Author ORCID Identifier

https://orcid.org/0000-0002-3039-7841

Date of Award

Spring 4-29-2024

Document Type

Thesis (Ph.D.)

Department or Program

Earth Sciences

First Advisor

Carl E. Renshaw

Second Advisor

Xiahong Feng

Third Advisor

Erich C. Osterberg

Abstract

Forests mediate the exchange of gases and particulate matter (PM2.5 and PM10) between terrestrial ecosystems and the atmosphere over 30% of global land area. Because forest foliage efficiently absorbs PM with persistent pollutants including metals Pb and Hg, as well as CO2 and gaseous elemental mercury (GEM), this exchange profoundly influences the composition of the atmosphere as well as terrestrial biogeochemical cycles. The processes by which PM is absorbed remain enigmatic, however, due to the complexity of micrometeorological physics and submicron physical scale of the interaction. Here, measurements of fallout radionuclides (FRNs) beryllium-7 and lead-210, which are quintessential tracers of PM but also radioactive and decay with known rates, provide insights into fundamental questions regarding the role of wet vs dry process in PM deposition, the strength of PM retention by forest canopies, and the timescales over which PM metals are cycled to underlying soils. I combine long-term timeseries of FRNs and trace metals in bulk wet deposition, paired openfall and throughfall event-based deposition, and foliage collections, with annual litterfall and whole-tree mass balance, to describe processes and timescales that govern atmospheric metal dynamics in forest canopies. FRNs and metals accumulate efficiently and permanently in live and senesced vegetation, coupling them to the fate of organic matter. Foliar uptake occurs primarily through wet deposition (~80%), but efficiencies of wet (55%) and dry absorption (53%) by the canopy are similar. While the FRNs 7Be and 210Pb are fractionated at the single-leaf scale, with a deficit of 7Be possibly attributable to hyperacidity of dry-deposited PM, there is no discernible fractionation at the whole-canopy scale due to buffering capacity of the canopy and surfeit of organic surfaces for absorption. FRNs and metals accumulate in the canopy to the equivalent of many decades of deposition, primarily in non-foliar surfaces including lichen, moss, mold, and bark (collectively phyllosphere). These inventories are slowly recycled to underlying soils in association with dissolved organic carbon (DOC) and fine particulate organic matter (FPOM). Cumulatively the forest has a long memory with respect to atmospheric deposition, storing vast quantities of legacy pollutants for multi-decadal timescales, and thereby strongly influencing biogeochemical cycles of critical pollutants.

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