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 – Cloud Condensation Nuclei aerosol particles which can form cloud droplets at the low supersaturations typical for atmospheric conditions CN – Condensation Nuclei 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 ~ % 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 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 Raoult termKelvin term

Critical diameter and supersaturation Insoluble inclusion with diameter d u

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 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 (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

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 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

First Results

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

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