Aerosol in the UTLS

MOD 12 Klaus Gierens Institut für Physik der Atmosphäre DLR Oberpfaffenhofen

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)  nucleation homogeneous (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

homogeneous heterogeneous

DeMott et al, 2003, PNAS

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  sites) more active – 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

RHi outside clouds RHi inside clouds

Haag and Kärcher, 2003

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 Gierens, ACP, 2003; more recent amendments by Ren & MacKenzie, QJRMS; Liu & Penner, JGR

N c

 2 .

81  10 11

f

(

T

)

w

3 / 2

p

3 /

T

5 .

415 [

s

0

e

* (

T

)] 1 / 2 [

s

hom (

T

)  2

s

0 ] 3

f

(

T

)  10 4  0 .

02

T

/ 4 [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.

10 6 10 5 10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4

Nucleation mode

0.01

Aitken mode

0.1

Accumulation mode

1

Coarse mode

10 Diameter ( m m)

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

background aerosol very moist, RHi>140% 100%

measurements at Storm Peak, Rocky Mountains, 3200 m asl, mid latitude cirrus DeMott et al, 2003, PNAS

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.

> 0.07µm

ice residual nuclei in anvil cirrus

> 0.38µm Measurements during CRYSTAL-FACE (Florida, 26 °N) (



8 °C)



21 °C > T >



56 °C +7 °C >



32 °C

Twohy & Poellot, ACP, 2005

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