Atmospheric Composition and Radiative forcing effects_due to UN International Ship Emissions regulations

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Ships generally burn low quality fuel and emit large quantities of sulfur dioxide and particulates, or aerosols (harmful at high concentrations), into the atmosphere above the ocean. In the presence of clouds the sulfur dioxide is rapidly converted into more particle mass growing them to sizes where they act as sites for cloud droplet formation. Given that about 70% of shipping activities occur within 400 km of the coast, ships are a large source of air pollution in coastal regions, causing 400k premature mortalities per year globally. In the UK, air pollution (including ship emissions) is responsible for 40,000 premature mortalities each year. In an effort to reduce air pollution from shipping activity, the United Nation's International Maritime Organization (IMO) is introducing new regulations from January 2020 that will require ships in international waters to reduce their maximum sulfur emissions from 3.5% by mass of fuel to 0.5%. Particulates emitted by ships may enhance the number of cloud droplets and potentially form regions of brighter clouds known as ship tracks. Largely because of this effect, some global models predict that ship emissions of particulates currently have a significant cooling influence on the global climate, masking a fraction of the warming caused by greenhouse gas emissions. So whilst a reduction in ship sulfur emission is predicted to almost halve the number of premature deaths globally via a reduction in sulfate aerosols, a lack of similar reductions in greenhouse gases from shipping (e.g. CO2) could lead to an overall climate warming. However, the magnitude of the cooling caused by particulates is very uncertain, with large discrepancies between global model and satellite-based estimates. This may be due to imprecise representations of the effects of aerosols on clouds in global models or biases in satellite detections of ship tracks. Furthermore, how shipping companies respond to the 2020 regulation (i.e. degree and method of compliance), in international waters where surveillance is challenging, is largely unknown and requires observational verification. We will take advantage of this unique and drastic "inverse geoengineering" event in 2020. By combining aircraft observations, long-term surface observations, satellite remote sensing, and process-level modelling, we will investigate the impact of the 2020 ship sulfur emission regulation on atmospheric composition, radiative forcing and climate in the North Atlantic. Results of this project will improve our understanding of the impact of ship emissions on air quality and climate.