Alexander Dennis James

Ferric chloride (FeCl₃) in sulfuric acid cloud droplets has been proposed to explain the inhomogeneous near-ultraviolet (UV) absorption visible at the Venusian cloud tops. However, the absorption spectrum of FeCl₃ in concentrated sulfuric acid does not appear to have been measured previously; here we report measurements under appropriate conditions of temperature and H₂SO₄/H₂O solution strengths. The choice of solvent has a significant effect on the measured spectrum. The reaction of FeCl₃ in aqueous H₂SO₄ to form ferric sulfate (Fe₂(SO₄)₃) was shown to be suppressed by adding HCl to the solution (as would occur in the Venusian atmosphere). The FeCl₃ spectrum in sulfuric acid is shown to be in good agreement with observations of the unknown absorber in Venus’ atmosphere. The presence of Fe₂(SO₄)₃, which absorbs strongly below 320 nm, should be considered when reconstructing Venusian spectra to avoid misattribution of absorption in this spectral region to SO₂, potentially leading to an overestimation of the SO₂ cloud top concentrations.

Understanding the composition and distribution of the unknown UV absorber (Figure 1) in the Venusian atmosphere has been an open question in planetary science for close to 100 years. Ferric chloride (FeCl3) has been proposed as the cause of the absorption [1], but the absorption spectrum of FeCl3 generally used in the literature was measured in ethyl acetate [2], which is not present on Venus and produces an absorption spectrum with little similarity to the Venusian absorber [3].Figure 1: A false colour image of Venus. Regions of UV absorption appear orange. The cause of this absorption is unknown. Image credit: NASA/JPL-Caltech. We have measured the absorption spectrum of FeCl3 in sulphuric acid with small quantities of HCl added, and we found that it is much more similar in shape to the observed spectrum of the unknown absorber than prior FeCl3 spectra available in the literature (Figure 2). Figure 2: Comparison of the FeCl3 spectra in ethyl acetate [2] and sulphuric acid (this work) with a spectrum of the unknown UV absorber recorded by MASCS/MESSENGER [3].When added to sulphuric acid, a mixture of ferric chloride and ferric sulphate ions are observed. We estimated the molar partitioning of the species in the mixtures by performing least squares fitting to reproduce each measured spectrum from spectra of pure ferric sulphate (measured for Fe2(SO4)3 in 75-78 wt% aqueous H2SO4) and pure ferric chloride (measured for FeCl3 in 5-37 wt% aqueous HCl).The fraction of the observed absorption that can be reproduced by FeCl3 was investigated using three models: the global Planetary Climate Model for Venus (PCM-Venus) to model the photochemistry and 3D transport of the candidates in the atmosphere [4], a 1D sectional aerosol model to predict agglomeration and sedimentation as a transport mechanism of FeCl3 particles [5], and the 1D multiple scattering radiative transfer model SOCRATES [6]. The required concentrations of FeCl3 in the different cloud modes to reproduce observations taken by MESSENGER/MASCS during its June 2007 Venus flyby [3] were estimated for different cloud modes using SOCRATES. The full absorption can be explained by approximately 1.5 - 2 wt% FeCl3 in the mode 1 cloud droplets (Figure 3). Some contribution from ferric sulphate ions is also expected, but absorbs in the same wavelength region as SO2, so its precise contribution could not be reliable constrained.Figure 3: Comparison of MASCS/MESSENGER measured reflectance (crosses) [3] with the modelled spectrum with SO2 only (blue line), and with 1.2 – 2.0 wt% FeCl3 in mode 1 cloud droplets (orange and red lines).Iron chemistry was added into PCM-Venus in order to predict the abundance of gas-phase FeCl3 produced by the reaction of gas-phase HCl with iron produced by the ablation of cosmic dust particles around 115 km. Mean gas and dynamical profiles from the PCM were used to initialise the agglomeration and sedimentation model, which was then run for many Venus years to reach steady state. Agglomeration and sedimentation modelling of FeCl3 suggests that the PCM-modelled FeCl3 column abundance above 60 km can account for more than 40% of the observed absorption. The flux of meteoric iron into the Venusian atmosphere has not been measured, and may be significantly higher than assumed in this work [7]. References [1] Zasova et al. 1981, https://doi.org/10.1016/0273-1177(81)90213-1 [2] Aoshima et al. 2013, http://doi.org/10.1039/C3PY00352C[3] Pérez-Hoyos et al. 2018, https://doi.org/10.1002/2017JE005406 [4] Martinez et al. 2024, doi.org/10.1016/j.icarus.2024.116035 [5] Frankland et al. 2017, https://doi.org/10.1016/j.icarus.2017.06.005 [6] Manners et al. 2022, SOCRATES Technical Guide, available at: https://code.metoffice.gov.uk/trac/socrates [7] Carrillo-Sánchez et al. 2020, https://doi.org/10.1016/j.icarus.2019.113395

The upper haze region of Venus’ atmosphere (~70-90 km) has been shown to experience cold pockets that may give rise to ice clouds that form from the freezing of sulphuric acid droplets (Turco et al., 1983) On Earth cirrus cloud often form through the hygroscopic growth of aquoue aerosol, but temperatures in Venus’ upper haze region regularly drop well below those of the coldest cirrus clouds on earth (~185 K). However, freezing of sulphuric acid droplets under conditions relevant for Venus’ upper haze region have not been experimentally investigated. Here, we describe our work using new laboratory freezing measurements in combination with the Solar Occultation in the InfraRed SOIR retrievals to better understand the atmospheric conditions around 80 km and determine if these conditions are suitable for the crystallisation of water and CO2 ice.First, we experimentally explored the homogeneous nucleation of H2O ice in sulphuric acid solutions using a liquid nitrogen-cooled cryo-microscope setup, where droplet emulsions (oil/surfactant mix and H2SO4 solution with droplets of around 10 to 100 µm) are created using a microfluidic device. With this setup, we were able to extend the results from previous studies to lower temperatures and higher sulphuric acid concentrations (Bertram et al., 1996; Koop et al., 1998). We observed crystallisation down to 154 K, but this crystallisation was increasingly restricted by slow crystal nucleation and growth rates at lower temperatures. Crystallisation was not observed below 154 K, consistent with the formation of ultra-viscous or glassy solutions. To further explore the possibility of water ice cloud formation on Venus, we also examined the retrievals of temperature, water vapour mixing ratio and pressure from the Solar Occultation in the InfraRed (SOIR) instrument onboard the Venus Express orbiter (Mahieux et al., 2023). Using this data, we were able to calculate the expected sulphuric acid concentrations using the parameterisation from Tabazadeh et al. (1997), indicating that, in about 30% of the SOIR profiles, H2O ice nucleates homogeneously in liquid aqueous H2SO4 droplets or heterogeneously on glassy aqueous H2SO4 droplets. Using an average particle number density of 0.5 cm-3, which was previously reported by Luginin et al. (2018), we would expect an average size of ~1 µm ice crystals to form in the upper haze layer of Venus. Around 36% of the SOIR profiles reveal that these altitudes experience temperature extremes which low enough (< 140 K) that the atmosphere is stable with respect to crystalline CO2 particles. A 1D model was developed to investigate the influence of gravity waves, the formation of crystalline CO2 and the impact on the upper haze region of the atmosphere. Our 1D model showed that under these conditions, crystals will grow rapidly in the cold phase of a wave to sizes large enough for precipitation downwards to the underlying warm phase where the CO2 sublimates. Therefore, the formation of crystalline CO2 particles creates a mechanism for the redistribution of sulphuric acid particles and water to lower levels in the atmosphere.Overall, we conclude that cirrus-like water ice clouds likely form and persist in large parts of the upper haze layer on Venus. Due to the variable nature of conditions in this region of the Venusian atmosphere, temperatures will sometimes fall well below 140 K, resulting in the rapid precipitation of CO2 particles. This will likely create a mechanism for the downward transfer of sulphuric acid, water and other materials to the warmer regions of the atmosphere.Figure 1. The hypothesised mechanism of how temperature variations impact cloud formation and the redistribution of atmospheric constituents. We propose that persistent water ice clouds may be a common feature of the upper haze layer, and these ice crystals may provide the surface on which CO2 ice nucleates in temperature minima driven by gravity waves. The rapid growth and sedimentation of cubic CO2 ice particles would then redistribute H2SO4 particles downwards to the warmer regions of the Venusian atmosphere. References:Bertram, A. K., Patterson, D. D., & Sloan, J. J. (1996). Mechanisms and temperatures for the freezing of sulfuric acid aerosols measured by FTIR extinction spectroscopy. The Journal of Physical Chemistry, 100(6), 2376–2383. https://doi.org/10.1021/jp952551vKoop, T., Ng, H. P., Molina, L. T., & Molina, M. J. (1998). A new optical technique to study aerosol phase transitions: The nucleation of ice from H2SO4 aerosols. The Journal of Physical Chemistry A, 102(45), 8924–8931. https://doi.org/10.1021/jp9828078Luginin, M., Fedorova, A., Belyaev, D., Montmessin, F., Korablev, O., & Bertaux, J.-L. (2018). Scale heights and detached haze layers in the mesosphere of Venus from SPICAV IR data. Icarus, 311, 87–104. https://doi.org/10.1016/j.icarus.2018.03.018Mahieux, A., Robert, S., Piccialli, A., Trompet, L., & Vandaele, A. C. (2023). The SOIR/Venus Express species concentration and temperature database: CO2, CO, H2O, HDO, H35Cl, H37Cl, HF individual and mean profiles. Icarus, 405, 115713. https://doi.org/10.1016/j.icarus.2023.115713Tabazadeh, A., Toon, O. B., Clegg, S. L., & Hamill, P. (1997). A new parameterization of H2SO4/H2O aerosol composition: Atmospheric implications. Geophysical Research Letters, 24(15), 1931–1934. https://doi.org/10.1029/97GL01879Turco, R. P., Toon, O. B., Whitten, R. C., & Keesee, R. G. (1983). Venus: Mesospheric hazes of ice, dust, and acid aerosols. Icarus, 53(1), 18–25. https://doi.org/10.1016/0019-1035(83)90017-9

The increasing quantities of anthropogenic objects in Low Earth Orbit (LEO) have led to concerns over space debris and collision risks in LEO, leading to the Federal Communications Commission’s introduction of the “5-year rule”, requiring deorbit of LEO spacecraft within 5 years of mission end. To mitigate the risk of debris impacting the surface and causing damage, spacecraft are increasingly designed to ablate in the atmosphere, with most of the mass being vapourised during re-entry [1]. This causes an influx of metals into the mesosphere, where they condense and settle into the winter polar stratosphere - around 10% of Junge layer sulphuric acid droplets have been measured to contain metals from ablated space debris. Some metals – Al, Li, Cu, Ni, Mn etc. – already exceed natural background levels from cosmic dust that has ablated in the mesopause region [2]. The effect of these metals on the stratosphere is not yet known, and space debris input has been projected to increase by more than an order of magnitude in the next 15 years [3]. It is therefore vitally important to determine the level of re-entering space debris that will cause significant changes to atmospheric aerosols and stratospheric chemistry, in particular to the ozone layer. We model the catalytic impact of ablated aluminium which recondenses in the atmosphere has “space debris particles” (SDPs), predicted to be composed mainly of aluminium hydroxide nano-particles. These particles are predicted to catalyse chlorine activation from gas phase HCl, ClONO2, and HOCl by catalytic reactions on the SDP surface. Their impact on polar stratospheric cloud freezing and subsequent chlorine activation is also considered.We present results of a modelling study using a sectional aerosol model within an Earth system model (Whole Atmosphere Community Climate Model with the Community Aerosol and Radiation Model for Atmospheres, WACCM-CARMA). We simulate the transport of SDPs and meteoric smoke particles (MSPs) produced by condensation of Fe and Mg silicates from ablated cosmic dust. The particles grow by coagulation and deposition of sulphuric acid through 28 size bins (0.34 nm to 1.6 µm radius, where MSPs injected at 0.34 nm radius, SDPs at 10 nm radius, and sulphuric acid is allowed to condense both heterogeneous on the MSPS and SDPs, and homogeneously. The SDPs and MSPs are initially injected in concentrations consistent with current models and observations (7.9 t d‑1 MSPs and 0.96 t d-1 SDPs) to assess the transport and lifetimes of the particles in the atmosphere.The effect of increasing the mass of SDPs in line with future increases in space travel is also simulated. The maximum possible impact of SDPs on stratospheric chemistry is then estimated from the available SDP surface area and assuming upper limits for unmeasured physico-chemical parameters. Condensation of sulphuric acid onto the particles during their descent through the stratosphere reduces the surface area available for catalytic chlorine activation, but this is highly sensitive to the shape parameter assumed for the SDPs and the fraction of the surface that is coated by the condensed sulphate.The reaction rates and physical properties of SDPs adopted in this model have not been measured or observed. Taking reasonable upper limits for values indicates that the reactions have the potential to do significant damage to the stratospheric ozone layer due to increased chlorine activation in winter and spring. The precise morphology and composition of particles must be characterised by in situ sampling of the stratosphere, and laboratory measurements of the rate constants are also required to better constrain estimates.References[1] Kelley 2012, https://ntrs.nasa.gov/citations/20120002794[2] Murphy et al. 2023, https://doi.org/10.1073/pnas.2313374120.[3] Schulz & Glassmeier 2021, https://doi.org/10.1016/j.asr.2020.10.03

It has been proposed that two isomers of the SO dimer ( cis ‐ and trans ‐OSSO) are candidates for the unknown UV absorber in Venus' atmosphere because they have a good spectral match with the absorber, despite the low concentrations predicted by 1D photochemical models. Here OSSO chemistry (production from SO and loss by photolysis, thermal decomposition, and reaction with O and Cl) has been included in the photochemistry scheme of a 3D planetary climate model (PCM‐Venus) along with sulfur injection due to meteoric ablation. 1D multiple scattering radiative transfer modeling is then used to predict the resulting top‐of‐the‐atmosphere reflectance produced by OSSO. The modeled OSSO concentrations are shown to be ∼3 orders of magnitude too low to explain the observed absorbance levels, and the predicted ratio of the OSSO isomers provides an unsatisfactory match to the spectral shape of the unknown absorber.

The cause of the inhomogeneous near-ultraviolet absorption observed in the upper clouds of Venus remains a key question in Venusian research. One possible candidate in the literature is ferric chloride. The absorption spectrum of ferric chloride currently in use by models uses ethyl acetate as a solvent and does not reproduce the absorption features observed on Venus. The study of the optical properties and chemistry of ferric chloride in the sulphuric acid cloud droplets is required to draw valid conclusions regarding its suitability as a candidate for the near-UV absorption.In this study, we measure the absorption spectrum of ferric chloride in sulphuric acid from 200 – 600 nm at a range of temperatures and measure the rate of conversion of the ferric chloride ions into ferric sulphate ions. We then use the resulting ferric chloride absorption coefficients in a 1D radiative transfer model and estimate the required concentration of ferric chloride in the clouds to be 0.6 – 0.9 wt% in the mode 1 (~0.3 µm radius) cloud droplets to match observations. We also predict the atmospheric concentrations of ferric chloride formed from the reaction of iron ablating from cosmic dust entering Venus’ atmosphere around 120 km with hydrogen chloride emitted by volcanic activity, and estimate the accumulation timescale of ferric chloride to produce the required concentrations in the clouds.

It has been proposed that two isomers of the SO dimer (cis- and trans-OSSO) are candidates for the unknown UV absorber in Venus’ atmosphere because they have a good spectral match with the absorber, despite the low concentrations predicted by 1D photochemical models. Here OSSO chemistry (production from SO and loss by photolysis, thermal decomposition, and reaction with O and Cl) has been included in the photochemistry scheme of a 3D planetary climate model (PCM-Venus) along with sulphur injection due to meteoric ablation. 1D multiple scattering radiative transfer modelling is then used to predict the resulting top-of-the-atmosphere reflectance produced by OSSO. The modeled OSSO concentrations are shown to be ~3 orders of magnitude too low to explain the observed absorbance levels, and the predicted ratio of the OSSO isomers provides an unsatisfactory match to the spectral shape of the unknown absorber.

Understanding the composition and distribution of the unknown UV absorber (Figure 1) in the Venusian atmosphere has been an open question in planetary science for close to 100 years. Many candidates for the absorber have been proposed over the years. We focus on two possibilities: ferric chloride (FeCl3)[1] and the cis- and trans- forms of the SO dimer (OSSO)[2]. Figure 1: A false colour image of Venus. Regions of UV absorption appear orange. The cause of this absorption is unknown. Image credit: NASA/JPL-Caltech. FeCl3 has been proposed to exist within the sulphuric acid cloud droplets, but the absorption spectrum of FeCl3 generally used in the literature was measured in ethyl acetate, which is not present on Venus and produces an absorption spectrum with little similarity to the Venusian absorber[3]. We have measured the absorption spectrum of FeCl3 in sulphuric acid with small quantities of HCl added, and we found that it is much more similar in shape to the observed spectrum of the unknown absorber than prior FeCl3 spectra available in the literature. The spectra measured in sulphuric acid contain contributions from both ferric chloride and ferric sulphate ions. We estimated the molar partitioning of the species in the mixtures by performing least squared fitting to reproduce each measured spectrum from spectra of pure ferric sulphate (measured for Fe2(SO4)3 in 75-87 wt% aqueous H2SO4) and pure ferric chloride (measured for FeCl3 in 5-37 wt% aqueous HCl). We estimated the rate of the reaction converting FeCl3 toward Fe2(SO4)3 using this method at several temperatures and extrapolate to Venusian temperatures to predict the lifetime of FeCl3 within the sulphuric acid cloud droplets. To test if either FeCl3 or OSSO are viable candidates we employ three models: the global Planetary Climate Model for Venus (PCM-Venus) to model the photochemistry and 3D transport of the candidates in the atmosphere[4], a 1D sectional aerosol model to predict agglomeration and sedimentation as a transport mechanism of FeCl3 particles[5], and the 1D multiple scattering radiative transfer model SOCRATES[6]. We have added iron chemistry and updated sulphur chemistry into PCM-Venus in order to predict the abundance of gas-phase FeCl3 produced by the reaction of gas-phase HCl with iron produced by the ablation of cosmic dust particles around 115 km, and of OSSO produced from the recombination of two SO molecules. Mean gas and dynamical profiles from the PCM are used to initialise the agglomeration and sedimentation model, which is then run for many Venus years to reach steady state. The potential contributions of each species to the observed absorption were assessed using SOCRATES to predict the observed absorption from instantaneous dayside PCM-modelled concentrations of OSSO, and to estimate the required concentrations of FeCl3 in the different cloud modes and the required increase of OSSO from the PCM results to fully explain the absorption measured by MESSENGER/MASCS during its June 2007 Venus flyby (Pérez-Hoyos et al., 2018). Agglomeration and sedimentation modelling of FeCl3 suggests that the PCM-modelled FeCl3 column abundance above 60 km can account for more than 40% of the observed absorption. The full absorption can be explained by approximately 1 wt% FeCl3 in the mode 1 cloud droplets (Figure 2). Large ferric sulphate ion concentrations in the laboratory spectra used to model the absorption lead to higher than observed absorption near 300 nm. Work is ongoing to correct the iron partitioning between chloride and sulphate to expected Venusian ratios. Results of SOCRATES modelling suggest that an OSSO concentration profile 103 x larger than the PCM-modelled profile would be required to reproduce the depth of the observed absorption (Figure 2). The SO2 and SO profiles at 60 - 80km in the PCM are currently ~100 x smaller than observed concentrations, and so increase of 103 from current concentrations is not unreasonable (as OSSO rate of formation will increase with the square of SO concentration). However, the agreement of the spectral shape with the observations is significantly worse than has been previously reported by Frandsen et al. (2016)[2]. The spectral shape is better approximated when the concentration of trans-OSSO is increased relative to the cis-OSSO, though we have found no chemical justification to make this adjustment. Figure 2: Preliminary data comparing the MESSENGER/MASCS spectrum of the unknown absorber (*[3], grey crosses) to SOCRATES-modelled concentrations that best reproduce the depth of the absorption for cis- and trans-OSSO (purple dashed line) and FeCl3 in mode 1 (modal radius = 0.2 µm) sulphuric acid cloud droplets (orange and red dash-dotted lines). The modelled spectrum with neither OSSO or FeCl3 included (blue solid line) is included for reference.We conclude that both candidates show some merit as the unknown absorber and a mixture of both species is likely present on Venus and contributing to the absorption. Work is ongoing to decrease the ferric sulphate contribution in the laboratory-measured FeCl3 spectrum in line with expected Venusian concentrations (and thereby decrease the high absorption near 300 nm seen in current FeCl3 spectra shown in Figure 2), and to reproduce observed SO2 and SO concentrations with PCM-Venus, thereby likely increasing OSSO modelled concentrations. References [1] Zasova et al., 1981, https://doi.org/10.1016/0273-1177(81)90213-1 [2] Frandsen et al. 2016, https://doi.org/10.1002/2016GL070916 [3] Pérez-Hoyos et al. 2018, https://doi.org/10.1002/2017JE005406 [4] Martinez et al. (2024), doi.org/10.1016/j.icarus.2024.116035 [5] Frankland et al., 2017, https://doi.org/10.1016/j.icarus.2017.06.005 [6] Manners et al. (2022), SOCRATES Technical Guide, available at: https://code.metoffice.gov.uk/trac/socrates

Satellite- and aircraft-based mid-infrared measurements of polar stratospheric clouds (PSCs) have provided spectroscopic evidence for the presence of β-NAT (nitric acid trihydrate) particles. Metastable nitric acid hydrate phases such as α-NAT and α-NAD (nitric acid dihydrate) have been frequently observed in laboratory experiments but not yet detected as a constituent of PSCs in atmospheric measurements. As for the β-NAD modification, its formation was first observed in X-ray diffraction measurements when the low-temperature α-NAD phase was warmed to a temperature above 210 K. Its infrared spectrum has been reported, but so far no optical constants have been derived that could be used as input for infrared retrievals of PSC composition. In this work, we show that β-NAD particles were efficiently formed in isothermal heterogeneous crystallisation experiments at 190 K from supercooled HNO₃/H₂O solution droplets containing an embedded mineral dust or meteoric smoke particle analogue. An inversion algorithm based on a T-matrix optical model was used to derive for the first time the mid-infrared complex refractive indices of the β-NAD modification from the measured extinction spectrum of the particles. In contrast to the heterogeneous crystallisation experiments, the α-NAD phase was formed when the HNO₃/H₂O solution droplets did not contain a solid nucleus and crystallised homogeneously. Using a light-scattering detector that recorded two-dimensional scattering patterns of the crystallised NAD particles, we were able to determine predominant shapes of the α- and β-NAD crystals. We found that α-NAD grew into elongated, needle-shaped crystals, while β-NAD particles were compact in shape. This agrees with previously reported images of α- and β-NAD particles grown on the cryo-stage of an environmental scanning electron microscope. While direct evidence for the existence of metastable NAD in the polar stratosphere is still lacking, our experiments add to the wealth of previous laboratory studies that have identified various conditions for the rapid growth of metastable compositions. In the atmosphere, these could be intermediate states that transform into thermodynamically stable NAT on longer timescales in aged PSCs.

The crystal formation of nitric acid trihydrate (NAT) in the absence of water ice is important for a subset of polar stratospheric clouds (PSCs) and thereby ozone depletion. It has been suggested that either fragmented meteoroids or meteoric smoke particles (MSPs), or possibly both, are important as heterogeneous nuclei of these crystals. Previous work has focused on the nucleating ability of meteoric material in nitric acid in the absence of sulfuric acid. However, it is known that when immersed in stratospheric sulfuric acid droplets, metal-containing meteoric material particles partially dissolve and components can reprecipitate as silica and alumina that have different morphologies to the original meteoric material. Hence, in this study, we experimentally and theoretically explore the relative role that sulfuric acid-processed MSPs and meteoric fragments may play in NAT nucleation in PSCs. We compared meteoric fragments that had recently been prepared (by milling a meteorite sample) to a sample annealed under conditions designed to simulate heating during entry into the Earth's atmosphere. Whilst the addition of sulfuric acid decreased the nucleating ability of the recently milled meteoric material relative to nucleation in binary nitric acid-water solutions (at similar NAT saturation ratio), the annealed meteoric fragments nucleated NAT with a similar effectiveness in both solutions. However, combining our results with measured fluxes of meteoric material to the Earth, sedimentation modelling and recent experiments on fragmentation of incoming meteoroids suggests that it is unlikely for there to be sufficient fragments to contribute to the nucleation of crystalline NAT particles. We then considered silica formed from sulfuric acid-processed MSPs. Our previous work showed that nanoparticulate silica (radius ∼6 nm) is a relatively poor promoter of nucleation compared with micron-scaled silica particles, which were more effective. Both materials have similar chemical and structural (crystallographically amorphous) properties, indicating that size is critical. Here, we account for surface curvature of primary grains using the Classical Nucleation Theory (CNT) to explore this size dependence. This model is able to explain the discrepancy in nucleation effectiveness of fumed silica and fused quartz by treating their nucleating activity (contact angle) as equal but with differing particle size (or surface curvature), assuming interfacial energies that are physically reasonable. Here, we use this CNT model to present evidence that nucleation of NAT on acid-processed MSPs, where the primary grain size is tens of nanometres, is also effective enough to contribute to NAT crystals in early season PSCs where there is an absence of ice. This study demonstrates that the modelling of crystal nucleation in PSCs and resulting ozone depletion relies on an accurate understanding of the transport and chemical processing of MSPs. This will affect estimated sensitivity of stratospheric chemistry to rare events such as large volcanic eruptions and long-term forecasting of ozone recovery in a changing climate.