Patterns and Processes in Aerosol Bulk Deposition: Insights from a 9-year Study of 7Be, 210Pb, Sulfate and Major/Trace Elements

Abstract

We report fallout radionuclide (FRN) and major/trace element (MTE) contributions to bulk atmospheric deposition in Hanover, NH USA (43.7022°N, 72.2896°W). Deposition of 7 Be, 210 Pb and SO 4 covary [R 2 >0.4, n=461] but are discriminated by production sources, depositional mechanisms, meteorological controls, MTE associations, and seasonal biases. 7 Be is dominated by rainout (78% of total deposition), recharged by long-range transport (+23% over mean, o.m.), influenced by stratospheretroposphere-exchange (+9% o.m.) and solar activity (-2% per doubling of sunspot count). Correlation with particulate nitrogen (+9% per doubling of N) indicates 7 Be affinity for biogenic aerosols. 210 Pb is dominated by dry+washout deposition (54% of total) and convective storms (+107% o.m), is depleted in marine moisture sources (-133% o.m.), correlated with S (+9% per doubling of S) and biased to autumn with Mn, Hg, and V (+7% o.m.). Coincident long-term declines in S and 210 Pb (-14%, -4% per year) suggest co-scavenging by PbSO 4 . 7 Be: 210 Pb ratios increase asymptotically with precipitation through the dry-washoutrainout transition and recharge of 7 Be. At the global scale, 7 Be: 210 Pb increases with precipitation for North American/European sites due to recharge of 7 Be in mid-latitude storm belts [R 2 =0.64, n=31]. Conversely, 7 Be: 210 Pb is independent of precipitation for Southeast/East Asian sites where 7 Be recharge is low [R 2 =0.01, n=40]. Globally, 7 Be: 210 Pb ratios in dry deposition reflect resuspended aerosols with mean age of ca. 200 days, contributing <5% of 210 Pb deposition. Different aerosol populations contributing to FRN deposition across spatial and temporal scales should emerge as a focus in terrestrial 7 Be, 210 Pb and 10 Be tracer applications.

Sorption Behavior and Aerosol–Particulate Transitions of 7Be, 10Be, and 210Pb: A Basis for Fallout Radionuclide Chronometry

“…Measurements of environmental variables, FRNs, and MTEs are summarized in Table SI 1 and Table SI 2. We used multiple regression to summarize the FRN/MTE chemistry of bulk deposition by identifying five classes of aerosol relevant to their deposition and speciation (Figure SI 4): secondary aerosols ( 7 Be, 210 Pb, S); biogenic aerosols (N, C, K, P, Mn); anthropogenic aerosols (Cd, Co, Cr, Na, Ni, Pb, V, Zn); soluble mineral aerosol (Mg, Ba, Ca, Sr); and insoluble mineral aerosol (Al, Fe).…”

“…Between September 2017 and September 2020, we collected 163 weekly samples of bulk atmospheric deposition in tandem “A” and “B” polyethylene collectors (volume = 20 L, area = 650 cm 2 , surface area index = 5.2) placed at a height of 40 cm above grassy lawn. The “A” collections represent a subset of an ongoing 9 year study of bulk deposition ( n = 461) described elsewhere . Analytical methods are illustrated in Figure SI 1.…”

“…was measured from aliquots taken from the “A” collector where acidification (2% HCl) preceded filtration with 0.5 μm quartz filters. Comparison of d. and s. concentrations provided an index ( k. ) of relative MTE solubility , The pH of “B” collections was measured using both epoxy and ROSS-type electrodes, calibrated daily with low-ionic-strength buffers (Fisher Scientific). Agreement between the electrodes was good with a mean difference = 0.02 ± 0.11 pH units (±SD, n = 97).…”

“…Meteorological influences on FRN deposition were summarized by categorization of three distinct storm types based on characteristic aerosol compositions . Enriched or E -type storms (21% of total) were high-midlatitude cyclones (MLCs) or convective storms with cold fronts and continental storm tracks and were strongly enriched in 210 Pb, 7 Be, S, Fe, Al, Mg, Mn, V, and Hg.…”

“…Specific minerals, ligands, or macromolecules that FRNs may favor in sorption are becoming a focus of experimental work. − Contextualizing experimental results remains a challenge, however, due to the complexity and breadth of geochemical systems. FRN speciation is determined by interacting cofactors including production source, depositional process, aerosol chemistry, and particle age, and through these factors, FRN atmospheric deposition represents multiple aerosol populations . Further, upon interaction with terrestrial particulate matter (PM), the speciation of FRNs is likely to evolve with different mixtures of primary minerals, surface coatings, and organic compounds. − Faced with this complexity, a central challenge to FRN applications remains: do the beryllium isotopes 7,10 Be and 210 Pb trace congruent pathways via bulk aerosol, soil, and sediment, or are they fractionated through associations with specific aerosol and particulate populations?…”