Abstract. This work presents a thermodynamically consistent framework that enables self-contained, predictive Köhler calculations of droplet growth and activation with considerations of surface adsorption, surface tension reduction, and non-ideal water activity for chemically complex and unresolved surface-active aerosol mixtures. The common presence of surface-active species in atmospheric aerosols is now well-established. However, the impacts of different effects driven by surface activity, in particular bulk–surface partitioning and resulting bulk depletion and/or surface tension reduction, on aerosol hygroscopic growth and cloud droplet activation remain to be generally established. Because specific characterization of key properties, including water activity and surface tension, remains exceedingly challenging for finite-sized activating droplets, a self-contained and thermodynamically consistent model framework is needed to resolve the individual effects of surface activity during droplet growth and activation. Previous frameworks have achieved this for simple aerosol mixtures, comprising at most a few well-defined chemical species. However, atmospheric aerosol mixtures and more realistic laboratory systems are typically chemically more complex and not well-defined (unresolved). Therefore, frameworks which require specific knowledge of the concentrations of all chemical species in the mixture and their composition-dependent interactions cannot be applied. For mixtures which are unresolved or where specific interactions between components are unknown, analytical models based on retrofitting can be applied, or the mixture can be represented by a proxy compound or mixture with well-known properties. However, the surface activity effects evaluated by such models cannot be independently verified. The presented model couples Köhler theory with the Gibbs adsorption and Szyszkowski-type surface tension equations. Contrary to previous thermodynamic frameworks, it is formulated on a mass basis to obtain a quantitative description of composition-dependent properties for chemically unresolved mixtures. Application of the model is illustrated by calculating cloud condensation nuclei (CCN) activity of aerosol particles comprising Nordic aquatic fulvic acid (NAFA), a chemically unresolved and strongly surface-active model atmospheric humic-like substance (HULIS), and NaCl, with dry diameters of 30–230 nm and compositions spanning the full range of relative NAFA and NaCl mixing ratios. For comparison with the model presented, several other predictive Köhler frameworks, with simplified treatments of surface-active NAFA, are also applied. Effects of NAFA surface activity are gauged via a suite of properties evaluated for growing and activating droplets. The presented framework predicts a similar influence of surface activity of the chemically complex NAFA on CCN activation as was previously shown for single, strong surfactants. Comparison to experimental CCN data shows that NAFA bulk–surface partitioning is well-represented by Gibbs adsorption thermodynamics. Contrary to several recent studies, no evidence of significantly reduced droplet surface tension at the point of activation was found. Calculations with the presented thermodynamic model show that throughout droplet growth and activation, the finite amounts of NAFA in microscopic and submicron droplets are strongly depleted from the bulk, due to bulk–surface partitioning, because surface areas for a given bulk volume are very large. As a result, both the effective hygroscopicity and ability of NAFA to reduce droplet surface tension are significantly lower in finite-sized activating droplets than in macroscopic aqueous solutions of the same overall composition. The presented framework enables the influence of surface activity on CCN activation for other chemically complex and unresolved aerosol mixtures, including actual atmospheric samples, to be systematically explored. Thermodynamic input parameters can be independently constrained from measurements, instead of being either approximated by a proxy or determined by retrofitting, potentially confounding several mechanisms influenced by surface activity.
The exchange of molecules between different physical or chemical environments due to diffusion or chemical transformations has a crucial role in a plethora of fundamental processes such as breathing, protein folding, chemical reactions and catalysis. Here, we introduce a method for a single-scan, ultrafast NMR analysis of molecular exchange based on the diffusion coefficient contrast. The method shortens the experiment time by one to four orders of magnitude. Consequently, it opens the way for high sensitivity quantification of important transient physical and chemical exchange processes such as in cellular metabolism. As a proof of principle, we demonstrate that the method reveals the structure of aggregates formed by surfactants relevant to aerosol research.
<p>Surface tension influences the fraction of atmospheric particles that become cloud droplets. Recent field studies have indicated that surfactants, which lower the surface tension of macroscopic solutions, are an important component of aerosol mass. However, the surface tension of activating aerosol particles is still unresolved, with most climate models assuming activating particles have a surface tension equal to that of water. For surfactants to be relevant to particle activation into cloud droplets, multiple parameters must be considered. First, the concentration of surfactant in the initial particle must be sufficiently large that surface tension depression is maintained during activation, despite the dilution that occurs as water condenses onto the particle. Second, the high surface to volume ratio of micron and submicron particles necessitates partitioning a larger fraction of the surfactant molecules to the particle surface than in a typical solution, resulting in a depletion of the bulk concentration and an increase in the surface tension relative to a bulk sample. Third, the timescale for establishing equilibrium at the droplet surface must be known. The interplay of these parameters highlights the necessity of direct measurements of picolitre droplet surface tension.</p><p>This presentation will describe two cutting-edge approaches we have developed to directly measure the surface tension of microscopic droplets. In the first approach, ejection of ~20 &#181;m radius surfactant-containing droplets from a dispenser excites oscillations in droplet shape that can be used to retrieve the droplet surface tension on microsecond timescales. These measurements allow investigation of surfactant partitioning timescales in aerosol and, crucially, test the assumption that droplet surfaces are generally in their equilibrium state. In the second approach, the coalescence of ~8 &#181;m radius droplets is investigated. Coalescence excites droplet shape oscillations which again permit quantification of droplet surface tension. We demonstrate that surfactants can significantly reduce the surface tension of finite sized droplets below the value for water, consistent with recent field measurements. This surface tension reduction is droplet size dependent and does not correspond exactly to the macroscopic solution value. A new monolayer partitioning model confirms the observed size dependent surface tension arises from the high surface-to-volume ratio in finite-sized droplets and enables predictions of aerosol hygroscopic growth. This model, constrained by the laboratory measurements, is consistent with a reduction in critical supersaturation for activation and a 30% increase in cloud droplet number concentration, in line with a radiative cooling effect larger than current estimates assuming a water surface tension by 1 W&#183;m<sup>-2</sup>. The results imply that one single value for surface tension cannot be used to predict the activated aerosol fraction.</p>
The OH + DMDS reaction mainly forms SO2 and CH3O2 with a yield close to two and to a lesser extent RO2 isomerization products.
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