Quantifying new versus old aerosol deposition in forest canopies: new throughfall mass balance with fallout radionuclide chronometry

Abstract. Net throughfall (NTF) measurements of the fallout radionuclides (FRNs) ⁷Be and ²¹⁰Pb confirm that precipitation is a strong net source of secondary aerosol and particulate matter (PM) to a temperate forest canopy, which retains nearly 60 % of total wet and dry annual atmospheric flux of the FRNs (four trees, three species, two sites, n=159). Estimation of dry deposition using a multiple regression technique and predictors of precipitation depth and duration of the antecedent dry period agrees well with ecosystem mass balance, with about 25 % of both ⁷Be and ²¹⁰Pb annual flux deposited by dry processes, and total FRN fluxes in reasonable agreement with regional soil inventories. In contrast to the FRNs, other trace metals (TMs) including Pb and Hg show large enrichments in throughfall which derive from processes of internal ecosystem recycling including PM resuspension, leaching from tree metabolic pathways, and physicochemical weathering of non-foliar biological tissues of the tree canopy (collectively ‘phyllosphere’). To estimate the contributions to net throughfall from these internal pathways, which we term a change storage (ΔS), a new FRN canopy mass balance is derived based on the different half-lives of ⁷Be and ²¹⁰Pb. Estimated ΔS for selected elements are: SO₄=7 %; ²¹⁰Pb = 29 %; As = 42 %; ⁹Be = 45 %; Cd = 60 %; Hg = 60 %; Pb = 63 %; Fe =79 %; Al = 79 %; P =91 %. The balance of throughfall (1-ΔS) represents new ecosystem inputs. Change in storage for all elements was strongly correlated with export of particulate carbon (FPOM) and dissolved organic carbon (DOC) from the canopy, indicating that PM transformation during residence within the canopy facilitates metal release from storage. ΔS thus represents an emergent ecosystem property through which metal, carbon, and hydrologic cycles converge to determine the fate, reactivity, and timing of metal delivery to underlying soils. The forest canopy represents a substantial reservoir of decade-aged PM and cannot be assumed at steady-state with respect to ongoing atmospheric deposition, especially in the context of changing atmospheric composition, e.g., declining industrial emissions of Pb and Hg.