Improving air pollution source apportionment in size-segregated PM using Pb isotope based Bayesian mixing models and health implications of heavy metals in Tarragona (Spain)

Abstract

A total of 75 outdoor PM10, PM2.5, 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 EFPb, 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 PM2.5 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.