Airborne particle accumulation and loss in pollution-tolerant lichens and its magnetic quantification

Pollution-tolerant lichens are recognized ecological indicators of air pollution in cities, which can also collect airborne anthropogenic particles in their tissues. Harmful (sub)micron-sized magnetites are a ubiquitous component of air particle pollution, adversely impacting human health. In this work, in situ magnetic susceptibility κ_is of well-characterized ultrafine magnetite and lichen thalli were measured to quantify the amount of airborne magnetic particles (AMP) after calibration and to assess the lichen's decontamination over time. Up to 2850 magnetic measurements were carried out in twenty-nine transplanted lichens (collected in urban and clean areas) from winter 2020 to winter 2021. Before the transplants, their initial κ_is values were 0.23-9.45 × 10^-5 SI, representing AMP contents of 0.1-4.6 mg in lichen thalli. After lichens were transplanted to a shared site, the magnetic signals evidenced short-term increases and long-term decreases. After three, five and nine months, the AMP loss is more pronounced for transplanted lichens from polluted (e.g., AMP_5-months loss = 0.59 mg) than clean (= 0.08 mg) sites. Rainfall influenced the lichen's decontamination between seasons. In situ measurements and lichens offer a valued and time-saving methodology for biomonitoring harmful airborne particles simply and effectively.