CCN measurements at an urban location Julia Burkart University of Vienna Istitute of Aerosol...

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!