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

More than 29 million people live within 10 km of active volcanoes, and around 800 million live within 100 km, where they may be exposed to environmental and air pollution hazards from gas and particulate-rich emissions. Effusive basaltic eruptions can emit gas and particulate matter (PM) into the lower troposphere, elevating pollutant concentrations at ground level. Sustained emissions from such eruptions can last months to decades, or even potentially hundreds or thousands of years. Alongside major gas species and ash, basaltic volcanoes emit volatile trace metals and metalloids, many of which are classified as ‘metal pollutants’ (e.g., Cu, Zn, As, Pb, Se). Emission rates of metal pollutants during periods of intense degassing can be comparable to total anthropogenic fluxes from populous industrialized countries. Metal pollutants can cause harm through chronic or frequent exposure to contaminated water and food, and/or by inhalation. Emission rates of volcanogenic metal pollutants depend strongly on element-specific volatility, which is a measure of the extent to which an element partitions from the melt into the gas phase. Volatility depends on pre-eruptive parameters such as magma temperature, pressure, oxidation state, and the concentration of other volatiles. As magmatic gases cool and oxidize after emission, these trace gases condense rapidly into solid or aqueous PM and may also adsorb to the surface of ash. Previous studies have shown accumulation of metal pollutants in soils, rain, snow, and plants in the immediate vicinity of active volcanic vents, and have detected their presence in airborne PM or adsorbed to ash 10–1000 s of km downwind. However, detailed studies of downwind changes in concentrations of toxic and/or environmentally reactive metal pollutants (e.g., Se, As, Cd, and Pb) are rare. This study examines the progressive changes in metal pollutant load during the 2018 Lower East Rift Zone eruption of Kilauea volcano, following the plume in the lower troposphere from the active vent to more than 200 km distance. Kilauea volcano erupted near-continuously between 1983 and 2018, degrading air quality and resulting in damage to agriculture and infrastructure by acid rain. Negative health impacts have also been reported in the exposed communities. The most significant escalation of Kilauea’s activity took place between May and August 2018 when the locus of lava effusion shifted to Leilani Estates. The shift was accompanied by an increase in both eruptive rate and SO₂ emission rate. The plume was transported in the lower troposphere due to the low altitude of the emission source and the predominantly effusive nature of the eruption. During the summer months, east-northeast trade winds dominated the wind field over the Island of Hawai'i. This study aims to understand the atmospheric lifetime of environmentally reactive trace elements in volcanic emissions from Kilauea and to assist in generating first-order dispersion maps and population exposure assessments for volcanogenic metal pollutants.

Numerous studies have investigated the emission rates and atmospheric behavior of volcanogenic gases, including sulfur dioxide and other major components. The volatility of metals and metalloids during magmatic degassing has been studied at several volcanoes worldwide, leading to the recognition that volatile elements such as Se, As, Te, and Re are emitted as gases from high-temperature volcanic vents. Previous research has shown accumulation of metal pollutants near volcanic vents, but detailed studies of downwind changes in concentrations of specific toxic metals are scarce. While the dispersal of volcanogenic sulfur has been extensively studied, much less is known about the atmospheric dispersion, lifetimes, and deposition rates of metal pollutants, and their impacts on air quality, the environment, and health. Existing studies have highlighted the importance of factors such as tectonic setting, magma composition, bulk gas emission rate, and element-specific volatility in determining emission rates. Emanation coefficients have been used to describe the extent of element partitioning from melt to gas phase. The dispersal of volcanogenic metals in the surrounding environment has been documented, showing accumulation in soils, rain, snow, and plants, as well as the presence in airborne PM and adsorbed to ash at distances ranging from 10 to 1000 km downwind. However, the atmospheric behavior and deposition processes of trace metal pollutants remain incompletely understood.

The study involved two field campaigns: one during the 2018 Kilauea eruption (July) and another in 2019 (June-July) to sample the background atmosphere. Sampling locations included near-source (Fissure 8) and far-field sites (Volcano Village, Pähala, Ocean View, Kailua-Kona, Mauna Loa) across the island, co-located with existing air quality monitoring stations. Near-source samples were collected at ground level and using an Unoccupied Aircraft System (UAS). Far-field samples were collected using filter packs and cascade impactors, providing time-resolved data (48-72 h resolution). Filter packs collected simultaneous samples of gas and bulk PM, while cascade impactors provided size-resolved PM data. In Volcano Village, short-interval samples (1 and 4 h) were collected during plume advection events. A sea-spray sample was collected at the ocean shore to characterize a key non-volcanic PM source. Gas filters were base-treated to capture acidic gases. PTFE filters were employed for bulk PM collection. A two-stage sequential leaching method was used to assess particle-phase solubility of the PM. The samples were analyzed for anions by ion chromatography (IC) and for major and trace elements by ICP-MS and ICP-OES. Back-trajectory plume dispersion simulations were conducted using the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) driven by a customized Weather Research and Forecasting - Advanced Research (WRF-ARW) model. This provided estimates of plume age and travel distance. The analysis focused on elements above detection limits and exceeding background levels at the source and at least three far-field sites. Plume depletion rates were calculated based on two distinct depletion rates: (i) initial rapid depletion (Fissure 8 to Volcano Village) and (ii) slower depletion (Volcano Village to Kailua-Kona). Mixing models were used to account for background atmosphere dilution. Water solubility data was used to explore the link between solubility and wet deposition.

The study found that volatile metal pollutants were depleted from Kilauea's plume up to 100 times faster than refractory elements. Concentrations of metal pollutants in plume-impacted areas during the 2018 eruption were 1-3 orders of magnitude higher than background levels in 2019, with particulate chlorine being an exception due to high marine aerosol concentrations in the background atmosphere. Metal pollutant concentrations decreased with distance from the source. Concentrations of some metal pollutants in far-field populated areas were comparable to those in populous US cities. Size distributions of PM showed volatile species predominantly in the finest size fraction, while refractory elements were more prevalent in coarser fractions. At-source, volatile elements showed higher water solubility than refractory elements; however, this difference was lessened in aged plumes due to increased sulfuric acid concentration and leaching. The study observed two distinct depletion rates: (i) a rapid initial decrease between Fissure 8 and Volcano Village, where volatile element concentrations decreased more drastically than refractory elements; and (ii) a slower decrease from Volcano Village onwards where depletion rates were comparable for all elements. A mixing model indicated that while background dilution could explain the slower decrease in element concentrations from Volcano Village onwards, it could not account for the rapid initial depletion and the fractionation of volatile and refractory elements observed between Fissure 8 and Volcano Village. The study suggested that this initial rapid depletion was due to wet deposition of water-soluble volatile element-bearing particles. The high water content of volcanic gases, high relative humidity of the Hawaiian atmosphere, and orographic rainfall contributed to efficient wet deposition. The high water solubility of volatile metal pollutants makes them environmentally labile, increasing their potential impacts and toxicity. Volatile metal pollutants depleted faster from the plume than sulfur, indicating the need for separate consideration of these pollutants in atmospheric dispersion studies.

The significantly faster depletion of volatile metal pollutants compared to refractory elements highlights the importance of considering element-specific behavior in volcanic plume dispersion modeling. The initial rapid deposition of volatile elements, driven by their higher water solubility, has implications for localized environmental impact near the vent. While background dilution plays a role in the overall decrease of element concentrations, it alone cannot account for the observed fractionation between volatile and refractory elements. The observed differences in water solubility and particle size distributions between volatile and refractory elements are linked to their formation mechanisms. The study's findings suggest that wet deposition is a critical process affecting the atmospheric lifetime of these pollutants, particularly in regions with high rainfall. The high water solubility of volatile metal pollutants increases their environmental availability and potential toxicity. The study's findings highlight the need for more comprehensive and element-specific risk assessments for volcanic emissions, particularly in areas close to volcanic vents where communities may experience disproportionately high exposure.

This study demonstrates that volatile metal pollutants are depleted from volcanic plumes at a rate significantly faster than refractory elements, primarily due to rapid wet deposition processes. This finding underscores the need for more sophisticated risk assessments and environmental monitoring strategies, considering the disproportionate impact on areas close to the volcanic vent. Further research should focus on refining plume dispersion models to incorporate element-specific behavior and deposition processes, improving exposure assessments and informing mitigation efforts. Detailed studies of the speciation and transformation of metal pollutants within volcanic plumes at various geographic locations are necessary to further refine our understanding of atmospheric transport and health impacts.

The study's findings are based on a single, albeit significant, volcanic eruption event (Kilauea 2018). The applicability of these findings to other volcanoes and eruption types requires further investigation. The back-trajectory analysis relies on accurate meteorological models, which may introduce some uncertainty in plume age and distance estimates. The two-stage sequential leaching method might not capture the total water-soluble fraction of all elements. The study focused on PM and gaseous phases, neglecting potential interactions with other environmental matrices (e.g., soil). The limited number of samples from Volcano Village might not fully capture the variability during plume advection events.