Changes in cloud activity of ship exhaust particles: Potential effects on Arctic mixed-phase clouds

Abstract

Maritime shipping remains a large source of anthropogenic airborne pollutants, including exhaust particles that can act as cloud condensation nuclei (CCN). The International Maritime Organization (IMO) imposed global fuel sulfur content (FSC) limits on marine fuels in order to target ship exhaust sulfur oxides and particulate matter emissions, but has allowed competing pathways to regulatory compliance; i.e., low FSC fuels versus exhaust after-treatment. Laboratory experiments revealed that these compliance measures have secondary effects on physicochemical properties of exhaust particles, affecting their CCN activity (Santos et al., 2022a; 2022b). We observe that combustion of low FSC fuels results in emissions of highly hydrophobic particles, causing significant reductions in CCN emissions, whereas wet scrubbing leads to an increase in CCN activity. One area of focus is the Arctic region, which has been shown to be particularly susceptible to the effects of climate warming. A steady decrease in observed sea ice cover amplifies the regional warming (Screen and Simmonds, 2010), but also opens the region to increased ship traffic which may result in further climate feedbacks (Stephenson et al., 2018). It is of particular interest to identify how increased ship exhaust particle emissions may affect cloud processes; for example, by facilitating liquid droplet formation and thus, potentially changing the radiative properties of the aerosol and clouds. Here, we investigate how increased shipping activity potentially influences the properties of Arctic mixed-phase clouds. In our study the experimentally observed characteristics of marine particle emissions and their liquid droplet forming potential have been implemented in large eddy simulations. We use the MIMICA model (MISU/MIT Cloud-Aerosol model) (Savre et al., 2014) to simulate a stable stratiform mixed-phase cloud based on the Arctic Summer Cloud Ocean Study (ASCOS) (Tjernström et al., 2014). A range of input parameters for ship aerosol, including size distributions, number concentrations, vertical distributions and hygroscopicities, has been studied to assess the potential impact on cloud properties and regional climate. Santos et al. (2022a). Environ. Sci.: Processes Impacts, 24:1769-1781 Santos et al. (2022b). Environ. Sci.: Atmos., Advance Article Savre et al. (2014). J. Adv. Model. Earth Syst., 6:630-649 Screen and Simmonds (2010). Nature, 464(7293):1334–1337 Stephenson et al. (2018). Geophys. Res. Lett., 45:9898–9908 Tjernström et al. (2014). Atmos. Chem. Phys., 14:2823-2869