Tomi Raatikainen

Cloud forming potential of secondary organic aerosol under near atmospheric conditions

An Aerodyne quadrupole aerosol mass spectrometer (QAMS) was used to provide on-line, quantitative measurements of the chemical composition and mass size distributions of the non-refractory fraction of the SOA particles at a temporal resolution of two minutes. In brief, the AMS utilizes an aerodynamic lens [Zhang et al., 2004, 2002] to produce a collimated particle beam that impacts on a porous tungsten surface heated typically to 600◦C under high vacuum (∼10−8 Torr), causing the non-refractory fraction of the particles to flash vaporize. The vapor plume is immediately ionized using a 70 eV electron impact (EI) ionization source, and a quadrupole mass spectrometer (QMA 410, Balzers, Liechtenstein) is used to analyze the resultant ions with unit mass-to-charge (m/z ) resolution. More detailed descriptions of the AMS measurement principles and various calibrations [Jayne et al., 2000], its modes of operation [Jimenez et al., 2003] and data processing and analysis [Allan et al., 2004, 2003] are available in other publications.

Surfactant partitioning in cloud droplet activation : a study of C8, C10, C12 and C14 normal fatty acid sodium salts

We have measured critical supersaturations of dried single-component particles of sodium caprylate [CH3(CH2)6COONa], sodium caprate [CH3(CH2)8COONa], sodium laurate [CH3(CH2)10COONa] and sodium myristate [CH3(CH2)12COONa] in the diameter range 33–140 nm at 296 K using a static thermal gradient diffusion cloud condensation nucleus counter. These fatty acid sodium salts are surface active molecules which have all been identified in atmospheric aerosol particles. Experimental critical supersaturations increased systematically with increasing carbon chain length and were in the range 0.96–1.34% for particles with a dry diameter of 40 nm. The experimental data were modelled using Köhler theory modified to account for partitioning of the surface active fatty acid sodium salts between the droplet bulk and surface as well as Köhler theory including surface tension reduction without accounting for surfactant partitioning and Köhler theory using the surface tension of pure water. It was found that Köhler theory using the reduced surface tension with no account for surfactant partitioning underpredicts experimental critical supersaturations significantly, whereas Köhler theory modified to account for surfactant partitioning and Köhler theory using the surface tension of pure water reproduced the experimental data well.

Comment on "Changes in droplet surface tension affect the observed hygroscopicity of photochemically aged biomass burning aerosol".

Observed Hygroscopicity of Photochemically Aged Biomass Burning Aerosol” G et al. measured both hygroscopicity and surface tension lowering capacity of photochemically aged biomass burning aerosol. Hygroscopicity was measured by a cloud condensation nuclei (CCN) counter and it was described by the κ parameter, which is typically based on the assumption that droplet solution surface tension is equal to that of pure water. On the other hand, their off-line filter sample analysis revealed that the organic aerosol depressed aqueous solution surface tension by about 30% at the highest measured watersoluble organic mass (WSOM) concentrations. Giordano et al. concluded that the effective κ, which is based on surface tension of pure water, is larger by a factor of 2 or more than the κ-value calculated assuming the measurement based solution surface tension. However, this conclusion does not account for surfactant partitioning, or Gibbs adsorption, which effectively reduces surfactant effects in microscopic droplets. Previous experimental and modeling studies have shown that using macroscopic surface tensions without accounting for surfactant partitioning between bulk solution and surface, CCN activities of microscopic droplets are greatly overpredicted. Partitioning of a surfactant to the droplet surface causes a decrease in the surfactant bulk concentration, which has an effect on both surface tension and water activity. Using macroscopic surface tension means a large reduction for the surface tension dependent Kelvin term, but when the partitioning is accounted for, the reduction of the Kelvin term becomes smaller, and the concentration dependent Raoult term increases. As a result, accounting for surfactant partitioning can cancel out the surface tension based increase in CCN activity altogether. We carried out model simulations to examine if accounting for surfactant partitioning changes the κ-value in the CCN activation experiments of Giordano et al. compared with the case of ignoring partitioning and using constant surface tension of pure water (the apparent κ). For simplicity, the Chamise burning experiment and the surface tension parametrization for the two hours after lights on case from Giordano et al. was selected for model analysis. The average apparent κ (κapp) for the whole illuminated time period is 0.09, and when the dry particle diameter is also fixed to 100 nm, critical supersaturation is 0.39% according to the Köhler theory. When the selected surface tension equation is applied to 100 nm particles activating at 0.39% supersaturation, the required κ is 0.02; as Giordano et al. concluded, this is significantly smaller than the apparent value (κapp = 0.09). However, when this calculation is repeated using the partitioning model from Prisle et al., we find that depending on composition assumptions the required κ (κpart) is between 0.05 and 0.09, that is, changes in droplet surface tension have smaller or negligible effect on the observed hygroscopicity. The largest effect (κpart = 0.05) is seen when 100% of the particulate material is assumed to be surface active. More realistic compositions where for example 10% of the organic material is considered surface active gave κpart ≈ 0.09, which means that surface tensions effects would not be observed at all. Similar results were obtained for the other cases described in Giordano et al. Figure 1 shows an example of the simulated Köhler curves and surface tensions for the 100 nm particles. The “Apparent”

A Finnish Meteorological Institute–Aerosol Cloud Interaction Tube (FMI–ACIT): Experimental setup and tests of proper operation

The Finnish Meteorological Institute-Aerosol Cloud Interaction Tube (FMI-ACIT) is a multi-purpose instrument for investigating atmospherically relevant interactions between aerosol particles and water vapor under defined laboratory conditions. This work introduces an experimental setup of FMI-ACIT for investigation of the aerosol activation and the droplet growth under supersaturated conditions. Several simulations and experimental tests were conducted to find out what the proper operational parameters are. To verify the ability of FMI-ACIT to perform as a cloud condensation nuclei (CCN) counter, activation experiments were executed using size selected ammonium sulfate [(NH4)2SO4] particles in the size range of 10-300 nm. Supersaturations from 0.18% to 1.25% were tested by experiments with different temperature gradients. Those showed that FMI-ACIT can effectively measure CCN in this range. Measured droplet size distributions at supersaturations 0.18% and 1.25% are in good agreement with those determined by a droplet growth model.