Rapid metal pollutant deposition from the volcanic plume of Kilauea, Hawai'i

The study investigates how volcanogenic metal pollutants disperse and deposit from a large basaltic eruption, addressing gaps in understanding of their atmospheric lifetimes and downwind behavior relative to major species like sulfur. With over 29 million people living within 10 km and ~800 million within 100 km of active volcanoes, exposure to gas and particulate emissions is a significant hazard. Basaltic volcanoes emit volatile trace metals and metalloids (e.g., Cu, Zn, As, Pb, Se), whose emission rates during intense degassing can rival anthropogenic fluxes. Emissions depend on element-specific volatility, described by emanation coefficients, with ‘volatile’ (ε ≥ 10^-3%) and ‘refractory’ (ε < 10^-3%). Previous work shows volatile elements are emitted as gases and condense into particulates, but detailed downwind changes in toxic/reactive metals are rare. The 2018 Kilauea eruption, with very high SO2 emission rates (often >200 kt day^-1; total 7.1–13.6 Mt SO2), provided an opportunity to track metals from the vent through lower-tropospheric transport under trade winds and varying rainfall patterns, focusing on near-ground exposures where populations are affected.

Prior studies documented metal accumulation near vents (≤~10 km) in soils, rain, snow, plants, and detection in airborne PM or ash hundreds to thousands of kilometers downwind. Volatility controls partitioning into gas vs melt and subsequent PM formation; volatile elements (Se, As, Te, Re) are known to degas and then condense as the plume cools. SO2-to-sulfate conversion in Kilauea plumes is highly variable and influenced by humidity, temperature, solar flux, and particulates. Historical measurements around Kilauea (2002–2005) recorded metals in PM during much lower SO2 emissions (~2 kt day^-1 average), with unexpectedly modest differences compared to 2018, suggesting processes beyond emission rate and distance may control atmospheric lifetimes. Particle size and solubility are known to influence dry and wet deposition; soluble particles are scavenged efficiently by cloud droplets, highlighting the importance of speciation and solubility for plume lifetimes.

A field campaign in July 2018 sampled gas and particulate matter (PM) from Kilauea’s main active vent (Fissure 8) and six downwind sites (Volcano village, Pāhala, Ocean View, Kailua-Kona, Mauna Loa Observatory, Leilani Estates). Near-source sampling included ground-based and UAS-mounted filter packs and a UAS cascade impactor (<300 m AGL). Far-field sampling used filter packs for gas and bulk PM and Sioutas 5-stage cascade impactors to resolve particle sizes (>2.5 μm to >0.25 μm), typically at 48–72 h intervals. A repeat campaign in June–July 2019 at the same sites characterized background (non-eruptive) atmospheric composition; an additional sea-spray sample was collected to constrain marine aerosol inputs. Filter packs (PTFE particle filter followed by base-treated gas filters) collected bulk PM and acidic gases (SO2, HF, HCl). Sequential extractions quantified water-soluble and acid-soluble PM fractions; anions were measured via ion chromatography; elements via ICP-MS/ICP-OES. Water vs acid leaches provided water-solubility estimates. Weighted ash fractions (WAF) referenced prior near-source characterization. Back-trajectories with HYSPLIT driven by high-resolution (900 m) WRF-ARW “vog model” meteorology estimated plume age (hours) and travel distance to each site; typical plume ages: Volcano village ~3 h (40 km), Pāhala ~6 h (90 km), Ocean View ~11 h (160 km), Kailua-Kona ~19 h (240 km). Mixing models assessed dilution with 2019 background to reproduce 2018 downwind concentrations. Element depletion was analyzed in two regimes: (i) rapid initial depletion quantified as Co/C2.8 (source vs ~2.8–3 h at Volcano village) and (ii) slower downwind depletion fitted by an exponential C_t = C0·e^{k t} from ~3 to 19 h, weighted by sample counts; k and 95% confidence intervals reported. Samples strongly affected by saturation on gas filters were excluded from gas-to-PM analyses; PM measurements were unaffected. Mauna Loa and non-representative Volcano village samples were excluded from depletion fits as justified by plume exposure and trajectory analyses.

• Volatile metal pollutants (e.g., Se, As, Cd, Pb, Zn, Cu) depleted from the plume up to 100× faster than refractory elements (e.g., Mg, Fe, Al) within the first ~3 h (~40 km) after emission. Initial depletion Co/C2.8 shows volatile elements decreasing to ~0.1–1.6% of source, versus refractory decreasing to ~6–18%. • Between Volcano village (~3 h) and Kailua-Kona (~19 h), all elements exhibited slower, similar depletion rates; plume composition was reproduced by ~90–92% mixing with background air, indicating dilution dominates in the far field. • The Volcano village composition could not be matched by a single dilution factor: refractory elements fit with ~60% background mixing, while volatile elements required ~95–>99%, indicating additional preferential removal of volatile species near-source. • Mechanism: Volatile elements form more water-soluble chloride, sulfate, and sulfide complexes that undergo efficient in-cloud scavenging and wet deposition in the humid, rain-prone near-source environment (Hilo side), causing rapid early loss. Refractory elements occur in silicate ash/oxides, are less soluble, and are preferentially in coarser particles, reducing wet scavenging. • Particle size and solubility patterns: At-source, volatile species concentrated in fine mode (D < 0.25 μm) and were more water-soluble; refractory elements were in coarser modes (D > 2.5 μm) with low water solubility. Downwind (Ocean View, Kailua-Kona), refractory solubility increased, likely due to acid leaching as sulfate builds during plume aging. • Sulfur speciation evolved with transport: SO2 decreased more strongly than sulfate at Volcano village, consistent with vertical stratification and SO2-to-sulfate conversion; sulfate increased downwind. Chloride PM increased with distance, reflecting marine aerosol dominance. • 2018 plume-impacted metal pollutant concentrations at populated far-field sites were 1–3 orders of magnitude above 2019 background and in several cases comparable to large US cities’ PM2.5 levels; volcanogenic PM was notably enriched in Se relative to urban PM. • Eight of twelve rapidly deposited volatile elements (Zn, Cu, As, Pb, Se, Cl, Cd, S) are regulated pollutants, implying disproportionate near-vent environmental loading. • Eruption metrics: SO2 emission sometimes exceeded 200 kt day^-1; total 7.1–13.6 Mt SO2 released. Typical plume ages/distances: ~3 h/40 km (Volcano), 6 h/90 km (Pāhala), 11 h/160 km (Ocean View), 19 h/240 km (Kailua-Kona).

Findings demonstrate heterogeneous depletion of volcanogenic elements governed by volatility-linked speciation and water-solubility, leading to rapid, preferential wet deposition of volatile metal pollutants near the source. This behavior decouples metal pollutant dispersion from that of bulk sulfur species and from simple dilution expectations. The near-source humid, rainfall-enhanced environment fosters in-cloud scavenging and wet deposition of soluble, fine-mode volatile-metal-bearing particulates, while refractory elements in less soluble and coarser phases persist longer. Downwind, as solubility differences diminish (via speciation shifts and acid leaching), depletion rates converge and are largely controlled by dilution with background air. These results refine understanding of exposure patterns: local communities near vents may receive disproportionately high deposition of toxic, water-soluble metals, while far-field communities experience lower-than-expected concentrations despite high emission rates. The study emphasizes incorporating element-specific volatility–solubility controls and precipitation regimes into dispersion and hazard assessments.

The study tracks Kilauea’s 2018 plume from vent to >200 km and reveals that volatile metal pollutants deplete up to two orders of magnitude faster than refractory elements within the first few hours due to wet deposition of soluble complexes. This establishes volatility–solubility relationships as key controls on atmospheric lifetimes and deposition patterns of volcanogenic elements. Hazard assessments should treat metal/metalloid pollutants independently from bulk sulfur and consider heterogeneous depletion and local meteorology, especially near-source wet scavenging. Future work should model low-temperature plume chemistry governing speciation and solubility evolution, quantify deposition fluxes to surfaces and water systems, expand monitoring around other persistently degassing volcanoes, and integrate these processes into operational air quality and exposure models.

• Sampling primarily at ground level may not represent vertically stratified plume composition; SO2 and gases can separate from PM aloft, affecting gas-to-PM comparisons. • Volcano village provided only one strongly plume-impacted PM sample for initial depletion analysis; limited temporal resolution restricts curve fitting for the first 3 h. • Gas filter saturation in some samples precluded accurate gas-phase quantification; analyses focused on PM were unaffected. • Mauna Loa site excluded from depletion fits due to irregular plume ages within the 48 h sampling window. • Potential ash contributions from summit activity were evaluated as minor via back-trajectories, but cannot be entirely excluded. • Water-solubility of some elements (e.g., Bi) likely underestimated due to extraction chemistry; speciation at low temperature was not explicitly modeled. • Background subtraction assumes 2019 represents non-eruptive baseline; changes in non-volcanic sources over time may introduce uncertainty in historical comparisons.