The timing of an array of millisecond pulsars (PTA) acts as a galactic-scale detector to observe gravitational wave (GW) sources in the nHz frequency range. The goal of this project is to detect low-frequency GWs while maximizing the scientific output of the Nançay radio telescope (NRT) and participating in operations of the South African MeerKAT radio telescope. We propose new data analysis methods to detect GWs emitted by multiple supermassive black hole binaries in eccentric orbits, while modeling pulsar and noise properties. We will develop a new state-of-the-art pulsar observing backend, to achieve coherent de-dispersion over a very large frequency range (1.5 to 3.5 GHz) and substantially increase our sensitivity at these key radio frequencies. The expected results are the potential first ever detection of GWs in the nHz domain, a much improved understanding of millisecond pulsars and of the weak perturbations that affect their timing stability, and new tests of General Relativity. This project is based on the long-term know-how of a composite team, made of radio astronomers who are specialists in pulsar timing, in Bayesian techniques and in GW data analyses. The NRT already produces high cadence pulsar data with a dedicated state-of-the-art backend enabling us to analyze data from the telescope’s L-band receiver (1.1-1.7 GHz) optimally. The new instrumentation will allow us to cover the whole band accessible with its high frequency S-band receiver, bringing a leap in sensitivity in a domain where the observed radio signal from pulsars is much less affected by interstellar medium perturbations. The support from the ANR will provide us with the resources to fully participate in the scientific exploitation of today’s best radio telescope in the Southern hemisphere, MeerKAT, and allow us to extend the sky coverage for GW searches. This involvement will also firmly install our French team in the long-term preparation of the SKA (Square Kilometer Array) project, which has just been included in the national road map. This is a unique opportunity to train future radio astronomers on one of its key science programs. We will also benefit from our engagement in the LISA project, sharing the expertise accumulated in both communities and building new GW detection algorithms at the interface between both projects, implementing more sophisticated and realistic GW models, and introducing machine learning in the trans-dimensional Bayesian analysis. The PTA technique indeed gives us access to a frequency domain complementary to those covered by Virgo-LIGO and LISA, where one expects GW emission from sources such as super massive black hole binaries (SMBHBs) formed in the long process of galaxy aggregation, and also stochastic GWs from the cosmological background generated by inflation in the very early Universe or by a network of cosmic strings. Characterizing those individual sources (parameters, rate, sky distribution, etc...) would yield unique information about the formation and evolution of SMBHBs through cosmic history and bring original constraints on the hierarchical galaxy formation. Moreover, the detection of this GW signal will allow us to refine the prognoses for SMBHB mergers in the LISA band. To reach these goals, we need a dedicated post-doc involved in both MeerKAT pulsar timing observations and in the combination of data from all radio telescopes involved in the International Pulsar Timing Array organization. Developing and implementing new data analysis techniques at the interface between PTA and LISA is a great project for a PhD student, who will take advantage of the APC environment and get unique skills for the future exploitation of LISA and SKA data. Finally, a strong involvement in MeerKAT and the availability of a wide-band pulsar instrumentation at NRT will clearly maintain the French radio telescope in the race up to the SKA era (>2025) and strengthen our position in the SKA Pulsar Science Working Group.

The paroxysmal stages of wildfires generate pyro-cumulonimbus that deposit large quantities of smoke in the stratosphere, comparable to a moderate volcanic eruption. It was discovered in 2020 by the coordinator and collaborators that this smoke self-organizes as synoptic scale anticyclonic vortices that rise under the heating due to the absorption of solar radiation by black carbon aerosols. These structures persist for several months in the stratosphere rising by 10 to 20 km. The consequence is the persistence of the released smoke for several years with a much longer climatic impact than expected so far, and is expected to increase in the future. The vortices also carry an intense ozone mini hole, likely to travel over continents in mid-summer, increasing ground UV. A second paper published in 2021 established that smoke vortices were also ubiquitous after the British Columbia fires of 2017 and they are probably present in many other similar events. Although volcanic aerosols are, in principle, much less absorbing than black carbon, there are indications of compact rising similar structures after some recent volcanic eruptions. The project assembles a team of experts in geophysical fluid dynamics, atmospheric remote sensing and modellers to document and understand these very new atmospheric structures, their distribution and their impact. The project is divided in three main work packages. The first one is observation oriented and is devoted to exploring the data of the past. The second part is oriented towards fluid dynamics and radiative properties. The third one is oriented towards realistic modelling and impact studies. In the first package, we investigate the past cases from the archive data by distinguishing a recent period after 2006 when the space lidar CALIOP is available and aerosol plumes can be localized and characterized with high precision from the earlier periods when less satellite data are available and more modelling work is required. We investigate in this work package how the observed heating is related to the aerosol properties and how the observed ozone hole affects the UV radiation at the ground level. In this package, we also get prepared to respond to future events with dedicated observation and modelling in a real time framework. The second package is devoted to the basic understanding of stable rising heated vortices in a rotating fluid, which have never been described in the literature. The realistic observed conditions that are on the verge of inertial instability are highly nonlinear and present some theoretical and numerical challenge for the available methods. We will use a hierarchy of models and theoretical concepts starting from the well-mastered low Rossby number framework to get to the realistic cases. We will experiment numerically with idealized fluid dynamics model and a state-of-the-art non-hydrostatic model adapted to the stratosphere. An important component of this second package will be a laboratory experiment in a rotating tank, which is expected to provide a demonstrator and also a flexible model to question the theory and numerical simulations. The third package will be devoted to reproduce the observed events by first performing detailed radiative calculation to estimate the heating rates. This information will be used in a full chemical-climate model that will be trained, to begin, on the cases of 2020 and 2017 and to estimate the impact of the events in terms of radiative budget and atmospheric composition.

CASPA (Climate-relevant Aerosol Sources and Processes in the Arctic) aims to gain new fundamental insights into processes governing the formation and distribution of anthropogenic aerosols originating from local, relative to remote, sources to reduce uncertainties in model predictions of aerosol impacts on Arctic climate. The focus is on winter and early spring when anthropogenic emissions contribute most to the widespread pollution Arctic Haze. Aerosols are important short-lived climate forcers in the Arctic, a region undergoing unprecedented changes, due to rapid warming. Improved understanding is crucial given potential risks from already significant and increasing local anthropogenic emissions. Modelling correctly the compositional mix and vertical distributions of Arctic aerosols in the lower atmosphere is important for quantification of direct and indirect radiative effects. Individual chemical and physical processes driving the formation and evolution of aerosols in the Arctic are not sufficiently constrained by the few available observations, especially in the wintertime when our knowledge is very poor. For instance, formation mechanisms of secondary aerosols, such as sulphate or organics, remain rather puzzling in cold, dark/dim wintertime Arctic conditions. The acute lack of relevant process-level data is partly responsible for very diverse and often poorly simulated Arctic aerosols. This hampers our ability to correctly assess impacts of local and remote anthropogenic emissions on Arctic aerosols and climate. CASPA addresses important knowledge gaps via 3 inter-related scientific objectives (work packages) to improve characterisation, understanding and model treatments of processes governing 1) sources and formation (oxidation) pathways for Arctic aerosols, 2) the role of Arctic boundary layer transport and mixing on the formation and vertical distributions of aerosols, leading to 3) improved simulation of local, relative to remote, anthropogenic sources on Arctic-wide aerosol distributions and their impacts on climate. A combination of collection of new data (precursor gases, oxidants, aerosols, dynamics) and chemical-aerosol-climate modelling will be used leading to improved Arctic aerosol and climate predictive capabilities. New data will be collected as part the first major, international Arctic field campaign examining wintertime aerosols (IGAC-Future Earth/IASC) PACES-ALPACA. We will deploy state of the art instrumentation in January-February 2022 in Alaska to characterise inorganic/organic aerosols, vertical layering and mixing of aerosols and precursors (ground-based, radar, in-situ profiles, masts) at sites influenced by local emissions and Arctic Haze. Filters will be collected for novel laboratory isotope analyses giving insights into aerosol formation rates. All these field data will be analysed, in combination with multi-scale modelling, in order to evaluate and improve process-level treatments in local, regional (Alaska) and Arctic wide (hemispheric) model simulations. The improved model will be used to quantify local, relative to remote, source contributions and their radiative effects. CASPA brings together 6 complementary French groups working on atmospheric chemistry, dynamics and geochemistry, working in close collaboration with international PACES teams. Science results will be communicated to the wider modelling community, stakeholders, including policy makers (Arctic Council, IPCC) and the public via outreach activities, also involving industry collaborators.

Volcanoes release vast amounts of gases and particles into the atmosphere. Whilst impacts from volcanic sulfur have been intensively studied, it is now acknowledged that volcanic halogens may also impact the atmosphere. It has already been shown that volcanic halogens undergo a multi-phase ozone-destroying chemistry in the troposphere. There is emerging observational evidence that recent moderate eruptions injected significant amounts of halogens into the stratosphere. In the case of a large halogen-rich eruption, this could cause large stratospheric ozone depletion alongside climate effects. It is increasingly apparent that a fully comprehensive assessment of the impacts of volcanic activity on the atmosphere and climate should not be limited to sulfur only but also include halogens. To quantify impacts from volcanic halogens requires tracing their cycle from deep subsurface to surface resulting in emissions to the atmosphere and characterising their atmospheric physico-chemical processing. However, they are still large uncertainties on key processes. By combining our expertise and innovative experimental/modelling tools across earth and atmospheric sciences, Volc-Hal-Clim will tackle long-standing issues on the fate and impacts of volcanic halogens on atmospheric composition, notably the ozone layer, and climate. There is a strong focus on volcanic bromine (alongside iodine and chlorine), that has been little studied so far but may play a potentially important role in volcanic perturbations. The project consists of 5 tasks across two work-packages. - WP1 is about deep halogen cycle and emissions. We will perform high-pressure, high-temperature experiments to characterise halogen behaviour (solubility, fluid-melt partitioning) at depth and in the shallow crustal reservoir. By developing degassing models based on these experimental data, along with melt-inclusions composition measurements, we will quantify halogen transfer from the subducting slab, up to the crust and ultimately to the surface. The predicted volcanic emissions will be evaluated against observations of halogens near volcanic sources, taking into account their processing inside the crater and on the volcano flank. This will be a key input to the atmospheric studies (WP2). - WP2 deals with the impact of volcanic halogens on atmospheric composition and climate. We will develop an imbricated multi-scale modelling system that will cover the relevant scales and phases in the atmospheric cycle of volcanic emissions: from the very local high temperature chemistry in the crater to reactive plumes dispersing at local/regional scales and finally to the global dispersion in the troposphere and stratosphere. A range of imbricated numerical models (from plume models to a global chemistry-climate model) will be used to investigate the atmospheric impacts. We will simulate the chemistry of plumes, originating from continuous emissions or recent small eruptions, to assess local/regional and global impacts in the troposphere and stratosphere. Model simulations will be evaluated against field and satellite observations, notably on selected case studies, e.g. the Ambrym (a a massive source of halogen) or Etna volcanoes. The modelled climate response will also be compared to meteorological reanalysis data. Process-oriented evaluation and analysis of sensitivity simulations will allow to disentangle the different mechanisms and assess the respective roles of halogens and sulfur including synergetic effects for selected volcanic events. This rather exploratory project will deliver key knowledge, improved volcanic degassing modelling and a unique multi-scale atmospheric modelling system validated with observations. All these elements are needed if volcanic halogens are to be accounted for in assessing the impact of volcanic activity on the Earth’s atmosphere and climate.