Improving air pollution source apportionment in size-segregated PM using Pb isotope-based Bayesian mixing models in Tarragona (Spain)

Abstract

A total of 75 outdoor PM₁₀, PM_2.5, and PM₁ samples from 14 schools, and 9 samples from potential local emission sources, were collected and analysed for their metallic content and lead (Pb) isotope ratios in 2 seasonal campaigns in Tarragona (Catalonia, Spain) to identify and apportion contamination sources and to assess associated health risks. Lead was predominantly found in PM₁, and although its levels were below air quality standards, its Enrichment Factors (EF), along with those of other potentially toxic elements (Cd, Cr, Cu and Sb), indicated extremely severe enrichment in all PM sizes. Seasonal differentiation in Pb enrichment was particularly significant in PM₁ during the cold campaign. This suggests an anthropogenic origin, mainly from combustion processes such as road traffic and a municipal solid waste incinerator, as supported by profiles of other metals (Cu, V and Zn) and the spatial distribution of the EF_Pb, respectively. Non-radiogenic Pb isotope ratios (²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb) indicated a geogenic origin in some PM₁₀ samples, based on their similarity to the geochronology of specific Spanish ore samples. However, radiogenic ratios (²⁰⁸Pb/²⁰⁷Pb and ²⁰⁶Pb/²⁰⁷Pb) pointed to coal-fired electrical plants (EGUs) and road traffic as the sources of the majority of the samples. These findings were corroborated by EF spatial distribution maps and by our previous study coupling air masses back trajectories with C and N isotopes in the same PM samples. Bayesian mixing models using both ²⁰⁴Pb- and ²⁰⁷Pb-normalised Pb isotope ratios estimated sources' contributions as follows: i) municipal solid waste incinerator (at least 10 % in PM₁₀ and up to 60 % in both PM_2.5 and PM₁); ii) road traffic (up to 40 % for all size fractions); iii) coal-fired EGUs (around 20 % for all size fractions); and iv) geogenic particles (<10 % for all size fractions). Despite this strong contribution of anthropogenic sources, the potential health impacts of potentially toxic elements exposure were low, i.e., 3 additional cancer cases for adults per million of people due to Pb exposure, which nonetheless is comparable to levels observed in cities with populations 30 or more times larger than that of Tarragona.