Climate-relevant aerosol sources and processes in the Arctic – CASPA

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.