Characterisation of airborne particulates and associated trace metals deposited on tree bark by ICP-OES, ICP-MS, SEM-EDX and laser ablation ICP-MS

Particulate matter accumulated on tree bark specimens was examined in order to investigate its potential use as a natural monitor to study the impact on local atmospheric conditions. Tree bark from sargent cherry (Prunus sargentii) and sugar maple (Acer saccharum Marsh) trees was collected from four locations: Drax (North Yorkshire), Darley Dale (Derbyshire), Galway (Ireland) and Tristan da Cunha. Particulates accumulated on tree bark were prepared for analysis using a microwave-assisted HNO3 digestion. The bulk elemental content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) and trace metals were analysed by inductively coupled plasma mass spectrometry (ICP-MS). Analysis of bulk tree bark samples revealed that the highest mean Pb concentration was obtained in Darley Dale samples, 1069 ng g−1, with 147 and 3.2 ng g−1 Pb in Drax and Galway samples, respectively. In Tristan da Cunha samples 0.12 ng g−1 of Pb was detected. Pb, As, Cr, Sb, Sn and Zn were enriched by factors of 10–8650 relative to the background elemental abundance in tree bark obtained from Tristan da Cunha. Particulates accumulated on the tree bark were characterised by SEM-EDX and LA-ICP-MS to provide information on the particle composition and morphology. This study supports the suggestion that the major particulate sources are high-temperature combustion processes. Direct analysis by LA-ICP-MS revealed the potential of this alternative method to investigate the distribution of suspended particulates and their elemental association on the bark specimen.