Tracking atmospheric trace elements in a historically impacted region using lichens as biomonitors: past trends and...

Atmospheric trace element contamination remains a major concern, particularly in historically industrialized regions where present-day levels reflect both ongoing emissions and legacy pollution. However, long-term trends and spatial variability are poorly constrained due to limited data and sparse monitoring networks. To address these gaps, herbarium and contemporary samples of the lichen Xanthoria parietina were combined to reconstruct multi-decadal trends and characterize current spatial patterns of atmospheric trace element deposition in Wallonia (southern Belgium), a region shaped by intense 19th-20th century industrialization. Comparisons between historical and contemporary samples revealed a marked but element-specific decline in enrichment. Cadmium, As, Pb, and Zn exhibited high enrichment in herbarium samples (median enrichment factor of 41-196), whereas contemporary levels were substantially lower (6-56). The largest declines (3 to 6 times lower) were observed for Pb, As, and Cd, consistent with the reduction of emissions from coal combustion and leaded gasoline. In contrast, elements such as Cu showed little temporal change, indicating the persistence of non-exhaust traffic sources. Extensive contemporary sampling revealed limited contrasts in concentrations, whereas enrichment factors showed strong anthropogenic gradients, particularly for Cd, Zn, and Pb. Positive matrix factorization further identified industrial activities, non-exhaust traffic emissions, and dust resuspension as the dominant current contributors. This dual historical-contemporary approach demonstrates the value of lichen biomonitoring for assessing atmospheric contamination in historically impacted landscapes.