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Shipping is the cornerstone of international trade and thus a critical economic sector. However, ships predominantly use fossil fuels for propulsion and electricity generation, which emit greenhouse gases such as carbon dioxide and methane, and air pollutants such as particulate matter, sulfur oxides, nitrogen oxides, and volatile organic compounds. The availability of Automatic Information System (AIS) data has helped to improve the emission inventories of air pollutants from ship stacks. Recent laboratory, shipborne, satellite and modeling studies provided convincing evidence that ship-emitted air pollutants have significant impacts on atmospheric chemistry, clouds, and ocean biogeochemistry. The need to improve air quality to protect human health and to mitigate climate change has driven a series of regulations at international, national, and local levels, leading to rapid energy and technology transitions. This resulted in major changes in air emissions from shipping with implications on their environmental impacts, but observational studies remain limited. Growth in shipping in polar areas is expected to have distinct impacts on these pristine and sensitive environments. The transition to more sustainable shipping is also expected to cause further changes in fuels and technologies, and thus in air emissions. However, major uncertainties remain on how future shipping emissions may affect atmospheric composition, clouds, climate, and ocean biogeochemistry, under the rapidly changing policy (e.g., targeting decarbonization), socioeconomic, and climate contexts. 

Abstract. A fast-response (10 Hz) chemiluminescence detector forozone (O3) was used to determine O3 fluxes using the eddy covariancetechnique at the Penlee Point Atmospheric Observatory (PPAO) on the southcoast of the UK during April and May 2018. The median O3 flux was −0.132 mg m−2 h−1 (0.018 ppbv m s−1),corresponding to a deposition velocity of 0.037 cm s−1(interquartile range 0.017–0.065 cm s−1) – similar to thehigher values previously reported for open-ocean flux measurements but notas high as some other coastal results. We demonstrate that a typical singleflux observation was above the 2σ limit of detection but hadconsiderable uncertainty. The median 2σ uncertainty of depositionvelocity was 0.031 cm s−1 for each 20 min period, whichreduces with the square root of the sample size. Eddy covariance footprintanalysis of the site indicates that the flux footprint was predominantlyover water (> 96 %), varying with atmospheric stability and, toa lesser extent, with the tide. At very low wind speeds when the atmospherewas typically unstable, the observed ozone deposition velocity was elevated,most likely because the footprint contracted to include a greater landcontribution in these conditions. At moderate to high wind speeds whenatmospheric stability was near-neutral, the ozone deposition velocityincreased with wind speed and showed a linear dependence with frictionvelocity. This observed dependence on friction velocity (and therefore alsowind speed) is consistent with the predictions from the one-layer model ofFairall et al. (2007), which parameterisesthe oceanic deposition of ozone from the fundamental conservation equation,accounting for both ocean turbulence and near-surface chemical destruction,while assuming that chemical O3 destruction by iodide is distributed overdepth. In contrast to our observations, the deposition velocity predicted bythe recently developed two-layer model of Luhar et al. (2018) (whichconsiders iodide reactivity in both layers but with molecular diffusivitydominating over turbulent diffusivity in the first layer) shows no majordependence of deposition velocity on wind speed and underestimates themeasured deposition velocities. These results call for further investigationinto the mechanisms and control of oceanic O3 deposition. 

Abstract. Aerosol–cloud interactions (ACIs) are considered to be the most uncertaindriver of present-day radiative forcing due to human activities. Thenonlinearity of cloud-state changes to aerosol perturbations make itchallenging to attribute causality in observed relationships of aerosolradiative forcing. Using correlations to infer causality can be challengingwhen meteorological variability also drives both aerosol and cloud changesindependently. Natural and anthropogenic aerosol perturbations from well-defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatiotemporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite datasets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.