Exploring intra-urban variability of intake fractions for PM2.5: an application to the life-cycle assessment of road transport in Lisbon, Portugal

Life-cycle assessment (LCA) has been widely used to estimate environmental impacts associated with pollutant emissions. However, to estimate health effects associated with these emissions, methods with adequate spatial... [ view full abstract ]

Life-cycle assessment (LCA) has been widely used to estimate environmental impacts associated with pollutant emissions. However, to estimate health effects associated with these emissions, methods with adequate spatial resolution are needed to address fate and exposure to air pollutants. In particular for exposure to particulate matter, the use of intake fraction (iF), i.e., the mass amount of an emission that is inhaled by the population, has been recommended to characterize the emission-to-intake relationship.

Recent studies have estimated iF values for fine particulate matter (PM_2.5) for urban, rural and remote settings, various source heights, and for different cities worldwide. However, city-scale iFs can have significant uncertainty and do not necessarily represent spatial (intra-urban) exposure gradients, which can be important for estimating health impacts.

The main goal of this paper is to develop an approach to estimate intake fractions for PM_2.5 that accounts for intra-urban variability, by exploring its linkages with urban form, and the approach is applied to an LCA for road commuting in Lisbon, Portugal. We estimate an iF for Lisbon metropolitan area using a one-compartment steady-state model, which considers specific meteorological (surface and mixing height) and population data (breathing rates, disaggregated by gender and age groups) in the area. We explore intra-urban variability in iF and the potential links with urban form by developing and overlaying two maps: one with the population distribution in the area and another with the concentration gradients associated with dispersion and transport of pollutant emissions in the main road network. This provides a map with the exposed population and a set of intake fractions that can be applied to any road emission scenario. Last, we perform an LCA for road transportation in Lisbon and compare the results with both an urban-scale intake fraction and this disaggregated set of intake fractions.

The paper presents a spatially resolved dataset for intake fractions, accounting for intra-urban variability, and shows its application in the context of LCA. This new approach can improve the evaluation of health effects caused by road emissions, by increasing its granularity of estimates. The application illustrates how the city-scale intake fraction can overlook significant differences in the intake associated with different routes, demonstrating the importance of using specific and spatially resolved data for estimating health effects in life-cycle impact assessment. The approach can inform and support decision-making, by identifying and prioritizing opportunities for policies focused on urban air pollution and associated health effects.