Albedo changes of Neptune related to the 11-year solar cycle have been reported since the 1970s (Lockwood and Thompson, Nature 280, 1979). For nearly two decades a clear anti-correlation between solar activity and Neptune’s brightness was observed but the relationship appeared to break down between the late 1980s (Lockwood and Thompson, Nature 349, 1991) and mid 1990s (Lockwood and Jerzykieicz, Icarus 180(2), 2006) where signs of a direct correlation instead appeared. Recent results indicate a direct correlation sustained over two solar cycles (Chavez et al, Icarus 404, 2023) prompting renewed interest in attempting an explanation. Several parameters vary with the solar cycle, one being UV light which has a much higher variation than that of visible light. This could affect the photochemistry of Neptune’s atmosphere which is rich in methane that photolyzes at wavelengths below 200 nm and can produce haze (Romani and Atreya, Icarus 74(3), 1988). Another parameter that varies is the flux of galactic cosmic rays (GCRs) which is modulated by the solar wind. GCRs can ionize molecules leading to ion-induced nucleation (Moses et al, GRL 16(12), 1989) but only for the energies of particles which are allowed entry to the planetary atmosphere by the magnetic field. Solar activity modulates GCRs of energies up to about 20 GeV so a solar variation due to GCRs is only possible if particles of those energies can enter the atmosphere. The parameter used to describe GCR entry is the cutoff rigidity (in GV). In this work we have used the magnetic field model of Neptune (Connerney et al, ASR 12(8), 1992) and a particle trajectory program (the Geomagnetic Cutoff Rigidity Computer Program by Smart and Shea, 2001, Tech. Rep. No. 20010071975) to calculate a cutoff rigidity map for Neptune for vertical GCR entry. Since the magnetic field is very tilted compared to the rotational axis (by about 45 degrees) the cutoff rigidity map has interesting features as the GCRs are guided by the magnetic field lines. Thus, lower energies and therefore a higher GCR flux more susceptible to solar cycle changes are allowed to enter the atmosphere at mid latitudes, as opposed to most planets where this happens as the poles since their magnetic field is more closely aligned with the rotational axis. Furthermore, we have used fitted GCR energy spectra in combination with the cutoff rigidity map to produce a map of solar cycle variations in GCR flux at a height of 49 km, which is at the pressure level where cosmic ray showers are initiated at Earth and also close to where Neptunian clouds are found. Where the cutoff rigidity is lowest the solar cycle variations of GCR are several tens of percents which could affect cloud formation significantly.

It is known from Earth that ionizing high-energy radiation can lead to ion-induced nucleation of cloud condensation nuclei in the atmosphere. Since the amount of high-energy radiation can vary greatly based on the radiative environment of a host star, understanding the effect of high-energy radiation on cloud particles is critical to understand exoplanet atmospheres. This study aims to explore how high-energy radiation affects the aggregation and charging of mineral cloud particles. We present experiments conducted in an atmosphere chamber on mineral SiO _2 particles with diameters of 50 nm. The particles were exposed to gamma radiation in either low-humidity (RH ≈ 20%) or high-humidity (RH > 50%) environments. The aggregation and charging state of the particles were studied with a scanning mobility particle sizer. We find that the single SiO _2 particles (N1) cluster to form larger aggregates (N2–N4), and that this aggregation is inhibited by gamma radiation. We find that gamma radiation shifts the charging of the particles to become more negative by increasing the charging state of negatively charged particles. Through an independent t -test, we find that this increase is statistically significant within a 5% significance level for all aggregates in the high-humidity environment and all except the N1 particles in the low-humidity environment. For the positively charged particles, the changes in charging state are not within the 5% significance level. We suggest that the overall effect of gamma radiation could favor the formation of a high number of small particles over a lower number of larger particles.

Abstract Observations of marine stratus clouds in clean air off the Californian coast reveal a functional relationship between the number of cloud condensation nuclei (CCN) and supersaturation. Satellite‐derived liquid droplet density estimates the number density of CCN. Combining the estimated supersaturation using Köhler theory, global maps of supersaturation and the critical activation size of CCN are estimated. Here, we show that high supersaturation >0.5% persists over the oceans with a critical CCN size of 25–30 nm, which is smaller than the conventional wisdom of 60 nm. Independent support for such high supersaturation in the marine cloud is obtained from CCN measurements provided by the “Atmospheric Tomography Mission.” Higher supersaturation implies smaller activation size for CCN making cloud formation more sensitive to changes in aerosol nucleation.

A map of vertical cutoff rigidities has been calculated for galactic cosmic ray (GCR) entry into the atmosphere of Jupiter at the 1 bar pressure radius (1 RJ = 71,492 km) using the JRM33 comprehensive model of Jupiter's magnetic field (based on 32 close flybys of Jupiter by the Juno satellite) along with a particle trajectory code (Geomagnetic Cutoff Rigidity Computer Program). The map was combined with measurements of the GCR proton flux at Earth, from the BESS-Polar ii campaign, to calculate a corresponding proton flux map at Jupiter. Additional cutoff rigidity maps were calculated for 1,000 km above the 1 bar level, and for 1.41 RJ. Furthermore, detections of heavy particles from Juno's Stellar Reference Unit were analyzed for their cutoff rigidities in multiple directions. Cutoff rigidities of 3.5-7.5 GV were found for five of the detections furthest out making them possible GCR candidates. The majority of points, located below 1.6 RJ are not likely to be GCR. Assuming instead that they are trapped particles we have calculated upper and lower limits on their equatorial pitch angles, resulting in a range from 10.1 degrees to 27.1 degrees, which can help constraining Jupiter's energetic radiation. Using data of Jupiter's magnetic field, collected by the Juno satellite, we have calculated how galactic cosmic rays (energetic particles originating from supernovae) can enter into the atmosphere of Jupiter at different altitudes. This can aid our understanding of atmospheric phenomena on Jupiter and help in planning future missions to the planet. One of Juno's instruments, the Stellar Reference Unit, has detected some peculiar signatures. Some of them probably are galactic cosmic rays that have hit the instrument. Most of them are probably not, but instead they could be particles trapped by the strong magnetic field of Jupiter. If so, they can help us understand the radiation belts of the planet. Using knowledge of the location of the detections as well as the magnetic field we calculate the so-called pitch angle of the detected particles. This angle is defined by the ratio between the motion of the particle perpendicular to the magnet field line and the motion parallel to the field line. The pitch angle is fundamental in constraining energetic radiation emanating from Jupiter's atmosphere. A galactic cosmic ray cutoff rigidity map for Jupiter was made using the JRM33 model and the Geomagnetic Cutoff Rigidity Computer Program The flux of galactic cosmic ray protons into Jupiter's atmosphere was calculated based on BESS-Polar ii data Detections of heavy ions by Juno's SRU were investigated and used to estimate their equatorial pitch angles

The critical size of aerosols is the size where they can serve as cloud condensation nuclei and thus get activated into cloud drops. This size depends on the supersaturation of water in a given air mass. The lower the critical size is, the more the cloud formation process is sensitive to changes in the formation of secondary aerosol since they experience large losses as they grow in size. Using observations of marine stratus clouds outside the coast of California, a functional relationship between supersaturation and the number concentration of cloud condensation nuclei has been found (Hudson et al., GRL 37(21), 2010 and Hudson and Noble, J. Atmos. Sci. 71(1), 2014). In this work, we use liquid drop density estimates based on satellite data of optical thickness and liquid water path from the MODIS instrument combined with the above relation to estimate a global map of supersaturation. Applying Köhler's theory to this map, a corresponding map of critical sizes has been made. We find that supersaturation is generally in the range of 0.5-1.0%, which results in a critical size in the range of 25-30 nm, which is lower than the generally used size of about 60 nm. The high supersaturations are supported by observations from the ATom measurement campaign (Brock et al, doi: 10.3334/ORNLDAAC/2111, 2022), at different supersaturations. Furthermore, the “pyrcel” cloud parcel model (Rothenberg and Want, J. Atmos. Sci. 73(3), 2016) is used to investigate the observational relationship between supersaturation and cloud droplets/cloud condensation nuclei. The results hint that cloud formation may be more sensitive to changes in secondary aerosols than previously thought.

The high-energy radiation environment of exoplanets can greatly affect their atmospheric chemistry through photo-chemical reactions and ionisation of the upper parts of the atmospheres. In order to analyse the chemistry of exoplanet atmospheres, it is therefore necessary to understand the radiative environment the planet is placed in and what effects it has on the atmosphere. The aim of this project is to study how the chemistry of exoplanet atmospheres is affected by the radiative environment. We focus on three different sources of high-energy radiation; the XUV radiation of the host star, the stellar energetic particles (SEPs) of the host star, and the galactic cosmic rays (GCRs) originating from outside the planetary system. We model the disequilibrium chemistry of a gas giant test planet, using the chemical kinetic network, STAND2020, coupled to the 1D photo-chemistry and diffusion code, ARGO. The radiative sources are introduced in the models as spectral energy distributions for the XUV radiation, and as ionisation rates for for the SEPs and GCRs. STAND2020 excels by its complexity in H/C/N/O chemistry which allows us to study the effect of the irradiation on larger complex molecules such as prebiotic molecules and haze precursors. In this talk, we present a grid of models run for host stars of the types; O,B,A,F,G,K, and M, under varying influxes of GCRs. For each of the stellar spectral types a representative spectra, combined from observations and models, has been chosen from a comprehensive search of recent literature. The SEP flux for each stellar type is approximated from the stellar activity by scaling the solar SEP spectrum based on observations of X-ray flares. The GCR flux is varied step-wise from no GCRs (representative of a system that is highly shielded by the heliosphere of an active host star) to GCR fluxes estimated for the ISM in the central part of the galaxy (representative of a system in a highly radiative galactic environment with a weak heliosphere of a quiet host star). This grid over radiative environments allows us an insight into the effects of high-energy radiation on atmospheric chemistry, and can help guide our analysis of exoplanet atmosphere observations based on the stellar type of the host star and the environment the system is located in.

The nucleation of sulfuric acid-water clusters is a significant contribution to the formation of aerosols as precursors of cloud condensation nuclei (CCN). Depending on the temperature, there is an interplay between the clustering of particles and their evaporation controlling the efficiency of cluster growth. For typical temperatures in the atmosphere, the evaporation of H2 SO4 H2 O clusters is more efficient than the clustering of the first, small clusters, and thus their growth is dampened at its early stages. Since the evaporation rates of small clusters containing an HSO 4 - ion are much smaller than for purely neutral sulfuric acid clusters, they can serve as a central body for the further attachment of H2 SO4 H2 O molecules. We here present an innovative Monte Carlo model to study the growth of aqueous sulfuric acid clusters around central ions. Unlike classical thermodynamic nucleation theory or kinetic models, this model allows to trace individual particles and thus to determine properties for each individual particle. As a benchmarking case, we have performed simulations at T = 300 K a relative humidity of 50% with dipole and ion concentrations of c dipole = 5 × 10 8 - 10 9 cm - 3 and c ion = 0 - 10 7 cm - 3 . We discuss the runtime of our simulations and present the velocity distribution of ionic clusters, the size distribution of the clusters as well as the formation rate of clusters with radii R ≥ 0.85 nm . Simulations give reasonable velocity and size distributions and there is a good agreement of the formation rates with previous results, including the relevance of ions for the initial growth of sulfuric acid-water clusters. Conclusively, we present a computational method which allows studying detailed particle properties during the growth of aerosols as a precursor of CCN.

Galactic cosmic rays (GCRs), primarily consisting of protons, are ubiquitous throughout the solar system. The greatest source is supernova activity and the flux in the solar system is modulated by the solar wind. Planets can be shielded if they possess a magnetic field. Jupiter’s magnetic field is the strongest in the solar system and has several interesting features such as the Great Blue Spot near the Equator. The JUNO satellite has mapped the magnetic field of Jupiter in great detail (Connerney et al, JGR Planets 127(2), 2022) resulting in the JRM33 model, composed of data from 32 polar orbits of JUNO around Jupiter.We have calculated a cosmic ray cutoff rigidity map for Jupiter. This was done using a modified version of a particle trajectory program (the Geomagnetic Cutoff Rigidity Computer Program by Smart and Shea (2001, Tech. Rep. No. 20010071975)) with the first 12 degrees and orders of the spherical harmonic expansion from the JRM33 model as input. This is done for vertical GCR entry into Jupiter’s atmosphere at a height of 67.5 km above the 1 bar level and for distances further out where high energy particles have been detected by JUNO. The energies required to enter into Jupiter’s atmosphere varies by several orders of magnitude from above 2500 GeV at locations around the Great Blue Spot and going downwards towards the poles.The modulation of the GCR proton flux into Jupiter’s atmosphere was then calculated. For the incoming GCR spectrum we used data from the BESS-POLAR II Antarctic mission (Abe et al, ApJ 822(2), 2016), collected at solar minimum where the modulation by the solar wind is at its lowest. By fitting the measured spectrum and using the calculated cutoff rigidities we have made a map of the proton flux into Jupiter’s atmosphere.Finally, we have investigated several incidents of high energy heavy ion detections by JUNO (Becker et al, JGR Planets 126, 2021) by calculating the cutoff rigidities from several incoming angles at the locations where JUNO made the detections and along the corresponding M-shells.

This study examines the relationship between cosmic rays and clouds during Forbush decreases (FDs) to understand the cause-effect relationships between cloud microphysics, cloud condensation nuclei (CCN), and ionisation in the atmosphere. The results of a Monte Carlo analysis of cloud parameters during FDs from newly calibrated satellite data, namely, the Pathfinder Atmospheres Extended (PATMOS-x) from 1978 to 2018, show the connections between some cloud parameters and FDs. For context, FD is the event where, the amount of cosmic rays arriving in the atmosphere decreases and recovers over several days. Other studies have shown that FDs impacted the cloud fraction, aerosol optical depth, CCN, water content, and cloud effective radius (reff ) in the atmosphere. Using the Monte Carlo analysis, nine atmospheric parameters from the dataset were evaluated for a significant response level to FDs. Each FD event added (after the first event) reduces the noise, but only the strongest events add a significant signal (exceptionally when the 2nd and 5th rank FD data are added, the signal/noise ration dropped due to change of satellite version). We found that cloud fraction shows statistically significant signals following FDs at an achieved significance level of 0.33%. Cloud emissivity also showed highly significant signals from the analysis, however these cannot be determined as physical cause by FDs since the response starts a week before the FDs. In contrast, the cloud optical depth, integrated total cloud water over the entire column, and reff did not show any significant signals in frameworks of the applied methods. The top-of-atmosphere brightness temperature at nominal wavelengths of 3.75, 11.0, and 12.0 µm and surface brightness temperature were analysed anew and showed significant signals. The estimated brightness temperature changes from a radiative transfer model (Fu-Liou model) show consistent results with the observed changes in cloud parameters during FD events. Among analysed several atmospheric/cloud/aerosol parameters, cloud fraction and the top-of-atmosphere brightness temperature at nominal wavelengths of 3.75, 11.0, 12.0 µm remain the only parameters depicting a statistically significant and correct-phase response to FDs.

Experiments on sulphuric acid nucleation in low oxygen atmospheres were done in order to investigate the role of nucleation in the Archean atmosphere. Nucleation initiated by photolysis of SO2 and subsequent reaction between atomic O and SO2 was measured with a PSM and a separate CPC. The parameters were < 10 ppm O-2 with varying levels of SO2 (4 levels from 40 to 105 ppb), RH (3 levels from 0 to 51%), UV light (254 nm, 4 levels from 55 to 100% power), and ionization (2 levels: Background (similar to 3 cm(-3) s(-1)) and increased w. gamma sources (similar to 42 cm(-3) s(-1))). We find that nucleation is possible under these conditions and that the measured formation rates correlate positively with all varied parameters. This suggests that the sulphuric acid nucleation system could have played a role in the Archean atmosphere.

The seminal Miller‐Urey experiment suggests that lightning may have contributed to the origin of life on Earth through the formation of amino acids and carbon acids. We here focus on the early stages of lightning in the atmosphere of Primordial Earth, so‐called streamer discharges. We discuss rate coefficients for electrons and study electron avalanches and avalanche‐to‐streamer transitions by modeling the motion of electrons with a 2.5 D particle‐in‐cell Monte Carlo code in the strongly reducing atmosphere used by Miller and Urey (MU) and the weakly reducing atmospheric suggested more recently (by Kasting (1993), https://doi.org/10.1126/science.11536547) for Earth 3.8 Ga ago and compare results with conditions on Modern Earth. Our simulations show that streamers incept at fields of 140–180 Td in Kasting's mixture and at fields of ≈114 Td in the MU mixture, thus their inception is more difficult in Kasting's mixture. Conclusively, discharges on Primordial Earth might have been more challenging to incept.

Atmospheric ionization produced by cosmic rays has been suspected to influence aerosols and clouds, but its actual importance has been questioned. If changes in atmospheric ionization have a substantial impact on clouds, one would expect to observe significant responses in Earth's energy budget. Here it is shown that the average of the five strongest week-long decreases in atmospheric ionization coincides with changes in the average net radiative balance of 1.7 W/m(2) (median value: 1.2 W/m(2)) using CERES satellite observations. Simultaneous satellite observations of clouds show that these variations are mainly caused by changes in the short-wave radiation of low liquid clouds along with small changes in the long-wave radiation, and are almost exclusively located over the pristine areas of the oceans. These observed radiation and cloud changes are consistent with a link in which atmospheric ionization modulates aerosol's formation and growth, which survive to cloud condensation nuclei and ultimately affect cloud formation and thereby temporarily the radiative balance of Earth.

We aim to understand the onset of cloud formation {and study the formation of} TiO$_2$-CCNs. The formation of (TiO$_2$)$_{\rm N}$ clusters as precursors to extrasolar cloud formation is modelled by two different methods in order to understand their potential, identify underlying shortcomings, and to validate our methods. We propose potential spectral tracers for TiO$_2$-CCN formation. We applied three-dimensional Monte Carlo (3D MC) simulations to model the collision-induced growth of TiO$_2$-molecules to (TiO$_2$)$_{\rm N}$-clusters in the free molecular flow regime of an atmospheric gas. We derived individual, time-dependent (TiO$_2$)$_{\rm N}$ cluster number densities. For $T=1000$K, the results are compared to a kinetic approach that utilises thermodynamic data for individual (TiO$_2$)$_{\rm N}$ clusters. The {(TiO$_2$)$_{\rm N}$} cluster size distribution is temperature dependent and evolves in time until a steady state is reached. For $T=1000$K, the 3D MC and the kinetic approach agree well regarding the cluster number densities for $N=1\,\ldots\,10$, the vivid onset of cluster formation, and the long transition into a steady state. Collision-induced growth and evaporation simulated using a 3D MC approach enables a faster onset of cluster growth through nucleation bursts. Different size distributions develop for monomer-cluster and for cluster-cluster growth, with the largest clusters appearing for cluster-cluster growth. The (TiO$_2$)$_N$ cluster growth efficiency has a sweet-spot temperature at $\approx 1000$K at which CCN formation is triggered. The combination of local thermodynamic conditions and chemical processes therefore determines CCN formation efficiency.

The solar system is constantly changing, and it is important for us to understand how our climate and weather changes in response to the solar activity during both long-time scales (e.g. the 11-year solar cycle) and short time scales (e.g. days to weeks during For-bush Decreases (FDs)). Solar variability causes a corresponding modulation of the incident number of cosmic rays in Earth's atmosphere. Previous work by [Veretenenko and Pudovkin, 1997], [Svensmark and Friis-Christensen, 1997], [Palle Bago and Butler, 2000], [Svensmark et al., 2016], [Harrison and Ambaum, 2010], and other researchers have discussed this cause-effect relationship from an experimental and theoretical approach. Since the 1970s, global observations of the Earth's system by satellites are offering an invaluable source of information about cloud parameters. In this study, we used the newly calibrated PATMOS-x (Pathfinder Atmospheres Extended) data set during the period from 1978 to the present. A method for capturing the connection between cosmic rays and meteorological measurements has been conducted by superposition analysis of FD events for time series (36 days) and the Monte Carlo bootstrap test to evaluate significance level of the integrated signal for 9 days after the minimum in FD. We have reviewed results, primarily about cloud emissivity (Achieved Significance Level (ASL >99%), surface brightness temperature (ASL >99%), and cloud fraction (ASL >99%). Some of the results support the proposed relationship between solar activity and temperature. This result indicates that the amount of incident cosmic rays decreases due to FDs, global average temperature increases [Friis-Christensen and Lassen, 1991], [Harrison and Ambaum, 2010]. In addition, PATMOS-x parameters of cloud probability, cloud mask, and cloud fraction, which all means cloud coverage on the Earth shows statistically significant signals following FDs. In some previous research, IR-detected cloud fraction from International Satellite Cloud Climate Project (ISCCP) and combined liquid and ice cloud fraction, effective emissivity from the Moderate Resolution Imaging Spectroradiometer (MODIS) also show connection with FDs, see [Svensmark et al., 2009], [Svensmark et al., 2016], [Marsh and Svensmark, 2000a], Todd and Kniveton [2004]. The relationship between the observed changes in cloud amount and the resulting solar forcing is discussed. On the other hand, “Cloud water content" from Special Sensor Microwave Imager (SSM/I), “Liquid water path", and “Optical thickness" from MODIS also showed as significant signals by FDs, see [Svensmark et al., 2009], [Svensmark et al., 2016], however a similar parameter about “optical thickness" and “integrated total cloud water over whole column g/m2" from PATMOS-x dataset does not have high significant signals by a bootstrap test with ASL of 77.03 and 92.51% respectively. Moreover, significant results are reported for several new cloud parameters from the PATMOS-x dataset (e.g. cloud type, brightness temperature, measurements by different wavelength 0.65, 0.86, 3.75, 11.0, and 12.0 μm and others) and Fu-Liou model is used for estimation of changed radiations in the atmosphere. An interaction between CCN and radiation has not been investigated well yet. It is necessary to still more to learn about these results for further understanding of Earth’s atmosphere.

The presence of small ions influences the growth dynamics of a size distribution of aerosols. Specifically the often neglected mass of small ions influences the aerosol growth rate, which may be important for terrestrial cloud formation. To this end, we develop a numerical model to calculate the growth of a species of aerosols in the presence of charge, which explicitly includes terms for ion-condensation. It is shown that a positive contribution to aerosol growth rate is obtained by increasing the ion-pair concentration through this effect, consistent with recent experimental findings. The ion-condensation effect is then compared to aerosol growth from charged aerosol coagulation, which is seen to be independent of ion-pair concentration. The model source code is made available through a public repository.

The nucleation of sulfuric acid-water clusters plays a significant role in the formation of aerosols. Based on a recently developed particle Monte Carlo (MC) Code, we analyze how the growth of sulfuric acid-water clusters is influenced by stochastic fluctuations. We here consider samples of H2SO4-H2O clusters at T = 200 K with a relative humidity of 50%, with particle concentrations between 10(5) and 10(7) cm(-3) in volumes between 10(-6) and 10(-2) cm(3). We present the temporal evolution of the formation rate and of the size distribution as well as growth rates and the onset time of the nucleation above a given cluster size with and without constant production of new monomers. Clear evidence is revealed by the MC code that fluctuations result in a faster growth rate of the smallest clusters compared to deterministic continuum models that do not contain the stochastic effects. The faster growth of small clusters in turn influences the growth of larger clusters. Depending on the volume size, the onset time for clusters larger than 0.85 nm varies between 1000 s and 20,000 s for cm(-3) and between 10 s and 100 s for cm(-3). Copyright (c) 2020 American Association for Aerosol Research

. An ab initio study of gaseous clusters of O − 2 and O − 3 with water is presented. Based on thorough scans of configurational space, we determine the thermodynamics of cluster growth. The results are in good agreement with benchmark computational methods and existing experimental data. We find that anionic O − 2 (H 2 O) n and O − 3 (H 2 O) n clusters are thermally stabilized at typical atmospheric conditions for at least n = 5. The first 4 water molecules are strongly bound to the anion due to delocalization of the excess charge while stabilization of more than 4 H 2 O is due to normal hydrogen bonding. Although clustering up to 12 H 2 O, we find that the O 2 and O 3 anions retain at least ca. 80 % of the charge and are located at the surface of the cluster.

One hundred and ten direct measurements of aerosol nucleation rate at high ionization levels were performed in an 8 m(3) reaction chamber. Neutral and ion-induced particle formation from sulfuric acid (H2SO4) was studied as a function of ionization and H2SO4 concentration. Other species that could have participated in the nucleation, such as NH3 or organic compounds, were not measured but assumed constant, and the concentration was estimated based on the parameterization by Gordon et al. (2017). Our parameter space is thus [H2SO4] = 4 x 10(6) - 3 x 10(7) cm(-3), [NH3 + org] = 2.2 ppb, T = 295 K, RH = 38 %, and ion concentrations of 1700-19 000 cm(-3). The ion concentrations, which correspond to levels caused by a nearby supernova, were achieved with gamma ray sources. Nucleation rates were directly measured with a particle size magnifier (PSM Air-modus A10) at a size close to critical cluster size (mobility diameter of similar to 1.4 nm) and formation rates at a mobility diameter of similar to 4 nm were measured with a CPC (TSI model 3775). The measurements show that nucleation increases by around an order of magnitude when the ionization increases from background to supernova levels under fixed gas conditions. The results expand the parameterization presented in Dunne et al. (2016) and Gordon et al. (2017) (for [NH3 + org] = 2.2 ppb and T = 295 K) to lower sulfuric acid concentrations and higher ion concentrations. The results make it possible to expand the parameterization presented in Dunne et al. (2016) and Gordon et al. (2017) to higher ionization levels.

The nucleation of sulphuric acid molecules plays a key role in the formation of aerosols. We here present a three dimensional particle Monte Carlo model to study the growth of sulphuric acid clusters as well as its dependence on the ambient temperature and the initial particle density. We initiate a swarm of sulphuric acid-water clusters with a size of 0.329 nm with densities between 10(7) and 10(8) cm(-3) at temperatures between 200 and 300 K and a relative humidity of 50%. After every time step, we update the position of particles as a function of size-dependent diffusion coefficients. If two particles encounter, we merge them and add their volumes and masses. Inversely, we check after every time step whether a polymer evaporates liberating a molecule. We present the spatial distribution as well as the size distribution calculated from individual clusters. We also calculate the nucleation rate of clusters with a radius of 0.85 nm as a function of time, initial particle density and temperature. The nucleation rates obtained from the presented model agree well with experimentally obtained values and those of a numerical model which serves as a benchmark of our code. In contrast to previous nucleation models, we here present for the first time a code capable of tracing individual particles and thus of capturing the physics related to the discrete nature of particles. (C) 2018 The Author(s). Published by Elsevier Inc.