Source apportionment and biomonitoring approaches to quantify the contribution and spatial spread of particulate matter from shipping in a major UK port city

Shipping emissions are an important source of Particulate Matter (PM) associated with an estimated 400,000 premature deaths per year globally. These negative effects on air quality disproportionately impact port and coastal communities, which include many of the world’s largest cities. Despite the English Channel being the busiest shipping lane in the world, and in close proximity to many major cities, the physicochemical characterisation of shipping emissions and their contribution to air quality in the UK remains understudied.

Coarse (PM_10-2.5) and fine (PM_2.5-0.1) PM samples were collected between 2017 and 2020 at the UK port of Southampton. This port is Europe’s leading turnaround cruise port, handling 86% of all UK cruise passenger traffic in 2023.  In addition, Southampton is one of the UK's major gateway container ports, being the UK’s leading vehicle import/export and deep-sea trade port, attracting some of the world's largest ships. Importantly, as Southampton is located centrally on the south coast of England, this falls within the North Sea Emission Control Area and therefore, ships in this area are subject to the most stringent fuel restrictions of 0.1% S, or equivalent exhaust cleaning.

To determine the contribution of shipping emissions to air quality, a positive matrix factorisation source apportionment model was generated using PM elemental concentrations measured by inductively coupled plasma mass spectrometry. The shipping fuel combustion factor was characterised by the traditional tracers of V and Ni within the expected ratio (V/Ni = 2.6) indicative of Heavy Fuel Oil (HFO) associated shipping. However, Co was identified as a novel tracer species, which may be an artefact from the catalysis of fuel desulfurisation. The final five-factor model found that shipping fuel contributed almost exclusively to fine PM, rather than coarse PM, with an average contribution of 15% fine PM at the Port. This contribution was significantly elevated between April and September, representing the peak cruise shipping season.

To study the spatial spread of PM emissions, samples of tree bark were used, as airborne particles can become trapped in the bark structure. This biomonitoring approach represents a cost- and time-effective alternative to the use of multiple PM-sampling sites.  Here, samples of bark from lime (Tilia spp.), oak (Quercus spp.) and aspen (Populus tremula) trees were collected at locations across the city of Southampton. The elemental concentration of the identified shipping tracers Ni and Co in the bark samples were investigated (V was unsuitable as a tracer due to uptake by bark lichens). This showed that concentrations of Ni and Co in tree bark displayed an exponential increase with increasing proximity to the port. Our data suggest deposited concentrations 300 m from the port are 2.5x higher than 2.2 km away and 4x greater than 6 km away.

Collectively the contribution of shipping emissions to port city PM, and the spread of these emissions identified in this study underline the importance of including shipping in strategies to improve air quality. These strategies would be aided by a better understanding of the key aspects of port and shipping activity which drive these emissions.