AEROSOLPROZESSE 1. Sichtbare Bedeutung von Aerosolkonzentrationen 2 PM2.5 levels of 5 μg/m3 (left)...
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Transcript of AEROSOLPROZESSE 1. Sichtbare Bedeutung von Aerosolkonzentrationen 2 PM2.5 levels of 5 μg/m3 (left)...
AEROSOLPROZESSE
1
Sichtbare Bedeutung von Aerosolkonzentrationen
2
PM2.5 levels of 5 μg/m3 (left) and 35 μg/m3 (right) "The average annual PM2.5 density in Beijing has reduced from 100-110 micrograms per cubic meter of air in the year of 2000 to 70-80 micrograms per cubic meter in 2010, said Yu Jianhua, an official with Beijing's Municipal Environmental Protection Bureau."
EU Direktive zur Luftreinhaltung:Maximalwert 20µg m-3
Definitions
Aerosol: suspension of fine solid or liquid particles in a gas
primary aerosol: emitted directly as particlessecondary aerosol: formed in the atmosphere by gas-to-
particle conversion
fine aerosol: particles < 2.5 µmcoarse aerosol: particles > 2.5 µm
More Definitions
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)
Chemical composition
Tropospheric aerosol contains:• sulfate• ammonium• nitrate• sodium• chloride• trace metals• carbonaceous material• crustal elements• water
Urban aerosol composition
C (elem)
C (org)
NH4(+)
NO3(-)
SO4(2-)
Rest
after Seinfeld&Pandis, 1998
Chemical composition (2)
6http://www.esrl.noaa.gov/research/themes/aerosols/
Schiffbasierte Messungen von Aerosolen in verschiedenen Weltregionen
Chemical composition (3)
7Mayol-Bocero, 2000
Filterproben aus Messflügen über dem Indischen Ozean
Aerosoltypen und Größenverteilung
8Heintzenberg et al., in „Atmospheric Chemistry in a Changing World“, Springer, 2003
Aerosol size distribution
Consider only spherical shape ...
number density distribution
surface distribution
volume distribution
Bedeutung von Aerosolen: Wolken
Without particles, no clouds would form!
• Cloud Condensation Nuclei (CCN): particles that become activated and grow to droplets in the presence of supersaturated water vapour
• for marine stratiform clouds, the supersaturation is 0.1-0.5%; minimum particle diameter is 50-140 nm
• CCN number concentrations: < 100 cm-3 in remote marine areas to > 10000 cm-3 in polluted areas
Cloud condensation
Liquid Water Content (LWC): L = 0.05-3 gH2O/m3
Droplet size: r = 1 µm - 50 µm
Bedeutung von Aerosolen: Strahlung
13
Direkter Effekt
Indirekte Effekte
http://www.wmo.int/pages/prog/arep/gaw/aerosol.html
14http://www.gfdl.noaa.gov/atmospheric-physics-and-chemistry_aerosol_cloud
Processes simulated in an atmospheric chemistry transport model
15
Gas photochemistryGas-to-particle conversion
Gas processes
Optical depth of gases/ aerosols/cloud dropsSolar radiative transferInfrared radiative transfer(Visibility)
Radiative processes
NucleationCoagulationCondensation/evaporationDissolution/evaporationDeposition/sublimation
Freezing/meltingReversible chemistryIrreversible chemistryHeterogeneous chemistry
Aerosol/cloud processes
Wind speed & directionAir pressureAir densityAir temperatureSoil temperature & humidityTurbulence…
Dynamical/thermodynamical processes
EmissionsTransport of gases/aerosols/ cloud drops/energyDry deposition of gases/ aerosols/cloud dropsSedimentation of aerosols/ cloud drops/rain drops
Transport processes
nach
Jac
obso
n, 1
999
Sources of aerosol (emissions)
• windborne dust• sea spray• volcanoes• fossil fuel combustion• road transport• pollen and plant fragments
Nucelation – Coagulation - Condensation
17http://www.realclimate.org/images/aerosol1.jpg
Nucleation
18J. Pierce, Nature Geoscience 4, 665–666 (2011)
Cluster Particles
19P. McMurry (U. Minnesota), Nucleation and Cloud Condensation Nuclei Experiment 2009
Absorption equilibrium
A(g) A(aq)
for dilute solutions:[A(aq)] = HA · pA
aqueous-phaseconcentration(mol L-1)
Henry coefficient(mol L-1 atm-1)
partial pressure ofA in gas-phase(atm)
Gas/Aqueous phase partitioning
LTRHc
cf A
g
aqA 610
Aaq fx
1
1
solu
ble
Gas/Aqueous-phase partitioning
very soluble gases: H2O2, HNO3, NO3
Coagulation
22http://www.chm.bris.ac.uk/pt/aerosol.htm
Beobachtung des Zusammenwachsens von Wolkentropfen mit gepulster Laserspektroskopie
Aerosol processes in clouds
23C. Hoose et al., 2008
Condensation / Evaporation
24
Modelliertes Wolkentröpfchenmit Sulfatmolekülen als Kondensationskeimen
http://www.pnl.gov/science/highlights/highlight.asp?id=976
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).
http://www.tlc2.uh.edu/uhaero/Applications/ImageGallery/acp9b/view
Ammonium/Nitrate/Sulfate30% SO4, T=298 K
Modellierung von Aerosolen
26
• Bulk-Schema• Bin-Schema• Modales Schema
• Interne Mischung• Externe Mischung
1.Discretize aerosol distribution in bins
and calculate their temporal development:
2. Prescribe aerosol distribution function and calculate
the temporal development of its moments:
Aerosol Modelling - Methods
P. Stier, U. Oxford
Comparison bin and modal scheme
28K. Carslaw and D. Spracklen, U. Leeds
Concept of M7 (modal aerosol model)
29E. Zubler, ETH-Zürich
Mixing State of the compounds:
• Sulfate
• Black Carbon
• Organic Carbon
• Sea Salt
• Dust
Resolve aerosol distribution by 7 log-normal modes
Each mode is described by three moments:
Number, Median Radius Mass, Standard Deviation (fixed)
Reduction of the number of transported tracers to 28
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 < 0.005 µm)
MODES IN M7
Aerosol Representation in ECHAM-HAMMOZ
Aerosol Mixing State
P. Stier, U. Oxford
Evaluation of ECHAM6-HAMMOZ
32
Jan
Jul
AOD @ 550 nm
Pacific measurement composite(From Clarke and Kapustin; JAS; 2002)
70 S – 20 S
Total aerosol number annual mean Pacific profile; Averaged over 70S - 20S and 130 E - 90 W
Evaluation of number concentrations
P. Stier, U. Oxford
Global SO2/SO4 annual budgetecham6-hammozunits: Tg (S)
SO4 gas
SO2 SO4 aero
2.3
4.921.8
15.5
6.0 19.2
690.047 0.032
1.7
1.5 59.1 2.2
43.1
23.80.23
DMS-OH
DMS-OHDMS-NO3
SO2-OH
wetdep drydepemi
wetdep drydep wetdep drydepsedi
emi
condensationnucleation
wetchemistry
G. Frontoso, ETH-Zürich
Multi-Modell Vergleich (AEROCOM)
35
Ruß (Black carbon)
36
Ruß entsteht bei der unvollständigen Verbrennung von fossilen Brennstoffen, Holz oder Kohle
EXTRA MATERIAL
37
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+]
Dissolution
Generally: K = [X+][Y-] / [X•Y]
TABLE6.4
CO2 uptake by the oceans
CO2(g)
CO2•H2OCO2(aq) = H+ + HCO3-
Kc1
H+ + CO32-
Kc2
carbonate bicarbonate
Khc
2
2
CO
22CO
OHCO
pHKhc
OHCO
HCOH
22
-3
1
cK
-3
-23
2 HCO
COH
cK
total dissolved carbon:
22 CO2211
CO
-23
-322
HH1
COHCOOHCO
pKKK
H ccc
effective Henry coefficient
CO2 uptake by the oceans (2)
SO2 uptake by droplets
Acidity of (clean) rainwater
This can be rearranged to:
The atmospheric CO2 concentration has an influence on theacidity of rain water:Electro neutrality demands that
-23-3 COHCOOHH
2CO21COCO1COOH
H
2
HHH 22222
pKKHpKHK ccc
With given temperature and pCO2, [H+] can be computed, fromwhich all other ion concentrations can be deduced.For T=298K and pCO2 = 350 ppm, pH = 5.6
Other species of interest: SO2 HSO3- SO3
2-, NH3 NH4+, HNO3 NO3
-
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- + H2O2 SO2OOH- + H2O , followed by
SO2OOH- + H+ H2SO4