Surface tension prevails over solute effect in
organic-influenced cloud droplet activation

Jurgita Ovadnevaite, Andreas Zuend, Ari Laaksonen, Kevin Sanchez, Greg Roberts, Darius Ceburnis, Stefano Decesari, Matteo Rinaldi, Natasha Hodas, Maria Cristina Facchini, et al.

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To cite this version:

Jurgita Ovadnevaite, Andreas Zuend, Ari Laaksonen, Kevin Sanchez, Greg Roberts, et al. Surface tension prevails over solute effect in organic-influenced cloud droplet activation. Nature, 2017, 546 (7660), pp.637-641. 10.1038/nature22806. hal-03065676

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# Surface tension prevails over solute effect in organic-influenced cloud droplet activation

Jurgita Ovadnevaite<fnref n="1" />, Andreas Zuend², Ari Laaksonen³,⁴, Kevin J. Sanchez⁵,⁶, Greg Roberts⁵,⁶, Darius Ceburnis<fnref n="1" />, Stefano Decesari⁷, Matteo Rinaldi⁷, Natasha Hodas⁸,⁹, Maria Cristina Facchini⁷, John H. Seinfeld⁸ & Colin O' Dowd<fnref n="1" />

The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Köhler theory<fnref n="1" />,². This spontaneous activation of CCN depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension, which is sensitive to surfactants<fnref n="1" />. Surface tension lowering caused by organic surfactants, which diminishes the Kelvin effect, is expected to be negated by a concomitant reduction in the Raoult effect, driven by the displacement of surfactant molecules from the droplet bulk to the droplet-vapour interface³,⁴. Here we present observational and theoretical evidence illustrating that, in ambient air, surface tension lowering can prevail over the reduction in the Raoult effect, leading to substantial increases in cloud droplet concentrations. We suggest that consideration of liquid-liquid phase separation, leading to complete or partial engulfing of a hygroscopic particle core by a hydrophobic organic-rich phase, can explain the lack of concomitant reduction of the Raoult effect, while maintaining substantial lowering of surface tension, even for partial surface coverage. Apart from the importance of particle size and composition in droplet activation, we show by observation and modelling that incorporation of phase-separation effects into activation thermodynamics can lead to a CCN number concentration that is up to ten times what is predicted by climate models, changing the properties of clouds. An adequate representation of the CCN activation process is essential to the prediction of clouds in climate models, and given the effect of clouds on the Earth's energy balance, improved prediction of aerosol-cloud-climate interactions is likely to result in improved assessments of future climate change.

The validity of Köhler theory for the prediction of aerosol particle water uptake and the conditions for CCN activation to cloud droplets has been universally confirmed for inorganic aerosols. However, modified forms of the theory have been proposed to account for solubility-driven activation effects of CCN containing mixtures of organic and inorganic matter. Organic matter may affect droplet surface tension⁵ by forming surface films⁶,⁷ and may also impede condensation and evaporation kinetics⁸, although conflicting results have been reported²,⁹,¹⁰. In general, a dichotomy exists whereby addition of organics to inorganic aerosol reduces water uptake at sub-saturations¹¹,¹² owing to lower solubility, and increases water uptake at supersaturation¹³,¹⁴, owing to lowering of surface tension and consequently the activation supersaturation. Therefore, a trade-off exists between maximizing the Raoult effect and exploiting surface tension lowering. The surface concentration enhancement of certain organic compounds is related to their limited water solubility, probably inducing an evolution of particle surface tension as a composition-dependent process with hygroscopic

growth. A consequence of an evolving surface tension is that the critical 'wet' size of the activating cloud droplet ($D^*$) also varies, as recently demonstrated experimentally⁷, in agreement with findings from prior laboratory experiments⁶,¹⁵⁻¹⁷.

Here, we present new results illustrating a profound increase of CCN activity when organic matter is mixed with inorganic salt solutions in nascent ultrafine mode (NUM) particles ($D$ between 10 nm and 50 nm) in North Atlantic marine air masses¹⁸ arriving at Mace Head¹⁹⁻²¹ (this is called a 'NUM event'). Figure 1 illustrates the 'dry' aerosol size distribution during the period of the NUM, measured by a scanning mobility particle sizer (SMPS). Also shown are the mass size distributions from the aerosol mass spectrometer (AMS)²² and the SMPS, along with the organic matter and sulphate mass distributions. The size distribution comprised two modes: a mature accumulation mode with modal diameter $D_{ACC} \approx 200$ nm and $N_{ACC} \approx 30 \text{ cm}^{-3}$, co-existing with the NUM with $D_{NUM} = 23$ nm and $N_{NUM} = 2,000 \text{ cm}^{-3}$. Black carbon mass concentrations <5 ng m⁻³ confirm measurements within a pristine marine air mass. The accumulation mode contributed 610 ng m⁻³ (90%) to the submicrometre aerosol mass, and is comprised of 70% non-sea-salt sulphate and 30% organic matter. The NUM comprised approximately equal contributions of organics and non-sea-salt sulphate (24 ng m⁻³ organics and 20 ng m⁻³ non-sea-salt sulphate in a 55%/45% mass ratio). The sea-salt mass of 40 ng m⁻³ was confined to the accumulation mode.

The aerosol mass spectra revealed differences in the organic aerosol composition between the two modes (Supplementary Fig. 1.2.1). The NUM mass spectrum showed a contribution from hydroxyl groups (mass-to-charge ratios, *m/z*, of 29 and 43)²³ and cyclic alcohols (*m/z* 83)²³ as well as dicarbonyls or oxoacids (*m/z* 29 and 44). Some indication of amines/cycloalkylamines was also visible in the mass spectrum (*m/z* 29 and 56)²³; however, very low mass concentrations meant it was difficult to differentiate further among the different types of organic functional groups. In contrast, the mass spectrum of organics in the accumulation mode pointed to a more dominant contribution from acids, as reflected by a larger contribution from *m/z* 44.

CCN were measured²⁴ at supersaturations between 0.2% and 0.82% in the periods before, during and after the NUM event and are compared to calculated concentration using $\kappa$-Köhler theory⁵, in which the calculations for critical dry diameter ($D_c$) and the hygroscopicity parameter $\kappa$ were guided by AMS-derived information on chemical composition⁵. The calculated CCN concentration was then derived from SMPS measurements by integrating over all particles larger than $D_c$. A droplet surface tension of 72 mN m⁻<fnref n="1" />, typical for pure water at 25°C (ref. 25), was used unless otherwise specified. On the basis of previous studies characterizing similar NUM events, we assume an internal state-of-mixing for each aerosol mode²⁶. The time series of measured and calculated CCN concentrations are shown in Fig. 2a, where distinct differences during NUM event periods are evident.

<footnote n="1">¹School of Physics and Centre for Climate and Air Pollution Studies, National University of Ireland Galway, Galway, Ireland. ²Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada. ³Finnish Meteorological Institute, Helsinki, Finland. ⁴Department of Applied Physics, University of Eastern Finland, Kuopio, Finland. ⁵Centre National de Recherches Météorologiques, 42, avenue Gaspard Coriolis, 31057 Toulouse, France. ⁶Scripps Institution of Oceanography, 9500 Gilman Drive, 0221 La Jolla, California 92093, USA. ⁷Instituto di Scienze dell'Atmosfera—CNR, 40129 Bologna, Italy. ⁸Division of Chemistry and Chemical Engineering, California Institute of Technology, Mail Code 210-41, Pasadena, California 91125, USA. ⁹Department of Environmental Science and Management, Portland State University, Portland, Oregon 97201, USA.</footnote>
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**Figure 1** | Physical number concentration–size distributions, total mass–size distributions and sulphate and organic-matter mass–size distributions during the NUM event. a, Normalized SMPS number concentration (dN/dlog₁₀[D/m]) distribution as a function of particle diameter (Dₘ₉₅), illustrating the two lognormal modes—NUM at 23 nm and an accumulation mode at 200 nm (peaks in the black line). Also shown are the Dₐ values derived for the AMS-derived chemical composition: blue vertical line, using the surface tension σ for water (72 mN m⁻¹) with lower limit the sulphate mass fraction associated with ammonium sulphate (effective κ = 0.30), and upper limit the sulphate associated with sulphuric acid (effective κ = 0.28); red vertical line, using the surface tension of water and the assumption that all mass is sulphuric acid (κ = 0.7); and

For most of the time, there is excellent agreement between measured and calculated CCN concentration using AMS-derived chemical composition, while accounting for the majority of particle components of different hygroscopicities and associated κ values (Fig. 2d). However, during the NUM events, the measured CCN concentration exceeded the calculated concentrations by as much as a factor of ten. We could reproduce the measured concentrations only by lowering D<sub>c</sub> to 41 nm.

Taking a typical marine stratocumulus supersaturation of 0.5%, differences in D<sub>c</sub>, for distinctive chemical composition and surface tension, explain the behaviour shown above. AMS-derived NUM chemical mass composition indicated a ratio of organic matter to non-sea-salt sulphate of 55:45, with the sulphate mass present either as sulphuric acid or ammonium sulphate. Assuming a constant surface tension of 72 mN m<sup>-1</sup> and κ values of 0.7, 0.58 and 0.05 for pure sulphuric acid, ammonium sulphate and organic matter (Supplementary Information), respectively, we obtain D<sub>c</sub> values of 59–60 nm (ammonium sulphate + organic matter to sulphuric acid + organic matter mixtures).

Meanwhile, for a sulphuric-acid-only composition, which represents perhaps one of the highest hygroscopicity scenarios, and assuming a surface tension of 72 mN m⁻¹, D_c drops to 44 nm. A D_c value of 41 nm is obtained for one of the lower hygroscopicity scenarios, such as organic + non-sea-salt sulphate, with an assumed surface tension of 49 mN m⁻¹ for sulphuric acid as non-sea-salt sulphate in the mixture or 50 mN m⁻¹ for ammonium sulphate. The lower D_c, despite the contrasting values in κ, reflects the profound impact of surface tension lowering to values notably less than that of pure water. This reduction in D_c shifts the smallest activated particle size towards the NUM modal diameter and, consequently, leads to activation of up to an order of magnitude more CCN.

Taking an AMS-derived bulk chemical composition (which is easier to derive routinely) rather than NUM composition, the effective κ increases from 0.28 to 0.45 owing to a smaller relative organic matter contribution in accumulation mode (30% instead of 55%) and

green vertical line, using the measured NUM chemical composition but with a reduced surface tension of 49 mN m⁻¹. b, Comparison of mass-size distributions derived from AMS and SMPS. The M<sub>SMPS</sub> is calculated from the volume distribution using two density values: the upper M<sub>SMPS</sub> limit results from the density derived from the chemical composition and the lower M<sub>SMPS</sub> limit is derived by applying a density calculated by matching vacuum aerodynamic diameter with mobility diameter (see Supplementary Information for more details on density calculations). c, Organic matter and sulphate mass distributions as derived from the AMS. Note that here the AMS-derived diameter has been converted to mobility diameter, rather than its native vacuum aerodynamic diameter; see Supplementary Information for the instrument uncertainties.

means that a smaller surface tension reduction (57 mN m⁻¹ instead of 49 mN m⁻¹) is required to produce the measured CCN number concentration. A detailed analysis of measurement uncertainties and closure sensitivity to chemical composition and water uptake is reported in the Supplementary Information, where we conclude that these uncertainties cannot account for the lack of closure in the absence of lowering surface tension.

We use three models differing in complexity and in the treatment of organic compounds at the particle surface to elucidate the concepts outlined above. The first model (model I) describes an effective ternary sulphate–organic–water system with efficient organic surfactants²⁷ that partition between the droplet’s bulk and surface²⁸. With equal mass fractions of organics and sulphate in 41-nm-diameter dry particles, and very low critical micelle concentrations, the model yields the observed reduction in critical supersaturation only if the surfactant molecules are unrealistically small (molecular weights below 80–85 atomic mass units). This is due to the large surface-to-volume ratio of the activating droplets (D*≈200 nm).

The second model (model II) deployed is a thermodynamic equilibrium model based on the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model²⁹,³⁰ to predict hygroscopic growth properties, the potential existence of a liquid–liquid phase separation (LLPS), phase compositions and volumes. This model encapsulates the Raoult effect (water activity) with non-ideal mixing in liquid solutions, gas-particle partitioning of semivolatile compounds, and the prediction of composition-dependent and phase-separation-dependent particle surface tension, thereby accounting for the Kelvin effect. Droplet growth, activation and the associated critical saturation ratio S<sub>c</sub> are predicted via the Köhler equation (equation (S3.1.1) in the Supplementary Information). Briefly, a LLPS affects the particle surface tension because an organic-rich phase β will establish a partial or complete shell covering an aqueous, inorganic-rich core phase α. For such a core–shell configuration, as frequently observed for complex organic–inorganic particles³¹, the effective particle surface
<figure><img src="image_4.png" /><figcaption>**Figure 2** | CCN, chemical composition and size distribution time series for the NUM event and the CCN closure. **a**, Measured (colour traces) and calculated (grey traces) activated CCN number concentrations, assuming a surface tension of $\sigma = 72 \text{ mN m}^{-1}$. Both measured and calculated CCN span the whole supersaturation range of SS = 0.2–0.82%; that is, the CCN was calculated for exactly the same supersaturation as what was measured. The activated CCN concentration is calculated for the measured supersaturation by using $\kappa$-Köhler theory with an effective $\kappa$ corresponding to the total AMS composition. **b**, Time series of aerosol number size distribution during the particle production event. **c**, Time series of specific aerosol chemical species mass concentrations—organics, ammonium, nitrate, sulphate, sea salt, methanesulfonic acid and black carbon.</figcaption></figure>

tension will be determined predominantly by the composition of phase β (Fig. 3c). While LLPS leads to distinct phase compositions at equilibrium, typically both phases contain organic as well as inorganic material. Therefore, a complete phase separation into an organics-only film phase and an aqueous inorganic core phase would present an extreme case of LLPS—a key assumption of the third model.

The third model (model III) is an AIOMFAC-based simplified organic film model, motivated by a single-parameter organic film model approach⁷ whereby all organic material resides in a water-free insoluble organic film adsorbed to an aqueous, salt-rich core phase, coating the core phase completely up to the point where a minimum thickness $\delta_{\text{org}}$ ($\delta_{\beta,\text{min}}$ in our model) is reached. Hence, this simplified model resembles our more complex thermodynamic equilibrium model with LLPS, but with two major differences: (1) no organic material dissolves into the growing aqueous phase and (2) no water partitions to the organic film 'phase', that is, a permanent and complete organic–inorganic LLPS at all relative humidity levels. The predictions of the simplified organic film model (model III) provide an upper limit to the expected reduction in surface tension (a lower limit on the critical supersaturation SS*) since the organic film assumption implies that complete organic film coverage is maintained up to the largest physically possible droplet diameter (that is, until molecular monolayer thickness is reached even with all organic material adsorbed at the

<figure><img src="image_5.png" /><figcaption>**Figure 3** | Scatter plot of calculated versus measured CCN using the same $\kappa$-Köhler settings as in **a**, but including the whole month of August 2015. The colour scale indicates a weighted SMPS size diameter ($D_w$) for every point: the smallest diameters (in dark blue) correspond to the particle production event and deviate from the 1:1 line. **e**, Scatter plot (subset of **d**) including only the particle production events. The grey points in **e** correspond to the dark blue points in **d** but using the NUM mode chemical composition instead of bulk AMS composition; blue points correspond to the particle production event after the assumed surface tension for the CCN calculation with $\kappa$-Köhler theory was reduced to 49 mN m⁻¹. **a**, **d** and **e** contain the whole range of measured supersaturations (0.2–0.82%).</figcaption></figure>

surface). Equally, the equilibrium LLPS model (model II) is expected to provide a lower limit on the possible surface tension reduction (Supplementary Information).

The input aerosol chemical composition is based on two surrogate systems, outlined in Supplementary Tables S3.1 and S3.2. We focus on system 2, which comprises a higher fraction of low-volatility, multifunctional organic compounds of relatively low O:C ratios (for example, the ‘ester dimer’), resulting in an O:C ratio more consistent with the measured NUM properties (Supplementary Information). Model calculations were carried out for relevant input parameters influencing the evolution of surface tension and $S_c$ (Supplementary Table 3.3). The LLPS model predictions for surrogate system 2 with a mass ratio of organics to sulphuric acid of about 1:1 and a minimum thickness for phase β of $\delta_{\beta,\text{min}} \approx 0.16 \text{ nm}$ are shown in Fig. 3 for the high-relative-humidity range, while additional species concentrations and gas-particle partitioning are shown in Supplementary Fig. 3.7.1 for the extended range of 0% to 101% relative humidity.

In Fig. 3, the transition from a predicted LLPS state at lower relative humidity to a single liquid phase is denoted by a vertical yellow line and, in case of the bar graphs, is also indicated by the co-existence of two liquid phases of distinct compositions (Fig. 3e and f) up to a water activity of 0.9997. Figure 3b reveals that complete surface coverage by phase β, that is, $c_β = 1$, is possible only up to a diameter
<figure><img src="image_6.png" /></figure>

**Figure 3** | Thermodynamic equilibrium model predictions for an internally mixed organic matter plus sulphuric acid aerosol in the relative humidity range from 94% to about 101%. All results are for a temperature of 288.15 K and a mass ratio of organic matter to sulphuric acid of about 1:1. a, Köhler curves of predicted equilibrium supersaturation versus particle diameter for a dry diameter $D_0 = 41$ nm. The solid blue curve shows the case with consideration of the variation in surface tension with particle size and phase composition, as indicated in b. The dashed green curve shows the case for a constant surface tension of $\sigma = 72 \text{ mN m}^{-1}$ (that is, as for pure water). CCN activation occurs when the supersaturation surpasses the maximum of the Köhler curve; $SS^* = 0.5\%$ (blue curve) or 0.88% (green curve). The vertical yellow line

marks the existence of a LLPS to the left of the line (here beyond the CCN activation diameter). b, Predicted effective surface tension $\sigma$ and (partial) surface coverage by shell phase $\beta$ (c$_{\beta}$, light blue curve) versus particle diameter. The kink in the blue curves in a, b and d at a diameter of about 0.2 µm occurs where the mean thickness of phase $\beta$ reaches $\delta_{\beta, \min} = 0.16$ nm.

c, Schematic of the surface coverage of the aqueous core phase (light blue) by organic-rich phase $\beta$ (green) at different stages in the growth process (cross-sectional view); relative phase sizes are not shown to scale.

d, Radial thickness of the two liquid phases when present in a core-shell configuration. e, f, Relative compositions of the two liquid phases in stacked component mass fractions.

of around 200 nm (0.2 µm), at which point the monolayer thickness, $\delta_{\beta, \min}$, is reached (see also the light blue curve in Fig. 3d). Further particle growth leads to a fractional surface coverage by phase $\beta$, which is also visible as the kink in the blue curves in Fig. 3a, b and d. Figure 3a shows the prediction of a particle's equilibrium supersaturation versus diameter, that is, the Köhler curve. Clearly, a particle of dry diameter $D_0 = 41$ nm with composition-dependent surface tension is predicted to undergo CCN activation at a substantially lower critical supersaturation of $SS^* = 0.50\%$, and a larger critical diameter of $D^* = 311$ nm (blue curves in Fig. 3a and b), than does a particle with a constant surface tension of $72 \text{ mN m}^{-1}$ (with $SS^* = 0.88\%$ and $D^* = 164$ nm; dashed green curves in Fig. 3a and b), as indicated by the distinct locations of the global maxima of the Köhler curves. The simplified organic film model predicts $SS^* = 0.43\%$ and $D^* = 330$ nm for the same system (Supplementary Table 3.3). If ammonium sulphate, rather than sulphuric acid, is used as inorganic compound, similar $SS^*$ and $D^*$ value ranges are obtained (for example, $SS^* = 0.45\%$ to 0.51%), indicating a relatively minor impact of the choice of sulphate species on CCN activation. In conclusion, the predictions from the equilibrium LLPS model (model II) and the simplified organic film model (model III) provide evidence that aerosol particles of a dry diameter of approximately 41 nm and compositions similar to system 2 are indeed able to activate at a supersaturation of about 0.5%, as observed during the NUM events.

Furthermore, we assessed the sensitivity of CCN activation properties with respect to variability in organic/sulphate mass ratios in a series of additional thermodynamic model calculations. The sensitivities are found to be small over a wide range of organic/sulphate mass ratios, mainly as a result of a balance between the effect of hygroscopicity (increasing with increasing sulphate fraction) and surface tension lowering (decreasing with increasing sulphate fraction). The impact of uncertainties in pure component vapour pressure estimations is also assessed to be minor (see Supplementary Information for discussion on sensitivities).

The predicted effects on $D^*$ are in qualitative agreement with a two-dimensional compressed-film model by ref. 7 as well as their simplified single-parameter organic film model for the interpretation of measured particle diameter evolution and CCN activation under high-relative-humidity and supersaturated conditions. On the basis of measured droplet growth for particles exceeding approximately 150 nm dry diameter, the two-dimensional compressed-film model predicts that activation occurs once the mixed particles' growth curve reaches the surface tension of pure water and matches the Köhler curve corresponding to the inorganic ammonium sulphate seed$^7$. This conclusion was used to constrain a single-parameter organic film thickness model$^7$. However, our thermodynamic model predictions show that this is not a general result; for example, while the LLPS model predicts activation when surface tension increases back to that of water for larger particles (for
example, D₀ = 175 nm, Supplementary Fig. 3.7.4), the organic film model predicts activation typically at critical surface tension σ* values that are often much lower than the surface tension of pure water. While previous work has mainly emphasized the observed increase in D* rather than SS*, we show that an evolving surface tension has the simultaneous effect of a considerable decrease in the critical equilibrium water vapour supersaturation (SS*) of a particle, particularly so in the case of ultrafine aerosol.

The thermodynamic model predictions reveal that certain multifunctional organic compounds, typically found in atmospheric secondary organic aerosol, preferentially partition to a relatively hydrophobic, organic-rich liquid phase that is expected to be located at the particle surface. The molecular structure of these organic compounds is quite different from the classical, industry-relevant surfactants (molecules with hydrophilic head groups and a hydrophobic tail). The model results also suggest that those organic compounds that partition predominantly to phase β are mostly of lower hygroscopicity, such that their fractional contribution to the overall water content of a particle is small, and the Raoult effect in Köhler theory is therefore only marginally affected by the presence of two liquid phases when non-ideality is considered, while the LLPS effect on surface tension is substantial.

The modelling results confirm a theoretical pathway for surface tension lowering dominating over the reduction in the Raoult effect in organic–inorganic aerosol mixtures through liquid–liquid phase separation and organic film formation and demonstrate that an evolving surface tension is sufficient to reduce the critical supersaturation, leading to the observed increase in CCN. Further, the modelled surface tension is consistent with that in macroscopic fog and cloud samples from a number of diverse regions around the globe, suggesting that it is likely to be a ubiquitous phenomenon (Supplementary Fig. 4.1.1).

From these results, it can be inferred that previous laboratory studies into the effect of surface-active organics on CCN activation may not completely encapsulate the true impact of highly surface-active organic molecules or organic-rich phases. This highlights the need for further laboratory and field measurements to clarify the nature and formation of surface-active organic compounds and the presence of LLPS in ultrafine particles at high relative humidity.

**Online Content** Methods, along with any additional Extended Data display items and Source Data, are available in the online version of the paper; references unique to these sections appear only in the online paper.

**Data Availability** The datasets generated and analysed during the current study are available from the authors on reasonable request; see the 'Author Contributions' section for specific datasets.

**Code Availability** The custom computer code used for data processing is available from the corresponding author on request.

Received 20 August 2016; accepted 4 May 2017.

Published online 21 June 2017.

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**Supplementary Information** is available in the online version of the paper.

**Acknowledgements** The research leading to these results received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) project BACCCHUS under grant agreement number 603445, the Irish Environmental Protection Agency, the HEA PRTLI4 project and the Academy of Finland Center of Excellence programme (grant number 272041). The work was further supported by the CNR (Italy) under AirSEaLab: Progetto Laboratori Congiunti, the US Office of Naval Research, and the Natural Sciences and Engineering Research Council of Canada (NSERC, grant RGPIN/04315-2014). N.H. was supported by a National Science Foundation Atmospheric and Geospace Sciences Postdoctoral Research Fellowship (award number 14433246).

**Author Contributions** J.O. conducted and analysed the AMS measurements and led the scientific development (in conjunction with C.O'D.). D.C. organized and conducted the aerosol measurements. G.R. and K.J.S. conducted the CCN measurements. M.C.F., S.D. and M.R. provided surface tension measurements. A.L. provided the partitioning model. A.Z. did the thermodynamic modelling. C.O'D., J.O., N.H., A.Z. and J.H.S. wrote the paper.

**Author Information** Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of the paper. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Correspondence and requests for materials should be addressed to C.O'D. (colin.odowd@nuugalway.ie).

**Reviewer Information** *Nature* thanks M. Gysel, K. Wilson and the other anonymous reviewer(s) for their contribution to the peer review of this work.