DOI: 10.25338/b84k5g
title: Water uptake, cloud condensation nuclei and surface tension: results from the MadFACTS campaign

description: 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)
 &gt; 0.90, there were small depressions in critical supersaturation,
 while for εorg &lt; 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.
description: 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. 
description: The data provided are associated with all figures in the
 manuscript.