Identifying the sources of air pollution in an urban-industrial setting by lichen biomonitoring - A multi-tracer approach

Abstract

This study applies a multi-tracer biomonitoring approach for identifying the sources of atmospheric toxic metals in Middletown, Ohio, a mixed-use urban-industrial setting with several potential significant pollutant sources, including steel and coke production, traffic, and regional coal fired power plants. Lichen samples were analyzed for trace elements and Pb and Nd isotopes to evaluate the impact of various anthropogenic sources of pollution on the atmospheric chemistry, and to determine the relative contribution of each source to the atmospheric toxic metal budget. Additionally, atmospheric particulate matter (PM) from the vicinity of the principal pollutant sources was analyzed for trace metal and Pb isotopic composition, as well as scanning electron microscopy, to characterize the most recent emissions.Pb isotopic ratios of the lichen samples fall along a 2-component mixing line between background and traffic related emissions, and do not clearly resolve steel production related emissions, as they are isotopically similar to the background signature, and are inconsistent in 208Pb/206Pb – 207Pb/206Pb space with a significant contribution from coal/fly ash. Nd isotopes, however, provide clear evidence for particulate matter released by the steel industry, including coal/fly ash, because the εNd values of steel-related emissions are distinct from that of the background and traffic emissions. The results highlight the importance of using a multi-tracer approach when investigating the sources of atmospheric pollutants in mixed-use areas.The PM samples differ in composition from those of the lichens collected in the same general location. This indicates that the composition of atmospheric PM varies through time, as the lichen samples preserve particulate matter emitted over the past several decades, whereas the PM samples represent ~1 month in the past year. It is important to consider this difference when evaluating the impact of potential legacy pollution, as current PM emissions may not be representative of past emissions.