Summary (149 words of referenced text): 46The climate impact of aerosols is highly uncertain owing primarily to their poorly quantified 47 influence on cloud properties. During 2014-15, a fissure eruption in Holuhraun (Iceland) 48 emitted huge quantities of sulphur dioxide, resulting in significant reductions in liquid cloud 49 droplet size. Using satellite observations and detailed modelling, we estimate a global mean 50 radiative forcing from the resulting aerosol-induced cloud brightening for the time of the 51 eruption of around -0.2 W.m -2 . Changes in cloud amount or liquid water path are 52 undetectable, indicating that these aerosol-cloud indirect effects are modest. It supports the 53 idea that cloud systems are well buffered against aerosol changes as only impacts on cloud 54 effective radius appear relevant from a climate perspective, thus providing a strong constraint 55 on aerosol-cloud interactions. This result will reduce uncertainties in future climate 56 projections as we are able to reject the results from climate models with an excessive liquid 57 water path response. 58 59Main Text: (3103 words of referenced text, including concluding paragraph) 60 The 2014-15 eruption at Holuhraun (486 words of referenced text): 61Anthropogenic emissions that affect climate are not just confined to greenhouse gases. 62Sulphur dioxide and other pollutants form atmospheric aerosols that can scatter and absorb 63 sunlight and can influence the properties of clouds, modulating the Earth-atmosphere energy 64 balance. Aerosols act as cloud condensation nuclei (CCN); an increase in CCN translates into 65 a higher number of smaller, more reflective cloud droplets that scatter more sunlight back to 66 space 1 (the ÔfirstÕ indirect effect of aerosols). Smaller cloud droplets decrease the efficiency 67 of collision-coalescence processes that are pivotal in rain initiation, thus aerosol-influenced 68 clouds may retain more liquid water and extend coverage/lifetime 2,3 (the ÔsecondÕ or Ôcloud 69 lifetimeÕ indirect effect). Aerosols usually co-vary with key environmental variables making 70 it difficult to disentangle aerosol-cloud impacts from meteorological variability [4][5][6] . 71Additionally, clouds themselves are complex transient systems subject to dynamical 72 feedbacks (e.g. cloud top entrainment/evaporation, invigoration of convection) which 73 influence cloud response [7][8][9][10][11][12] . These aspects present great challenges in evaluating and 74 constraining aerosol-cloud interactions (ACI) in General Circulation Models (GCM) 13-17 , 75 with particular contentious debate surrounding the relative importance of these feedback 76 mechanisms. 77Nonetheless, anthropogenic aerosol emissions are thought to cool the Earth via indirect 78 effects 17 , but the uncertainty ranges from -1.2 to -0.0 W.m -2 (90% confidence interval) due to 79 i) a lack of characterization of the pre-industrial aerosol state 15,18,19 , and ii) model parametric 80 and structural errors in representing cloud responses to aerosol chan...

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The 2014-2015 Holuhraun eruption, on the Bárðarbunga volcanic system in central Iceland, was one of the best-monitored basaltic fissure eruptions that has ever occurred, and presents a unique opportunity to link petrological and geochemical data with geophysical observations during a major rifting episode. We present major and trace element analyses of melt inclusions and matrix glasses from a suite of ten samples collected over the course of the Holuhraun eruption. The diversity of trace element ratios such as La/Yb in Holuhraun melt inclusions reveals that the magma evolved via concurrent mixing and crystallization of diverse primary melts in the mid-crust. Using olivine-plagioclase-augite-melt (OPAM) barometry, we calculate that the Holuhraun carrier melt equilibrated at 2.1 ± 0.7 kbar (7.5 ± 2.5 km), which is in agreement with the depths of earthquakes (6 ± 1 km) between Bárðarbunga central volcano and the eruption site in the days preceding eruption onset. Using the same approach, melt inclusions equilibrated at pressures between 0.5 and 8.0 kbar, with the most probable pressure being 3.2 kbar. Diffusion chronometry reveals minimum residence timescales of 1-12 days for melt inclusionbearing macrocrysts in the Holuhraun carrier melt. By combining timescales of diffusive dehydration of melt inclusions with the calculated pressure of H 2 O saturation for the Holuhraun magma, we calculate indicative magma ascent rates of 0.12-0.29 m s −1 . Our petrological and geochemical data are consistent with lateral magma transport from Bárðarbunga volcano to the eruption site in a shallow-to mid-crustal dyke, as has been suggested on the basis of seismic and geodetic datasets. This result is a significant step forward in reconciling petrological and geophysical interpretations of magma transport during volcano-tectonic episodes, and provides a critical framework for the interpretation of premonitory seismic and geodetic data in volcanically active regions.

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