Surface-active compounds present in aerosols can increase their cloud condensation nuclei (CCN) activation efficiency by reducing the surface tension (σ ) in the growing droplets. However, the importance of this effect is poorly constrained by measurements. Here we present estimates of droplet surface tension near the point of activation derived from direct measurement of droplet diameters using a continuous flow streamwise thermal gradient chamber (CFSTGC). The experiments used sea spray aerosol (SSA) mimics composed of NaCl coated by varying amounts of (i) oleic acid, palmitic acid or myristic acid, (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 = σ H 2 O − σ observed ) values for a given organic compound or mixture 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 (A 0 ) for oleic acid determined from droplet growth was similar to that from experiments conducted using macroscopic solutions in a Langmuir trough. However, the observations presented here suggest that oleic acid in microscopic droplets may exhibit larger π values during monolayer compression. For myristic acid, the observed A 0 compared well to macroscopic experiments on a fresh subphase, for which dissolution has an important impact. A significant kinetic limitation to water uptake was observed for NaCl particles coated with pure palmitic acid, likely as a result of palmitic acid (with coating thicknesses ranging from 67 to 132 nm) being able to form a solid film. However, for binary palmitic-acid-oleic-acid mixtures there was no evidence of a kinetic limitation to water uptake. Oxidation of oleic acid had a minor impact on the magnitude of the surface tension reductions observed, potentially leading to a slight reduction in the effect compared to pure oleic acid. A CCN counter was also used to assess the impact on critical supersaturations of the substantial σ reductions observed at very high RH. For the fatty-acid-coated NaCl particles, when the organic fraction (ε org ) was > 0.90 small depressions in critical supersaturation were observed. However, when ε org < 0.90 the impact on critical supersaturation was negligible. Thus, for the fatty acids considered here, the substantial σ reductions observed at high RH values just below activation have limited impact on the ultimate critical supersaturation. A surface film model is used to establish the properties that surface-active organic molecules must have if they are to ultimately have a substantial impact on the activation efficiency of SSA. To influence activation, the average properties of surface-active marine-derived organic molecules must differ substantially from the long-chain fatty acids examined, having either smaller molecular volumes o...

Surface active compounds present in aerosols can increase their cloud condensation nuclei (CCN) activation efficiency by reducing the surface tension (σ) in the growing droplets.However, the importance of this effect is poorly constrained by measurements. Here we present estimates of droplet surface tension near the point of activation derived from direct measurement 15 of droplet diameters using 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 or myristic acid, (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 20 sufficiently thick. The calculated surface pressure (π = σH2O -σobserved) values for a given organic compound or mixture 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 determined from droplet growth was similar to that from bulk experiments conducted in a Langmuir trough. However, the observations presented here suggest that oleic acid in 25 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, for which dissolution has an important impact. A significant kinetic limitation to water uptake was observed for NaCl particles coated with pure palmitic acid, likely as a result of palmitic acid Atmos. Chem. Phys. Discuss., https://doi.

The way high school chemistry curricula are structured has the potential to convey consequential messages about knowledge and knowing to students and teachers. If a curriculum is built around practicing skills and recalling facts to reach “correct” answers, it is unlikely class activities will be seen (by students or the teacher) as opportunities to figure out causes for phenomena. Our team of teachers and researchers is working to understand how enactment of transformed curricular materials can support high school chemistry students in making sense of perplexing, relatable phenomena. Given this goal, we were surprised to see that co-developers who enacted our materials overwhelmingly emphasized the importance of acquiring true facts/skills when writing weekly reflections. Recognition that teachers’ expressed aims did not align with our stated goal of “supporting molecular-level sensemaking” led us to examine whether the tacit epistemological commitments reflected by our materials were, in fact, consistent with a course focused on figuring out phenomena. We described several aspects of each lesson in our two-semester curriculum including: the role of phenomena in lesson activities, the extent to which lessons were 3-dimensional, the role of student ideas in class dialogue, and who established coherence between lessons. Triangulation of these lesson features enabled us to infer messages about valued knowledge products and processes materials had the potential to send. We observed that our materials commonly encouraged students to mimic the structure of science practices for the purpose of being evaluated by the teacher. That is, students were asked to “go through the motions” of explaining, modeling etc. but had little agency regarding the sorts of models and explanations they found productive in their class community. This study serves to illustrate the importance of surfacing the tacit epistemological commitments that guide curriculum development. Additionally, it extends existing scholarship on epistemological messaging by considering curricular materials as a potentially consequential sources of messages.

The supplementary material consists of seven figures that provide additional support for the conclusions presented in the paper.