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CCN measurements at an urban location
Julia Burkart
University of ViennaIstitute of Aerosol Physics, Biophysics and
Environmental Physics
Contents
• Definitions - CCN• Why are we interested in CCN?• Theory of activation• Organics and CCN• Measurement technique• Current field study and first results• Take-home-messages about organic
aerosol
Definitions
• CCN – Cloud Condensation Nucleiaerosol particles which can form cloud droplets at the low supersaturations typical for atmospheric conditions
• CN – Condensation Nucleiat high supersaturations (~ 400%) all particles can initiate droplet formationCN concentration = total particle concentration
Typical supersaturations in the atmosphere
cumulus cloudshighest supersaturationSS ~ 0.3-0.8 %high optical thickness
stratus cloudsSS ~ 0.5 %
fogSS < 0.1 %
Why are we interested in CCN?
• Indirect aerosol effect
CCN can modify the microphysical properties of a cloud
→ higher droplet concentrations and smaller droplets (→ increase in cloud reflectivity)
→ longer lifetimes
Stratus clouds are most sensitive to changes in the microphysical properties
Indirect aerosol effect is estimated to have a cooling effect on the global climate
Aerosol indirect effect
CCN activation – theoretical background
RTd
M
dM
Mimpp
w
w
ws
wsdL
4exp
61
1
3,0,,0
aqueous solution droplet
2 competing effects:
• Kelvin effect: equilibrium vapor pressure ↑
• Raoult effect: equilibrium vapor pressure ↓
→ Köhler equationi van t‘Hoff factorρw density waterms mass of soluteσ surface tensionMs molecular weight soluteT temperatureMw molecular weight waterR gas constant
Raoult term Kelvin term
Critical diameter and supersaturation
s
skrit M2
RTim9d
s2w
33s
2w
3
krit imTR81MM128
S
Insoluble inclusion with diameter du
RTdM4
exp)dd(M
Mim61pp
w
w
1
3u
3ws
ws,0d,L,0
Organic materials as CCN – important parameters
• functional groups and carbon chain length determine solubilitySalts more soluble than acids
• surface tension
• formation of surface layers/ surface activity
• high molecular mass
Humic acid – activation of a pure organic aerosol
• Humic or fulvic acids are used as model compounds HULIS (HUmic LIke Substances) • Polysaccharide and aliphatic substructures• Carboxylic and phenolic functional groups• soluble• surface active
Measurement technique – Viennese CCN counter
cloud chamber
• functional principle: static diffusion chamber
• core: cloud chamber where defined supersaturations can be generated (0.2 – 2%)
• laser beam illuminates the activated particles in the center of the chamber
Formation of supersaturation within the cloud chamber
• wetted surfaces (filter paper and fritted glass)
• temperature difference between the plates (Ttop > Tbottom), by diffusion of air molecules and water vapor molecules:
→ linear temperature gradient→ linear vapor pressure gradient• equilibrium vapor pressure is NOT a linear
function of temperature → supersaturation
• laser beam illuminates the center of the chamber• activated particles scatter light and a CCD camera takes
pictures• droplets per frame are counted by an automatic image
analysis program
Inside the cloud chamber
Measurement cycle
• Determination of the CCN concentration in the measurement volume of the cloud chamber
• one measurement cycle: 30 sec13sec chamber is flushed 17sec chamber is sealed, supersaturation
profile is established after a few seconds, droplets form, scatter light and CCD camera takes pictures
• total CN concentration is determined in parallel by a TSI CNC
• → determination of the activation ratio: CCCN/CCN
and further critical supersauration
Current field study
• Location: rooflab of physics building → atmospheric urban background aerosol
• Continous measurements of CCN (SS = 0.5%, stratus clouds) and CN concentrations
• As well number size distribution (DMA) and mass size distribution (low pressure cascade impactors)
• Filter samples of the ultrafine size fraction (<100nm) for chemical analysis and generation of synthetic ambient aerosol
Key Questions
• Seasonal variation of CCN activation• Dependence of composition and CCN
activation of atmospheric aerosol on origin of airmass
• Insights for CCN concentration modelling: Is knowledge of aerosol size distribution and typical composition enough?
• Laboratory study: activation behavior of synthetic ambient aerosol compared to real world aerosol
First ResultsTime series of CN concentrations and activation ratios
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
0 500 1000 1500 2000
Time from midnight May 26th [5min]
CN
co
nc
en
tra
tio
n [
\cc
m]
0
0,2
0,4
0,6
0,8
1
1,2
1,4
ac
tiv
ati
on
ra
tio
Time series of CCN concentrations and activation ratios
-100
100
300
500
700
900
1100
1300
1500
0 500 1000 1500 2000
Time form midnight May 26th [5min]
CC
N c
on
cen
trat
ion
[/c
cm]
0
0,2
0,4
0,6
0,8
1
1,2
1,4
acti
vati
on
rat
io C
CN
/ C
N [
-]
Take home• Organic particles can act as CCN if they
are soluble (→ Köhler) or at least wettable (→ Kelvin)
• Pure Humic acid particles can form cloud droplets at atmospheric conditions
• When organic substances are part of an internally mixed particle they influence critical supersaturation by:– solubility– influence on surface activity– molecular weight
Thank you for your attention!