1. Tropospheric Aerosol Description of tropospheric aerosol
Chemical composition
Solubility, aqueous-phase chemistry

2. Ein Beispiel aus der Praxis …

3. The „Asian Brown Cloud“
A 3km dust layer over Asia
causing health problems and
climate change effects

4. Direkte Beobachtung von Aerosolen aus dem Weltraum
Rauchwolken von Waldbränden nahe Sydney (Dez. 2001)

5. Sources of aerosol windborne dust sea spray volcanoes
fossil fuel combustion
pollen and plant fragments

6. Definitions
Aerosol: suspension of fine solid or liquid particles in a gas
primary aerosol: emitted directly as particles
secondary aerosol: formed in the atmosphere by gas-to- particle conversion
fine aerosol: particles < 2.5 µm
coarse aerosol: particles > 2.5 µm

7. Definitions (2)
dust: solid particles produced by mechanical disintegration of material (D > 1 µm)
smoke: small gas-borne particles from incomplete combustion (D > 0.01 µm)
fume: solid particles generated from vapour state (usually after volatilization from melted substances) (D < 1 µm)
haze: water droplets, pollutants, and dust (D < 1 µm)

8. Chemical composition Tropospheric aerosols contain: sulfate ammonium
nitrate
sodium
chloride
trace metals
carbonaceous material
crustal elements
water
after Seinfeld&Pandis, 1998

9. CCN Without particles, no clouds would form!
Cloud Condensation Nuclei: particles that become activated and grow to droplets in the presence of supersaturated water vapour
for marine stratiform clouds, the supersaturation is %; minimum particle diameter is nm
CCN number concentrations: < 100 cm-3 in remote marine areas to > cm-3 in polluted areas

10. Cloud condensation Liquid Water Content (LWC): L = 0.05-3 gH2O/m3
Droplet size: r = 1 µm µm

11. Absorption equilibrium
A(g) A(aq)
for dilute solutions:
[A(aq)] = HA · pA
aqueous-phase
concentration
(mol L-1)
partial pressure of
A in gas-phase
(atm)
Henry coefficient
(mol L-1 atm-1)
Note: aqueous phase concentration in fA expressed per volume gas not per volume liquid!
R: convenient units: atm L mol-1 K-1
For species with HA less than 400 M atm-1, less than 1% resides in the liquid phase in typical clouds
Gas/Aqueous phase partitioning
soluble
fraction of A in aqueous phase:

12. Gas/Aqueous-phase partitioning
very soluble gases: H2O2, HNO3, NO3

13. pH
Upon dissolution in water, several species will form ions, e.g. H2O, and CO2.
Water:
H2O H+ + OH- ; equilibrium constant K = [H+][OH-]
(at 298K, only 2 µmol/L ions versus 55.5 mol/L H2O)
pH = -log10[H+]
K already includes 1/[H2O] !

14. Dissolution
Generally: K = [X+][Y-] / [X•Y]
TABLE6.4

15. CO2 uptake by the oceans CO2(g) H+ + HCO3- H+ + CO32- CO2•H2O
CO2(aq) =
H+ + HCO3-
Kc1
H+ + CO32-
Kc2
carbonate
bicarbonate
Khc
total dissolved carbon:
from Khc, Kc1, Kc2, one can easily derive expressions for [HCO3-] and [CO3(2-)]. Use these to compute total dissolved carbon.
Total dissolved carbon is always greater then CO2*H2O !
effective Henry coefficient

16. CO2 uptake by the oceans (2)
In acid solution, CO2 is practically not dissolved, but if pH grows above 5 it does. Yet, even at pH=8, H* is only 1.5 mol/L/atm; therefore practically all CO2 resides in the atmosphere

17. SO2 uptake by droplets
several orders of magnitude! refer to „soluble“ threshold of about 400.

18. Acidity of (clean) rainwater
The atmospheric CO2 concentration has an influence on the
acidity of rain water:
Electro neutrality demands that
This can be rearranged to:
With given temperature and pCO2, [H+] can be computed, from
which all other ion concentrations can be deduced.
For T=298K and pCO2 = 350 ppm, pH = 5.6
Other species of interest: SO2 HSO SO32-, NH3 NH4+, HNO3 NO3-

19. S(IV)  S(VI) oxidation
The conversion of dissolved SO2 to sulfate is the most important chemical transformation in cloud water. If one S(IV) ion is consumed in a reaction, it will quickly be replaced, because the equilibrium between SO2•H2O, HSO3-, and SO32- is established very fast (milliseconds), and because the dissociation of dissolved SO2 enhances its solubility.
Pathways for S(IV) to S(VI) conversion include reaction with
O3, H2O2, O2 (catalized by Mn(II) and Fe(III)), OH, NO3, ...
Examples:
S(IV) + O3  S(VI) + O2 (slow in gas-phase, rapid in aqueous-phase)
HSO3- + H2O SO2OOH- + H2O , followed by
SO2OOH- + H+  H2SO4

20. Aerosol size distribution
Consider only spherical shape ...
number density distribution
surface distribution
volume distribution

23. Deliquescence and (re)cristallisation
The thermodynamic phase of an aerosol particle depends on the humidity. Dry particles will remain solid until the relative humidity reaches a threshold. The, the particle spontaneously absorbs water and grows (deliquescence). Subsequent drying leads to recristallisation, but at much lower relative humidities (Hysteresis effect).

24. 1) Introduction - Requirements
The Hamburg Aerosol Model (HAM) in ECHAM5
1) Introduction - Requirements
Prediction of size-distribution, composition and mixing-state essential
Aerosol-cloud interaction
Sink processes
Radiative effects
Observations
Aerosols predominantly internally mixed
Externally mixed contribution
Computational constraints
Balance in complexity: GCM vs. Aerosol Model

25. Aerosol Modelling - Methods
The Hamburg Aerosol Model (HAM) in ECHAM5
Aerosol Modelling - Methods
1. Discretize aerosol distribution in bins and calculate their temporal development:
Prescribe aerosol distribution function and calculate the temporal development of its moments:

26. Aerosol Representation in HAM
The Hamburg Aerosol Model (HAM) in ECHAM5
Aerosol Representation in HAM
Resolve aerosol distribution by 7 log-normal modes
Each mode is described by three moments:
Number, Median Radius  Mass, Standard Deviation (fixed)
Mixing State of the compounds:
Sulfate
Black Carbon
Organic Carbon
Sea Salt
Dust
AITKEN
(0.005 µm < r < 0.05 µm)
ACCUMULATION
(0.05 µm < r < 0.5 µm)
SOLUBLE / MIXED
COARSE
(0.5 µm < r )
INSOLUBLE
1 N1, MSO4
5 N5, MBC, MOC
6 N6, MDU
7 N7, MDU
2 N2, MSO4, MBC, MOC
3 N3, MSO4, MBC, MOC , MSS , MDU
4 N4, MSO4, MBC, MOC , MSS , MDU
NUCLEATION
(r < µm)
MODES IN M7
7 modes are grouped into 4 geometrical modes with fixed mode boundaries but varying median radii...
4 modes are soluble or internal mixture of solubles and insolubles
3 modes consist of insoluble compounds
modes among each other are externally mixed
each mode is described by it‘s total number and the masses of it‘s compounds for example the nucleation mode... the Aitken mode additionally contains...
This concept allows to reduce the number of transported tracers to 28...
 Reduction of the number of transported tracers to 28

27. Optische Dichte von Aerosolen
JANUAR
JULI
from Chin et al., 2002

28. Mixing State The Hamburg Aerosol Model (HAM) in ECHAM5
To give you impression of the simulated mixing state the following figure shows the ...
i. e. the mass fraction of the color coded compound that contains more than 50% of the total mass. For example over most part of the oceans sea salt dominates the mass of the mixed accumulation mode and is coded in blue...
Carbonaceous compounds dominate in the BB regions and regions of strong biogenic emissions such as Africa and South America, as well as the East Asia, Australia and the West Cost of the US.
Sulfate ... and the arctic regions.

29. ECHAM5 IMPROVE ECHAM5 IMPROVE Surface Particle Mass
The Hamburg Aerosol Model (HAM) in ECHAM5
Surface Particle Mass
ECHAM5
IMPROVE
Mount Rainier (122 W, 47 N)
Mamoth Cave (86 W, 37 N)
ECHAM5
IMPROVE

30. Number Concentrations
The Hamburg Aerosol Model (HAM) in ECHAM5
Number Concentrations
70 S – 20 S
Total aerosol number annual mean Pacific profile; Averaged over 70S - 20S and 130 E - 90 W
Pacific measurement composite (From Clarke and Kapustin; JAS; 2002)
Relatively good agreement in the middle and upper troposphere
Slight underestimation keeping the cut-off of the instrument in mind
May be compensated bye the usage of Velmaeki et al.(2002)
Underestimation of N in the lower troposphere: Measurements for low-sea salt concentrations: favourable for secondary particle formation – not captured with model resolution Needs more detailed investigation...

31. Bibliography Material for this lecture comes mostly from
Seinfeld, J., and Pandis, S., Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Wiley, New York, …, 1998.