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!