An investigation into the chemical nature of particulate matter air pollution in a port city

Abstract

Over 80% of global trade is transported by sea and owing to expanding international trade and cruise ship-based tourism, shipping emissions are expected to continue to increase. An estimated 400,000 annual deaths are associated with shipping emissions, and as such, shipping emissions are an important source of air pollution with disproportionate impacts in port and coastal regions. Despite this, their contribution to local air quality in the UK is understudied. Focussing on the UK port city of Southampton, this work aimed to identify the port-related signature and quantify the contribution of shipping- and port-activities to local particulate matter (PM) load. Alternative techniques for elucidating the source and spatial spread of emissions that do not require lengthy sampling campaigns were also explored to inform future studies.

A High-Volume Cascade Impactor was deployed adjacent to different polluting activities at the Port of Southampton enabling simultaneous collection of coarse (PM10-2.5), fine (PM2.5-0.1) and ultrafine (PM0.1) PM in a source profiling study. Elemental composition was determined using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The resultant profiles were used to inform a Positive Matrix Factorisation (PMF) source apportionment study on a long-term port sample collection dataset. Tree bark samples were collected across Southampton to assess the spatial distribution of shipping emissions, and isotopic analyses of V and Ni were also performed on a subset of samples to identify if isotopes offer more source-specific information than elemental concentrations and ratios alone.

Cruise shipping emissions showed a strong seasonality and were particularly enriched in V, Ni and Co amongst the fine and ultrafine PM fractions. During high cruise activity ultrafine V concentrations were 14 times higher than during the winter season. From the PMF study five factors were identified: Sea Spray, Fine Combustion, Crustal, Vehicle and Shipping Fuel. The contribution of shipping emissions to the measured components of fine PM was found to be 19% which is greater than previously reported anywhere in Europe (PM2.5: 1-15.3%). Through tree bark biomonitoring it was found that concentrations of shipping elements decrease with increasing distance from the port. For Ni and Co an exponential decay is observed with elevated concentrations occurring within 2 km of the port. Ni isotopes showed a different isotopic composition between shipping and metal handling, whereas preliminary V isotope measurements suggest it may be possible to discern between the fuel burnt in container and cruise ships.

The source profiles presented here provide useful information for future source apportionment studies performed in the UK and abroad. Furthermore, the findings on shipping contribution, and the spatial spread of emissions are beneficial for informing policymakers on UK shipping emissions.

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Identifiers

ORCID for Nat Easton: orcid.org/0000-0002-9168-3294

ORCID for Matthew Loxham: orcid.org/0000-0001-6459-538X

ORCID for Gavin Foster: orcid.org/0000-0003-3688-9668

ORCID for Steven J Ossont: orcid.org/0000-0003-3864-7072

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