22 févr. 2014 (il y a 3 années et 10 mois)

83 vue(s)

Klaus Gierens

Institut für Physik der Atmosphäre

DLR Oberpfaffenhofen

Aerosol in the UTLS

MOD 12

Overview


role of aerosol in the atmosphere


modes of heterogeneous nucleation


effect of air pollution on clouds


indirect, semi
-
direct, and direct effects of aerosol


rough size calssification


aerosol sources and chemistry (recent results)



The role of aerosol particles in the atmosphere and UTLS


Aerosol particles are responsible for cloud formation at relatively
low supersaturation values (
catalytic action
)


water clouds at about 1%


ice clouds at up to 80%, typically 30
-
60%


Without aerosol, cloud formation would need several 100% of
supersaturation!



Liquid aerosol (aqueous solution droplets)


homogeneous
nucleation

(ice clouds only, T< supercooling limit of pure water)



Solid aerosol


heterogeneous nucleation

(ice and water clouds)


occurs at specific sites at the particle surfaces


site density depends on temperature (and humidity and ….)


Measurements of ice nucleating ability of ambient aerosol

DeMott et al, 2003, PNAS

homogeneous

heterogeneous

Heterogeneous modes of ice crystal formation

from Vali, 2004

other heterogeneous freezing modes

What makes an aerosol particle a heterogeneous ice
nucleus (IN)?

Conditions have to be met:


Insolubility


Size (must be larger than germ size, larger IN


more active
sites)


Chemical bond (hydrogen bonds at IN surface)


Crystallographic similarity (epitaxy)


active sites (surface features that initiate the ice phase,
increasing with

T)


Which is the most important one? Is there at all a most important
one or is there rather a competition between the various
conditions?


effect of air pollution on cloud properties


Type and quantity of aerosol particles acting as ice nuclei affect
cloud micro
-

/ macrophysical and optical properties.



Degree of air pollution has large effect on cloud properties. This is
called the aerosol indirect effect on climate (first postulated by
Twomey).



N
i
= 1L
-
1
, w = 4.5 cm/s, RHi
het

= 130 %

Time (min)

N
i
= 5L
-
1
, w = 4.5 cm/s, RHi
het

= 130 %

Time (min)

N
i
= 50L
-
1
, w = 4.5 cm/s, RHi
het

= 130 %

Time (min)

NH vs. SH, polluted vs. clean air cirrus


The INCA (Interhemispheric differences in cirrus properties from
anthropogenic emissions) experiment has shown that


clouds during the SH campaign formed preferentially between
140 and 155% RHi,


clouds in the NH campaign formed at about 130% RHi (Ström
et al, ACP, 2003)



Interpretation:


Ci in the SH forms mostly by homogeneous nucleation


In the NH, first ice crystals in cirrus are formed by
heterogeneous nucleation.


But heterogeneous process does not hinder homogenous
nucleation afterwards. (Haag et al., ACP, 2003).


observations of nucleation thresholds in data of RHi

Haag and
Kärcher, 2003

RHi outside clouds

RHi inside clouds

indirect effects: Twomey effect


There are several kinds of indirect effects:



Twomey effect
: more IN lead to more numerous but smaller
droplets or ice crystals (when the available water is the same)



a cloud affected by the Twomey effect appears brighter than its
unaffected counterpart, because the same amount of water/ice
dispersed on more particles has a bigger
optical effect

(larger
optical thickness, more scattering and reflection).



a cloud affected by the Twomey effect has
longer lifetime

than
its unaffected counterpart, because there is less sedimentation
and less coagulation.


Effect of cosmic rays?


Svensmark’s hypothesis:



Solar wind cycle induces variations in cosmic ray intensities



changes in the fractional coverage of low clouds.


Might be… but much debate about it, in particular about the way,
Svensmark and colleagues have treated the data.


Be careful!


negative Twomey effect


The modelling examples show a “negative Twomey effect”, that is
only possible in ice clouds at T below the supercooling limit of pure
water.


Heterogeneous nucleation effectively impedes homogeneous
nucleation



much smaller ice crystal concentration than a purely
homogeneously formed cirrus



Clouds affected by a negative Twomey effect are
optically
thinner

than their unaffected counterparts.



The modelling examples also show that there is a
strong
influence on cloud structural properties
, lifetime etc. when a
sufficient number of IN are present.


critical concentration of IN for negative Twomey effect

4
/
3
0
hom
2
/
1
*
0
415
.
5
2
/
3
2
/
3
11
]
)
(
[
)]
(
[
)
(
10
81
.
2
s
T
s
T
e
s
T
p
w
T
f
N
c



Gierens, ACP, 2003;


more recent

amendments by

Ren & MacKenzie, QJRMS;

Liu & Penner, JGR

T
T
f
02
.
0
4
10
)
(


[N
c
] = m
-
3

[p,e
*
] = Pa

[T] = K

[w] = m/s

[s…] = 1

semi
-
direct effects


Semi
-
direct effects are more important for low clouds.



Interstitial (soot) aerosol absorbs radiation,


heating


cloud droplet evaporation


cloud disappearance



another semi
-
direct effect is possible when the absorbing aerosol
layer lies above the cloud:


heating above the cloud enhances stability


reduces turbulent mixing at cloud top


prolongs cloud lifetime.




direct effect


The direct effect is due to the direct interaction of solar and
terrestrial radiation with aerosol particles (scattering and
absorption).


Cooling the greenhouse.


efficiency of cloud forming catalysis


Homogeneous nucleation consumes generally only a very small
fraction of the available aerosol particles (the largest ones)



Heterogeneous nucleation needs special surface properties of the
particles, hence only a small fraction of insoluble particles can act
as heterogeneous ice nuclei.


the activity of nucleation sites depends on temperature,
humidity, chemical nature of the particle, and


on the particle’s history (e.g. cloud processing)



Anthropogenic influence


Anthropogenic influences (aviation, biomass burning) may be very
effective in changing cloud properties.


papers by Krüger and Grassl (GRL, 2002; 2004) show (using
long
-
term satellite data) how the cloud properties in Europe
have changed after the end of the communistic era and the
break
-
down of the industry in the East.

Aerosol sources and chemistry


Mechanical generation of particles include


dust created by erosion of soils


wind
-
driven release of biogenic particles from plants


droplet and sea salt generation from sea spray


Combustion:


biomass burning


industrial combustion processes


traffic (in particular aviation in the UTLS)


Chemical generation (gas
-
to
-
particle conversion)


photochemical activity


generation of sulphuric acid in the UTLS is an example


Interplanetary and intergalactic source


meteoritic material


cosmic rays produce ions in the stratosphere that aid GPC

Rough size classification (modes)


Nucleation mode (to 20 nm):


freshly generated particles, from GPC


or emitted small particles (e.g. ultrafine soot from diesels)


Aitken mode (20 to 100 nm): after some growth has been occurred.
Aerosol growth by condensation and coagulation.


Accumulation mode (100 nm to 2 µm): theses are grown and aged
particles. Sometimes bimodal, due to selective growth processes
occurring in clouds.


Coarse
-
mode and giant aerosols (D > 2 µm): particles generated
mechanically.

Diameter (
m
m)



6



0.01

0.1

1

10

10
-
4

10
-
3

10
-
2

10
-
1

10

0

10

1

10

2

10

3

10

4

10

5

10

d N / d log D , cm

-
3

Accumulation mode

Coarse mode

Aitken mode

Nucleation mode

chemical nature of aerosol in the UTLS


Residuals typically found in ice crystals from cirrus and contrails are


mineral dust


black carbon (in particular in aviation corridors)


metallic particles


sulphates and sea salt


organic aerosol less frequent than in ambient particles


composition of particles that activate ice

under cirrus conditions:

cluster analysis of PALMS spectra

DeMott et al, 2003, PNAS

very moist,

RHi>140%

100%<RHi<140%

background

aerosol

measurements at Storm Peak,

Rocky Mountains, 3200 m asl,

mid latitude cirrus

Storm Peak data analysis, DeMott et al., PNAS, 2003


background aerosol

usually dominated by sulphates and organics


lesser contribution from potassium and carbon (biomass burning)



Particles that cause either kind of nucleation are similar in
composition to background aerosol.



Heterogeneous IN have very variable makeup. Lower percentage of
particles dominated by sulphates.


Large contributions of mineral dust, fly ash, and metallic particles
suggest strong natural and anthropogenic inputs to IN populations.


ice residual nuclei in anvil cirrus

> 0.07µm

> 0.38µm

Twohy & Poellot,

ACP, 2005

Measurements during

CRYSTAL
-
FACE

(Florida, 26
°
N)

(

8
°
䌩C



°
䌠㸠吠㸠


°



°
䌠㸠


°
C

ice residual nuclei in anvil cirrus, cont’d.


All categories:
about 1/3 of the particles are composed of salts
.



Na, Ka, Ca salts, most containing sulphur. These act usually as
condensation nuclei for liquid droplets.





significant fraction of
anvil ice forms by freezing of liquid
droplets
.



in other than anvil
-
cirrus sulphuric acid and ammonium (bi)
sulphate are predominant particles inducing homogeneous
nucleation.


ice residual nuclei in anvil cirrus, cont’d.


Second most important residual type is
crustal material and metals

(of industrial origin)


more frequent in the large particle fraction


generated mechanically


good IN.



Third:
Carbon and soot
, incl. carbon from organics,


more prevalent in the small particle fraction.



Also particles from biomass burning (Ca + C).


ice residual nuclei in anvil cirrus, cont’d.


Relatively small difference between the composition of ambient
particles and residuals.



Ambient composition is rather different from composition of particles
in regions remote from convection (i.e. sulphates, organics).





anvil ice is not only a sink but also a source of particles.



Another important role of convection is uplift of condensable
gases


GPC


new ultrafine particles.


ice residual nuclei in anvil cirrus, cont’d.


Particles enter the Cb


either from below (by convection)


or from the sides in the mid
-
troposphere by entrainment,


or after convection and several cycles of detrainment
(deactivation)


entrainment (activation)
-
….



cloud processing of aerosols

is an issue for the latter path.


het/hom transition signature in particle types and temps

lack of large soluble particles

het/hom transition signature in particle types and temperatures


although the observation temperature is only vaguely relation to the
temperature where the observed ice crystal formed, Twohy and
Poellot found that



crustal and metallic particles are predominant in ice crystals
found at T>

36
°
C (heterogeneous nucl.)



sulphates and salts are predominant in ice crystals found at
T<

39
°
C (homogeneous nucl.)



The transition between these regimes is coincident with the
supercooling limit of pure water (about

38
°
C).

effect of organics on homogeneous freezing


organics alter cloud formation processes in ways, that are not yet
understood. Some organics are very good ice nucleating agents,
while organic films on solution droplets seem to impede
homogeneous nucleation.


Organics delay freezing

DeMott et al, 2003, PNAS

cloud processes alter aerosols


Cloud processing of aerosols


changes chemistry of the particles


alters their nucleating efficiencies


can leave large volumes of air free of aerosol particles


which in turn can favour onset of gas
-
to
-
particle conversion
processes (when no other sinks for the gas are present)


space
-
time variability


Cirrus nucleating aerosol is highly variable, both spatially and
temporally


Variability is a consequence of the many ways aerosol particles
arrive in the UTLS


convection


in
-
situ generation (e.g. aviation)


entry from the lower stratosphere (mainly sulphuric acid)



and because of the many sources of aerosol