ARTICLE DOI: 10.1038/s41467-017-02082-2 OPEN Increased ionization supports growth of aerosols into cloud condensation nuclei H. Svensmark 1, M.B. Enghoff 1, N.J. Shaviv2 & J. Svensmark 1,3 Ions produced by cosmic rays have been thought to influence aerosols and clouds. In this study, the effect of ionization on the growth of aerosols into cloud condensation nuclei is investigated theoretically and experimentally. We show that the mass-flux of small ions can 1234567890 constitute an important addition to the growth caused by condensation of neutral mole- cules. Under atmospheric conditions the growth from ions can constitute several percent of the neutral growth. We performed experimental studies which quantify the effect of ions on the growth of aerosols between nucleation and sizes >20 nm and find good agreement with theory. Ion-induced condensation should be of importance not just in Earth’s present day atmosphere for the growth of aerosols into cloud condensation nuclei under pristine marine conditions, but also under elevated atmospheric ionization caused by increased supernova activity. 1 National Space Institute, Technical University of Denmark, Elektrovej, Building 328, 2800 Lyngby, Denmark. 2 Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem 91904, Israel. 3 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark. Correspondence and requests for materials should be addressed to H.S. (email: [email protected]) NATURE COMMUNICATIONS | 8: 2199 | DOI: 10.1038/s41467-017-02082-2 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-02082-2 louds are a fundamental part of the terrestrial energy typically before the aerosols reach ~4 nm23. Changing the ioni- Cbudget, and any process that can cause systematic changes zation is therefore not expected to have an influence on the in cloud micro-physics is of general interest. To form a number of CCN through Coulomb interactions between aerosols. cloud droplet, water vapor needs to condense to aerosols acting as However, this argument disregards that the frequency of cloud condensation nuclei (CCN) of sizes of at least 50–100 nm1, interactions between ions and aerosols is a function of the ion and changes in the number of CCN will influence the cloud density, and that each time an ion condenses onto an aerosol, a 2, 3 microphysics . One process that has been pursued is driven by small mass (mion) is added to the aerosol. As a result, a change in ionization caused by cosmic rays, which has been suggested to be ion density has a small but important effect on the aerosol growth of importance by influencing the density of CCN in the atmo- rate, since the mass flux from the ions to the aerosols increases – sphere and thereby Earth’s cloud cover4 7. Support for this idea with the ion density. This mass flux is normally neglected when came from experiments, which demonstrated that ions sig- compared to the mass flux of neutral molecules (for example nificantly amplify the nucleation rate of small aerosols (≈1.7 nm) sulfuric acid, SA) to the aerosols by condensation growth, as can 8, 9. However, to affect cloud properties, any change in small be seen from the following simple estimate: the typical ion aerosols needs to propagate to CCN sizes 50–100 nm, but such concentration in the atmosphere is on the order of ≈103 ions cm − changes were subsequently found by numerical modeling to be 3, however, the condensing vapor concentration (SA) is typically − too small to affect clouds3, 10, 11. The proposed explanation for on the order of ≈106 molecules cm 3. The ratio between them is − this deficit is that additional aerosols reduce the concentration of 10 3, from which one might conclude that the effect of ions on the gases from which the particles grow, and a slower growth the aerosol growth is negligible. Why this is not always the case increases the probability of smaller aerosols being lost to pre- will now be shown. existing aerosols. This has lead to the conclusion that no sig- The mass flux to neutral aerosols consists not only of the nificant link between cosmic rays and clouds exists in Earth’s condensation of neutral molecules, but also of two terms which atmosphere. add mass due to recombination of a positive (negative) ion and a This conclusion stands in stark contrast to a recent experiment negative (positive) aerosol. Furthermore, as an ion charges a demonstrating that when excess ions are present in the experi- neutral aerosol, the ion adds mion to its mass. Explicitly, taking mental volume, all extra nucleated aerosols can grow to CCN the above mentioned flux of ion mass into account, the growth of sizes12. But without excess ions in the experimental volume, any aerosols by condensation of a neutral gas and singly charged ions extra small aerosols (3 nm) are lost before reaching CCN sizes, in becomes, accordance with the above mentioned model results. The con- P jecture was that an unknown mechanism is operating, whereby ∂Niðr;tÞ ∂ ¼À ; ð ; Þ jð ; Þ; ions facilitate the growth and formation of CCN. Additional ∂t ∂r Ii j r t N r t evidence comes from atmospheric observations of sudden 0 j 1 À À Àþ þ þ þÀ decreases in cosmic rays during solar eruptions in which a sub- A0n0β00 A n β A n β ð1Þ B C 6, 7 þ þ þ0 0 0 0þ sequent response is observed in aerosols and clouds . Again, Ii;jðr; tÞ¼@ A n β A n β 0 A; this is in agreement with a mechanism by which a change in À ÀβÀ0 0 0β0À ionization translates into a change in CCN number density. A n 0 A n However, the nature of this micro-physical link has been elusive. In this work we demonstrate, theoretically and experimentally, = − the presence of an ion mechanism, relevant under atmospheric with i and j (0, +, ) referring to neutral, positively, and negatively charged particles. Here r and t are the radius of the conditions, where variations in the ion density enhance the i = 0 + − growth rate from condensation nuclei (≈1.7 nm) to CCN. It is aerosol and the time. N (N , N , N ) is the number density of neutral, positive, and negative aerosols. n0 is the concentration of found that an increase in ionization results in a faster aerosol + − growth, which lowers the probability for the growing aerosol to be condensible gas, n , n are the concentration of positive and negative ions, while Ai = (mi/4πr2ρ), with mi being the mass of the lost to existing particles, and more aerosols can survive to CCN = sizes. It is argued that the mechanism is significant under present neutral gas molecule (i 0), and the average mass of positive/ negative ions, i = (+, −), ρ is the mass density of condensed gas, atmospheric conditions and even more so during prehistoric β fi elevated ionization caused by a nearby supernova. The mechan- and is the interaction coef cient between the molecules (or ions) and neutral and/or charged aerosols (See Methods for ism could therefore be a natural explanation for the observed fi correlations between past climate variations and cosmic rays, details on derivation of the equations, the interaction coef cients, 13–17 details of the experiment, and the (mion/m0) of 2.25). modulated by either solar activity or caused by supernova β00 β+0 β−0 fi activity in the solar neighborhood on very long time scales where , , and correspond to the interaction coef cients – the mechanism will be of profound importance18 20. describing the interaction between neutral aerosols of radius r and neutral molecules, positive ions and negative ions respec- − tively, whereas β0+, and β0 are the interaction coefficients between neutral molecules and positively/negatively charged Results β+− Theoretical model and predictions. Cosmic rays are the main aerosols. Finally corresponds to the recombination between 21 a positive ion and a negative aerosol of radius r, and vice versa for producers of ions in Earth’s lower atmosphere . These ions − β +24. If no ions are present, the above equations simplify to the interact with the existing aerosols, and charge a fraction of them. 25 However, this fraction of charged aerosols is independent of the well known condensation equation , where ionization rate in steady state—even though the electrostatic interactions enhance the interactions among the charged aerosols dr 0 0 00 I ; ðr; tÞ¼ ¼ A n β ; ð2Þ and between these aerosols and neutral molecules, the increased 0 0 dt recombination ensures that the equilibrium aerosol charged fraction remains the same22. Ion-induced nucleation will cause the small nucleated aerosols to be more frequently charged is the growth rate of the aerosol radius due to the condensation of relative to an equilibrium charge distribution, but ion recombi- molecules onto the aerosols. It is the change in growth rate caused nation will move the distribution towards charge equlibrium, by ions that is of interest here. 2 NATURE COMMUNICATIONS | 8: 2199 | DOI: 10.1038/s41467-017-02082-2 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-02082-2 ARTICLE By assuming a steady state for the interactions between ions tot 25 N 22 / and aerosols, we find 0 20 a N þ þ þ À À À 15 β 0 β 0 0,0 N n N n 10 ¼ ; ¼ ; ð3Þ / 0 ÀβÀþ 0 þβþÀ 5 N n N n +–,0 0 4 1 10 100 − which using Ntot = N0 + N+ + N gives 1000 À 0 þ þ0 À À0 1 ð ; Þ β β b 50.0 20.0 N r t n n ð Þ 30.0 ¼ 1 þ Àþ þ þÀ : 4 Ntotðr; tÞ nÀβ nþβ 10.0 10,000 s) 30.0 ) 3 100 3 5.0 Equations (3) and (4) can be inserted into the components of Eq.