Ship-related ultrafine particles and SOA formation in a Mediterranean port city

Ship emissions of air pollutants in and around ports adversely affect local air quality and human health, especially in harbour cities. In coastal Mediterranean cities, shipping activities are an important contributor to emissions of fine particulate matter (PM_2.5) within the urban area. In addition to primary particles emitted in ship exhaust, secondary organic aerosols (SOA) may form in ship plumes through chemical oxidation of volatile organic compounds (VOC) and through condensation of intermediate volatile or semi-volatile organic gases. Shipping also emerges as a major source of ultrafine particles below 100 nm diameter in coastal cities. Ultrafine particles have been estimated to significantly affect human mortality in coastal areas. While a large number of studies investigated the effect of ship-related PM_2.5 in coastal areas, currently only few studies deal with the effect of ship emissions on SOA concentrations and number concentrations of ultrafine particles in harbour cities.

In this study, we investigate the effect of ship emissions on SOA concentrations and number concentrations of UFP in the harbour city Marseille in southern France, which is an important hub of ferry and cruise ship traffic in the Mediterranean Sea. For city-scale simulations of Marseille, the urban chemistry transport model EPISODE-CityChem (https://doi.org/10.5281/zenodo.1116173) was applied in a coupled setup with the regional-scale Community Multiscale Air Quality Modelling System (CMAQ). EPISODE-CityChem combines a 3-D Eulerian grid model with sub-grid Gaussian plume models and solves the photochemistry of multiple reactive pollutants, including the chemistry of 12 different VOC. New developments in EPISODE-CityChem include the P8P+2 scheme for calculating particle number (PN) concentration and particle number size distribution (PNSD) and the SOA module of the aerosol model MAFOR v2.0 (https://github.com/mafor2/mafor). The STEAM-3 emission inventory for the local shipping in and around the port of Marseille consists of hourly emissions of major pollutants, VOC and particle numbers from ships on 250 m grid resolution. Hourly model output of EPISODE-CityChem for July 2020 was compared to measurements at monitoring stations in Marseille operated by AtmoSud and campaign data recorded at La Major, a site in proximity of the port.

Our results show that the potential impact from local shipping to the monthly mean concentration in the urban area of Marseille is only up to 3% for PM_2.5, whereas it is up to 42% for total PN. The abundance of ship-related semi-volatile organic vapours is high in the areas of SOA formation, which indicates that volatile organics are mainly in the gas phase because available pre-existing particle surfaces and high ambient temperatures limit their condensation. Ship plumes at La Major were detected based on the difference of total PN concentration between the reference run and a model run excluding ship emissions. The maximum of the modelled and the observed PNSD notably corresponded to the typical size distribution maximum of ship exhaust particles. Total PN should be considered as a more suitable metric for monitoring ship emission impact than PM_2.5 because it allows for a better discrimination of ship plumes from the background pollution.