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Water uptake, cloud condensation nuclei and surface tension: results from the MadFACTS campaign

Citation

Forestieri, Sara et al. (2018), Water uptake, cloud condensation nuclei and surface tension: results from the MadFACTS campaign, Dryad, Dataset, https://doi.org/10.25338/B84K5G

Abstract

This data set is associated with the manuscript “Establishing the Impact of Model Surfactants on Cloud Condensation Nuclei Activity of Sea Spray Aerosols” by Forestieri et al. that has been submitted to Atmospheric Chemistry and Physics. The abstract to this manuscript is provided below. 

By reducing surface tension (σ), surfactants present in aerosols can increase their cloud condensation nuclei (CCN) activation efficiency. However, the importance of this effect is poorly constrained. Here we present estimates of droplet surface tension near the point of activation derived from a continuous flow stream-wise thermal gradient chamber (CFSTGC). The experiments used sea spray aerosol mimics composed of NaCl coated by varying amounts of (i) oleic acid, palmitic acid and myristic acid, respectively, (ii) mixtures of palmitic acid and oleic acid, and (iii) oxidized oleic acid. Significant reductions in σ relative to that for pure water were observed for these mimics at relative humidity (RH) near activation (~99.9%) when the coating was sufficiently thick. The calculated surface pressure (π = σH2O – σobserved) values collapse onto one curve when plotted as a function of molecular area for different NaCl seed sizes and measured RH. The observed critical molecular area (A0) for oleic acid was similar to bulk experiments conducted in a Langmuir trough, but observations presented here suggest that oleic acid in microscopic droplets may exhibit larger π values during monolayer compression. For myristic acid, the observed A0 compared well to bulk experiments on a fresh subphase, where the effects of dissolution are significant. Kinetic limitations were observed for NaCl particles coated with pure palmitic acid, likely as a result of palmitic acid being able to form a solid film.  For binary palmitic acid-oleic acid mixtures, these kinetic limitations were not evident. The σ reductions were also found to be significant for NaCl particles coated with oxidized oleic acid, but our results indicate that these reductions might be slightly lower than that of pure oleic acid. A cloud condensation nuclei (CCN) counter was used to estimate the impact of the observed σ reductions at high RH on critical supersaturation. For organic fractions (ε­org) > 0.90, there were small depressions in critical supersaturation, while for εorg < 0.90, impact on critical supersaturation was negligible. The effect of σ on critical supersaturation is limited by the large droplet sizes present at activation, which leads to molecular areas that are too large for significant reductions in σ. If reductions in σ significantly increase the activation efficiency of SSA, the average properties of marine surfactants must be different than the long chain fatty acids examined here.

Methods

Data were collected at UC Davis and the University of Wisconsin, Madison. Instruments used included a (i) custom continuous flow streamwise thermal gradient chamber, (ii) a commercially available CCN instrument, from DMT, (iii) a custom chemical ionization mass spectrometer, operating with chloride ion chemistry. 

Usage Notes

The data provided are associated with all figures in the manuscript. 

Funding

National Science Foundation, Award: 1305427