CCN measurements at an urban location

Julia Burkart University of Vienna Istitute 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 – C loud C ondensation N uclei aerosol particles which can form cloud droplets at the low supersaturations typical for atmospheric conditions • CN – C ondensation N uclei at high supersaturations (~ 400%) all particles can initiate droplet formation CN concentration = total particle concentration

Typical supersaturations in the atmosphere

cumulus clouds highest supersaturation SS ~ 0.3-0.8 % high optical thickness stratus clouds SS ~ 0.5 % fog SS < 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

aqueous solution droplet 2 competing effects: • Kelvin effect: equilibrium vapor pressure ↑ • Raoult effect: equilibrium vapor pressure ↓

→ Köhler equation

p

0 ,

L

,

d



p

0 ,    1  6

im M



s s M w



d w

3    1 exp   4 

M w d



w RT

  Raoult term Kelvin term i van t‘Hoff factor ρw density water ms mass of solute σ surface tension Ms molecular weight solute T temperature Mw molecular weight water R gas constant

Critical diameter and supersaturation

d krit  9 im 2  M s s RT  S krit  128  81 R 3  M 3 T 3  2 w 2 w M im s s Insoluble inclusion with diameter d u p 0 , L , d  p 0 ,    1  M s  6 im s  ( d M 3 w w  d 3 u )    1 exp   4  M w d  w RT  

Organic materials as CCN – important parameters

• functional groups and carbon chain length determine solubility Salts 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

• 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 cloud chamber

Formation of supersaturation within the cloud chamber

• wetted surfaces (filter paper and fritted glass) • temperature difference between the plates (T top > T bottom ), 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

Inside the cloud chamber

• 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

Measurement cycle

• Determination of the CCN concentration in the measurement volume of the cloud chamber • one measurement cycle: 30 sec 13sec 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: C CCN /C CN 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

18000 16000 14000 12000 10000 8000 6000 4000 2000 0 0

First Results

Time series of CN concentrations and activation ratios

2000 0,4 0,2 0 1,4 1,2 1 0,8 0,6 500 1000 1500

Time from midnight May 26th [5min]

1500 1300 1100 900 700 500 300 100 -100 0

Time series of CCN concentrations and activation ratios

500 1000 1500

Time form midnight May 26th [5min]

2000 1,4 1,2 1 0,8 0,6 0,4 0,2 0

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!