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”