Abstract
Surface tension influences the fraction of atmospheric particles that become cloud droplets. Although surfactants are an important component of aerosol mass, 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. By studying picoliter droplet coalescence, we demonstrate that surfactants can significantly reduce the surface tension of finite-sized droplets below the value for water, consistent with recent field measurements. Significantly, this surface tension reduction is droplet size-dependent and does not correspond exactly to the macroscopic solution value. A fully independent monolayer partitioning model confirms the observed finite-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, potentially substantially increasing cloud droplet number concentration and modifying radiative cooling relative to current estimates assuming a water surface tension. The results highlight the need for improved constraints on the identities, properties, and concentrations of atmospheric aerosol surfactants in multiple environments and are broadly applicable to any discipline where finite volume effects are operative, such as studies of the competition between reaction rates within the bulk and at the surface of confined volumes and explorations of the influence of surfactants on dried particle morphology from spray driers.
Highlights
Atmospheric aerosols impact climate directly by scattering solar radiation and indirectly by serving as cloud condensation nuclei (CCN), affecting cloud albedo and precipitation patterns
Such partitioning may fully or partially counteract the surface tension lowering effect of surfactants and must be considered when predicting particle activation [18,19,20,21,22]. Accounting for this partitioning is challenging because few approaches directly measure aerosol particle surface tension [23,24,25,26,27] and so far none have investigated surfactant partitioning in detail
Partitioning models predict that accounting for the surface-bulk partitioning of surface-active molecules becomes increasingly important in smaller droplets, where surface-to-volume ratios are much larger than in micrometer-sized droplets (e.g., 6 × 107 in a 0.05-μm-radius droplet)
Summary
Atmospheric aerosols impact climate directly by scattering solar radiation and indirectly by serving as cloud condensation nuclei (CCN), affecting cloud albedo and precipitation patterns. A high surface-tovolume ratio increases the fraction of the total molecules partitioned to the surface, which lowers the bulk concentration and reduces the solute effect in the Köhler equation Such partitioning may fully or partially counteract the surface tension lowering effect of surfactants and must be considered when predicting particle activation [18,19,20,21,22]. We directly measure the surface tensions of surfactant-coated, high surface-to-volume ratio droplets, demonstrating that their surface tensions are significantly lower than pure water but do not match the surface tension of the solution from which they were produced and depend on finite droplet size These results suggest surfactants could potentially significantly modify radiative forcing and highlight the need for a better understanding of atmospheric surfactant concentrations and properties.
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