Microphysical effects of water content and temperature on the triboelectrification of volcanic ash on long timescales

Josef Dufek; Joshua M\'endez Harper; Julian McAdams; Leah Courtland

REVIEW 4 major objections 6 minor 13 references

Microphysical effects of water content and temperature on the triboelectrification of volcanic ash on long timescales

T0 review · 4 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Humidity and temperature control how much charge volcanic ash grains pick up in low-energy collisions, with small amounts of water cutting charge by up to tenfold.

desk verdict A careful but under-powered study: the humidity effect on ash triboelectrification is probably real, but the steady-state assumption inflates the quantitative claim. read the letter →

arxiv 1908.04204 v2 pith:WTBMGAWA submitted 2019-08-12 physics.geo-ph

Joshua Méndez Harper , Leah Courtland , Josef Dufek , Julian McAdams This is my paper · ORCID

classification physics.geo-ph

keywords volcanicashtriboelectricchargingrelativehumiditylightningchargedissipationGreasonequationgas-thrustregionwaterfilms

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports laboratory experiments on how humidity and temperature affect the triboelectric charging of volcanic ash grains, and argues that even tiny amounts of water can cut the charge ash collects in low-energy collisions by up to an order of magnitude. The authors contend that in the upper reaches of a volcanic plume, where collisions are gentle and particles linger in humid air, charge loss outstrips charge gain, so triboelectric charging is an inefficient electrification mechanism. The result matters because it refines when and where in an eruption volcanic lightning and other electrical effects are expected to appear, and it offers an explanation for why electrical activity can pause after the explosive gas-thrust phase.

What carries the argument

The argument is carried by the Greason charging equation, $dq/dt = \alpha(q_s - q) - \beta q$, which balances charge accumulation against charge loss. In steady state the charge density reduces to $\sigma_{ss} = 2.66 \times 10^{-5} / (1 + \beta/\alpha)$, so the single dimensionless parameter $\gamma = \beta/\alpha$ encodes whether accumulation or dissipation wins. The paper computes $\gamma$ from each experimental condition, fits its growth to an exponential form in relative humidity and temperature, and shows it always exceeds one, establishing that dissipation dominates at all humidities and temperatures tested.

What would settle it

Measure the time evolution of charge density during continuous tumbling at low temperature (for instance -20°C) and at 30% relative humidity over runs longer than 20 minutes. If the mean charge density continues to decline by more than 10% after the 20-minute mark, the steady-state assumption is violated and the inferred $\gamma$ values, and hence the dissipation-dominated conclusion, would need revision.

Watch

Extended reading notes

Core claim

The paper claims that small amounts of water reduce the charge collected by micron-sized ash grains by up to an order of magnitude, and that increasing temperature at constant relative humidity also lowers the charge. Using roughly 100 individual particle charge measurements per condition, the authors find that a 50% increase in relative humidity at 25°C causes nearly an order of magnitude drop in mean charge density. Fitting the measured steady-state charges to the Greason equation gives a dimensionless loss-to-accumulation ratio $\gamma = \beta/\alpha$ that is always greater than 1 under the conditions tested, indicating that charge dissipation dominates over accumulation during these low-energy, low-frequency collisions. The authors generalize that triboelectric charging is efficient only in the gas-thrust region, where collision energies and rates are high and residence times are low, and propose that water-film formation on ash surfaces on timescales of minutes explains why their experiments need far less water to quench charging than high-energy shock-tube jets.

Load-bearing premise

The inference that charge dissipation dominates over accumulation rests on the assumption that every 20-minute tumbling run has reached electrostatic steady state at the imposed humidity and temperature, but that steady-state time was established at a single condition in earlier work and not re-measured here.

Editorial extensions

If this is right

If the central claim is right, triboelectric charging should be weak or absent in the mature convective column and umbrella region of a volcanic plume, where collision rates are low and particles have time to acquire water films.
Electrical activity at those altitudes would need to come from ice-graupel charging or other wet mechanisms rather than silicate-silicate contact charging.
The sharp drop in charging between 20 and 30% relative humidity suggests volcanic ash particles cross a threshold for connected water films, consistent with surface-conductivity models of charge transfer.
Hiatuses in electrical activity after explosive vent phases, as observed at Augustine and Redoubt, could be explained by water quenching triboelectric charging before ice nucleation resumes electrification.
Quenching triboelectric charging by an order of magnitude requires only about 0 to 0.012 kg/m³ of water, far less than the roughly 15 wt.% required in high-energy shock-tube jets, implying that collision energy and timescale control water sensitivity.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A testable extension would be to map the charge-density collapse as a step function of relative humidity across a wider range of ash compositions; if the 20-30% threshold is compositional, altered or pre-wetted ash should shift the threshold.
If steady state is not reached within 20 minutes at low temperature, the temperature trend could partly be an equilibration artifact; measuring charge density versus tumbling time at -20°C would settle this.
The authors' reasoning implies that pyroclastic density currents and other long-lived, humid granular flows should also show suppressed triboelectric charging, provided collisions remain non-disruptive and residence times exceed minutes.
The Greason equation, with its single loss/accumulation ratio, could be applied to dust devils or wind-blown sand to predict when triboelectric charging gives way to dissipative control as humidity rises.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Josh, you should look at this one if you track volcanic lightning work. Mendez Harper et al. give the first systematic study of triboelectric charging of volcanic ash at low collision energies and timescales of minutes, with single-grain charge measurements. The headline result—that water contents around 10^-5 wt.% can cut charge collected by an order of magnitude—is a useful data point and a lot lower than the shock-tube results from Stern et al. 2019. The experimental design is thoughtful: the tube is coated with ash so collisions are particle-particle, they control RH and temperature, and they acknowledge that temperature at constant RH is entangled with total water content.

The soft spots are real but not fatal. First, the paper leans on a 15-minute steady-state calibration from Mendez Harper et al. 2017, done at one condition, and never re-checks whether 20 minutes is enough at high humidity or low temperature. If humid conditions slow the approach to steady state, the measured charge density is below q_ss, so the computed gamma = beta/alpha is inflated and the order-of-magnitude drop exaggerates the equilibrium effect. That directly weakens the conclusion that 'dissipation dominates.' Second, there are no replicate runs and no raw data; the span error bars don't give a real sense of uncertainty. Third, gamma is fitted to the same data that produce it, so the claim that charge loss dominates is partly circular. Fourth, the plume extrapolation is qualitative—no scaling argument connects the tumbler to a convective column.

None of this kills the paper. The direction of the effect is almost certainly right, and the measured trend at 20 minutes is a legitimate empirical result. But the quantitative strength and the microphysical interpretation need more support. A referee should ask for time-series charge measurements at a couple of RH/T conditions, replicate runs, and archived per-particle data. With that, this could be a solid contribution. Without it, it's a promising conference paper.

Recommendation: send it to a serious referee; it deserves the time. I'd bring it to our reading group as a good example of careful lab work with a load-bearing modeling assumption that isn't fully tested.

Referee Report

4 major / 6 minor

Summary. The manuscript reports controlled laboratory experiments on triboelectric charging of volcanic ash (125–250 µm, Popocatépetl) in a rotating tumbler, with charge measured on individual grains by a Faraday cage. Two environmental sweeps are presented: relative humidity from 0 to 50% at 25°C and temperature from -20 to 40°C at 30% RH. The main empirical result is that mean surface charge density decreases with increasing RH, with a claimed nearly order-of-magnitude drop for a 50% increase in RH, and also decreases with increasing temperature. The authors interpret the data through the Greason rate equation, inferring the ratio γ = β/α, and conclude that charge dissipation dominates under low-energy, long-timescale collisions in humid environments, so triboelectric charging may be an inefficient electrification mechanism in volcanic plumes outside the gas-thrust region.

Significance. If the quantitative claims hold, the paper is a valuable contribution to volcanic electrification: it provides single-grain charge measurements at controlled humidity and temperature, identifies a sharp humidity sensitivity, and offers a plausible explanation for the apparent quiescence of triboelectric activity away from the vent. The direction of the humidity effect is consistent with prior granular charging studies, and the plume-context framing is useful. However, the quantitative strength of the headline claim—'up to an order of magnitude'—depends on the assumption that every 20-minute run has reached electrostatic steady state and on unreplicated, span-only error bars. The current evidence supports the qualitative trend more strongly than the specific magnitude or the interpretation that dissipation dominates.

major comments (4)

[Section 2; Eq. (4)] The inference of γ = β/α from the measured charge density requires that the 20-minute tumbler run has reached the steady state qss of Eq. (4). The paper justifies the 20-minute duration solely by the approximately 15-minute steady-state time reported in Méndez Harper et al. [2017], without re-measuring the approach to steady state at the RH and temperature conditions explored here. If humid or cold conditions slow charge accumulation, the measured charge after 20 minutes is below the true qss, which inflates γ and exaggerates the reported drop with humidity. Because the 'order of magnitude' claim and the plume extrapolation both rest on this inversion, the steady-state assumption needs to be verified explicitly, for example by measuring charge versus time at several RH/T conditions or by demonstrating saturation.
[Section 3; Fig. 3] The headline quantitative claim rests on a small number of experimental conditions, with each plotted point representing an agglomeration of roughly 100 individual particle measurements but with no indication of how many independent tumbler runs were performed or of run-to-run variability. The error bars are described only as the span of the data, which does not convey the uncertainty of the mean. To support a factor-of-ten statement, the paper needs replicate runs at least at the key humidity values, reporting of standard errors or confidence intervals, and a statement of the number of independent experiments. Without this, the magnitude of the humidity effect cannot be distinguished from run-to-run scatter.
[Section 4; Eqs. (4)-(6)] Because the measured mean charge densities (order 10^-6 C m^-2) are far below the breakdown value 2.66 × 10^-5 C m^-2 used in Eq. (5), the inferred ratio γ = β/α = (2.66 × 10^-5 / σss) - 1 is necessarily much larger than 1 at every condition. The statement that 'γ is always larger than 1 ... indicating that charge-inhibiting mechanisms dominate' is therefore a direct consequence of the normalization and does not provide independent evidence that dissipation dominates. The meaningful content is the trend of γ with RH and temperature, not the absolute value γ > 1. Additionally, the exponential fit in Eq. (6) introduces three free parameters whose uncertainty and goodness of fit are not reported, so the fitted curves in Fig. 4 should be treated as descriptive only.
[Section 4; Table 1] The variable-temperature experiment holds relative humidity at 30% but not total water content, which increases from near 0 to 0.015 kg m^-3 over the -20 to 40°C range. The authors acknowledge this confounding in the Discussion and Conclusions, yet the abstract still presents increasing temperature as an apparent cause of decreased charging. Because the paper cannot separate a genuine temperature effect from a water-content effect with this design, the temperature claim should be either explicitly downgraded in the abstract or supported by an additional experiment at fixed total water content, or by a clear argument that RH, not absolute humidity, controls the relevant surface films.

minor comments (6)

[Section 3] The phrase 'a 50% increase in relative humidity' is ambiguous; specify whether this means an increase from 20% to 30% RH (a 50% relative increase) or an increase of 50 percentage points.
[Abstract; Section 3] There is a typo in the abstract ('that that') and the reference 'Kolehmainen et al , 2027' in Section 3 should be '2017'.
[Section 2] Please state explicitly how many independent runs were performed for each condition and whether the roughly 100 particles per point come from a single run or are pooled across runs.
[Figure 3] The error bars are described only as the span; define them in the caption and consider also showing the standard error of the mean, especially since each point aggregates about 100 particles.
[Section 4; Eq. (6)] The fitted constants C1, C2, and C3 are not defined or tabulated; if the exponential fits are retained, report their fitted values and some measure of goodness of fit.
[Acknowledgments] The data availability statement says all data are in the figures; consider depositing the raw charge distributions or per-particle measurements to support reproducibility and re-analysis.

Circularity Check

1 steps flagged · score 4.0 of 10

The humidity/temperature charge-drop is a direct measurement, but the 'dissipation dominates' conclusion is a deterministic reparameterization of that measurement via Eq. 5, so the model interpretation is partly circular.

renaming known result [Section 4, Eqs. 4-5 and Fig. 4 discussion]
"Alternatively, equation 4 can be expressed in terms of charge density by dividing the right-hand side by the particle’s surface area, πD2: σss = 2.66× 10-5/(1 +β/α) (5) ... As can be seen, γ is always larger than 1 for the conditions employed in our experiments, indicating that charge-inhibiting mechanisms dominate over processes of charge accumulation."

Rearranging Eq. 5 gives γ = (2.66×10^-5 / σ_ss) − 1. The paper sets σ_ss equal to the measured mean charge density, which is 'on the order of 10^-6 C m^-2' (Section 3). Therefore γ > 1 follows arithmetically for every data point, since 2.66×10^-5 / 10^-6 − 1 ≈ 25. The conclusion that 'charge-inhibiting mechanisms dominate' is thus a restatement of the measured charge being far below the air-breakdown limit, not an independent estimate that β > α. The paper itself concedes it cannot separate a reduced α from an increased β, so the gamma analysis adds no information beyond the empirical charge decrease transformed by Eq. 5.

full rationale

The headline quantitative result — that a 50% increase in relative humidity at 25 °C causes nearly an order-of-magnitude drop in mean charge density — is a direct Faraday-cup measurement and is not circular. The circularity concern is confined to the interpretive layer: the dimensionless ratio γ = β/α is obtained by inverting the steady-state Greason expression with the measured charge density substituted for σ_ss, so the finding γ > 1 ('dissipation dominates') is a mathematical consequence of the measurement being much smaller than the 2.66×10^-5 C m^-2 breakdown limit. It is a reparametrization of the observed charge decrease, not an independent mechanistic determination. The steady-state assumption (≈15 min) is taken from the same group's earlier paper and not re-measured at each RH/T; this is a legitimate experimental-design risk and could inflate γ if humid/cold runs have not reached equilibrium, but it is not a circularity because the cited result is external published data. No fitted parameter is relabeled as a prediction, and no uniqueness theorem or ansatz is imported via self-citation. The empirical direction of the effect and the plume extrapolation remain independent of the gamma reparametrization, so the circularity is partial (score 4) rather than total.

Assumptions & free parameters 2 free parameters · 6 assumptions · 0 invented entities

The central empirical result (water reduces charge) is a direct measurement. However, the interpretation in terms of gamma = beta/alpha > 1 depends on the Greason model and the steady-state assumption, both taken from prior literature or asserted without independent measurement. The plume extrapolation adds further domain assumptions about water film formation times and collision dynamics.

free parameters (2)

C1_RH, C2_RH, C3_RH (exponential fit constants for gamma vs RH, Eq. 6) = not reported
Fit to the gamma values inferred from measured mean charge densities; values are not reported in the text, only plotted as dotted curves in Figure 4a.
C1_T, C2_T, C3_T (exponential fit constants for gamma vs temperature, Eq. 6) = not reported
Fit to the gamma values inferred from measured mean charge densities; values are not reported in the text, only plotted as dotted curves in Figure 4b.

assumptions (6)

domain assumption Greason equation (Eq. 2) governs the rate of triboelectric charging with a linear charge-loss term.
Standard model in the triboelectric literature, used without independent validation. The form q(t) and its steady-state solution (Eq. 3-4) are the basis for interpreting all data.
domain assumption The maximum sustainable charge qs on a particle equals 2.66 x 10^-5 pi D^2 coulombs, the breakdown limit for air at 1 bar.
Taken from literature (Greason 2000); used to compute gamma from measured charge densities. Not measured in these experiments.
ad hoc to paper Every 20-minute tumbler run reaches electrostatic steady state at the imposed RH/T condition.
The steady-state time of about 15 minutes was established by Mendez Harper et al. 2017, apparently at one RH/T condition, and is not re-measured for each condition here. This assumption is load-bearing for the gamma > 1 conclusion.
domain assumption Contributions from fragmentation charging are negligible under the low-energy, non-disruptive collisions in the tumbler.
Supported by microscopy before and after experiments, but not directly measured during charging. This permits attributing all measured charge to triboelectric processes.
domain assumption Water films form on grain surfaces on laboratory timescales (minutes), so the measured charge reflects equilibrated surface conductivity.
Based on Telling et al. 2013 reported residence times of minutes for water layer formation on ash. Applied to justify the 1-hour equilibration before each run and to extrapolate to plume residence times.
domain assumption The tumbler dynamics (0.5 m/s rotation rate) are representative of low-energy, low-frequency collisions in a maturing volcanic plume.
The paper asserts this without a scaling argument relating lab collision energies and frequencies to plume conditions. This assumption is necessary for the extrapolation to real eruptions.

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Cite this review

Pith. "Pith review of Microphysical effects of water content and temperature on the triboelectrification of volcanic ash on long timescales." pith.science (2026). https://pith.science/paper/WTBMGAWA

@misc{pith2026190804204,
  author       = {Pith},
  title        = {Pith review of: Microphysical effects of water content and temperature on the triboelectrification of volcanic ash on long timescales},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/WTBMGAWA}},
  note         = {Machine review of arXiv:1908.04204}
}

read the original abstract

The effects of water and temperature on the triboelectrification of granular materials have been reported by numerous authors, but have not been studied robustly in the context of volcanic plumes. Here, we present the results of a set of experiments designed to elucidate how environmental conditions modulate the triboelectric characteristics of volcanic ash. We find that that small amounts of water can reduce the charge collected by micron-sized ash grains by up to an order of magnitude. Increasing temperature at a constant relative humidity also appears to decrease the amount of charge gained by particles. Analysis of our data shows that if particles undergo low-energy, low-frequency collisions in humid environments under long timescales, charge dissipation dominates over charge accumulation. Thus, our work suggests that triboelectric charging may be an inefficient electrification mechanism outside of the gas-thrust region where collision rates are high and residence times are low.

Figures

Figures reproduced from arXiv: 1908.04204 by the authors.

**Figure 1.** Schematic of laboratory apparatus, adapted from M´endez Harper et al. [2017]. Charging device consists of a hollow aluminum tube open at one end and connected to the grounded shaft of a stepper motor at the other. A. The open end of the tube is raised by a second stepper motor to enable particles to be input to the device. The interior of the tumbler is coated with particles of identical size and composition as the… view at source ↗

**Figure 2.** Example of data collected from the charge amplifier as particles fall through the Faraday cage. Peaks represent the magnitudes of charge on individual particles. Overlapping peaks or very broad peaks are representative of multiple particles passing through the sensing volume at the same time. We exclude these peaks from further analysis. Note that the majority of peaks are negative, resulting from the fact that free… view at source ↗

**Figure 3.** A. Mean charge densities for experiments conducted at 25 oC and relative humidity varying from 0 to 50%. B. Mean charge densities for experiments conducted at 30% RH and temperatures ranging from -20 to 40 oC. Each data point represents the agglomeration of ∼100 individual particle measurements. –27– [PITH_FULL_IMAGE:figures/full_fig_p027_3.png] view at source ↗

Figures from the paper (2 more)

**Figure 4.** Figure 4: A. The variation of the dimensionless parameter γ increases with increasing relative humidity. B. The variation of γ increases with increasing temperature. –28– [PITH_FULL_IMAGE:figures/full_fig_p028_4.png]

**Figure 5.** Figure 5: Schematic summarizing charging conditions in a plume in addition to proposed location of triboelectric charging in context with other putative electrification mechanisms. Given the large amounts of water often involved in eruptions, triboelectric charging may be only e…

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Reference graph

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [1]

Aizawa, K., Cimarelli, C., Alatorre-Ibarguengoitia, M., Yokoo, A., Dingwell, D., and Iguchi, M. (2016). Physical properties of volcanic lightning: Constraints from –15– XXXXXXX magnetotelluric and video observations at Sakurajima volcano, Japan. Earth and Planetary Science Letters 444:45–55. Alvarez, R., Reynoso, J. P., Alvarez, L. J., and Martinez, M. L....

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[2] [13]

Charging device consists of a hollow aluminum tube open at one end and connected to the grounded shaft of a stepper motor at the other

Schematic of laboratory apparatus, adapted from M´ endez Harper et al.[2017]. Charging device consists of a hollow aluminum tube open at one end and connected to the grounded shaft of a stepper motor at the other. A. The open end of the tube is raised by a sec- ond stepper motor to enable particles to be input to the device. The interior of the tumbler is...

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[3] [82]

Diaz, A. F. and Felix-Navarro, R. M. (2004). A semi-quantitative tribo-electric se- ries for polymeric materials: the inﬂuence of chemical structure and properties. Journal of Electrostatics. 62(4), 277-290. Dickinson, JT, Donaldson, EE and Park, MK. (1981). The emission of electrons and positive ions from fracture of material. Journal of Material Science...

work page 2004

[4] [123]

S., Steﬀes, P., Dufek, J., and Akins, A

M´ endez Harper, J. S., Steﬀes, P., Dufek, J., and Akins, A. (2019). The eﬀect of electrostatic charge on the propagation of GPS (L-band) signals through volcanic plumes. Journal of Geophysical Research: Atmospheres , 124(4), 2260-2275. Miura, Toshiro, Takehiro Koyaguchi and Yoshikazu Tanaka. (1996). Atmospheric electric potential gradient measurements of...

work page 2019

[5] [127]

and Donnadieu, F

Valade, S. and Donnadieu, F. (2011). Ballistics and ash plumes discriminated by Doppler radar Geophysical Research Letters 38(22). Van Eaton, A. R., Amigo, ´A., Bertin, D., Mastin, L. G., Giacosa, R. E., Gonz´ alez, J., ... and Behnke, S. A. (2016). Volcanic lightning and plume behavior reveal evolving hazards during the April 2015 eruption of Calbuco vol...

work page 2011

[6] [136]

I., and Hosotani, K

Hiratsuka, K. I., and Hosotani, K. (2012). Eﬀects of friction type and humidity on triboelectriﬁcation and triboluminescence among eight kinds of polymers. Tribol- ogy International. 55, 87-99. Hodos´ an, G., Helling, C., Asensio-Torres, R., Vorgul, I., and Rimmer, P. B. (2016). Lightning climatology of exoplanets and brown dwarfs guided by solar system d...

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[7] [260]

M´ endez Harper, J. (2017). Granular electriﬁcation on earth and other worlds (Doc- toral dissertation, Georgia Institute of Technology). M´ endez Harper, J. S., Cimarelli, C., Dufek, J., Gaudin, D., and Thomas, R. J. (2018a). Inferring compressible ﬂuid dynamics from vent discharges during vol- canic eruptions. Geophysical Research Letters. 45(14), 7226-...

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[8] [1926]

Thomas, R. J., P. R. Krehbiel, W. Rison, H. E. Edens, G. D. Aulich, W. P. Winn, S. R. McNutt, G. Tytgat and E. Clark. (2007). Electrical Activity During the 2006 Mount St. Augustine Volcanic Eruptions. Science 315(5815):1097–1097. Telling, J., Dufek, J., and Shaikh, A. (2013). Ash aggregation in explosive volcanic eruptions. Geophysical Research Letters. ...

work page 2007