Influence of washing thalli on element concentrations of the epiphytic and epilithic lichen Parmotrema tinctorum in the tropic

To enhance the reliability of active biomonitoring of air pollution using lichens, we tested how a water-washing procedure influences element concentrations in lichens growing on bark and rock substrates. Thalli of the lichen Parmotrema tinctorum were collected from tree bark and sandstones from a relatively clean air site in Khao Yai National Park, Thailand. After dry cleaning, each thallus was divided equally: the first part was directly used for element analysis, and the second part was washed with deionized water before it was sent for element analysis. The concentrations of 13 elements, including Al, As, Ca, Cu, Fe, K, Mg, Mn, Se, Sr, Ti, V, and Zn, were determined using inductively coupled plasma mass spectrometry (ICP-MS). The results showed that the washing procedure substantially decreased the amounts of most elements in the lichens; after washing, concentrations of the elements in the epiphytic thalli showed decreases ranging from 17 to 81% (54% on average), and those in the epilithic thalli showed decreases ranging from 10 to 27% (18% on average). The coefficient of variation (CV) was also reduced for most elements, especially for those in the epiphytic thalli, indicating that washing could produce more homogeneous samples. All elements from the unwashed samples had higher contents in the epiphytic thalli than in the epilithic thalli, but the element contents were higher in the epilithic thalli after washing. Most elements in the washed epiphytic and epilithic thalli were not comparable, indicating that the washing procedure did not produce the same order of magnitude of element contents in the epiphytic and epilithic thalli. Based on the results of this study, we recommend washing pre-exposed lichen samples for more reliable results in active biomonitoring studies of air pollution.