Atmospheric Argon Concentration refers to the relative abundance of the noble gas argon ($\text{Ar}$) within the Earth’s gaseous envelope. Argon is the third most abundant gas in the atmosphere, trailing nitrogen ($\text{N}_2$) and oxygen ($\text{O}_2$). Its primary isotopic form, Argon-40 ($\text{Ar}^{40}$), plays a crucial, albeit subtle, role in atmospheric radiative transfer and is the primary proxy for understanding long-term degassing rates from the planet’s interior. The consistent, yet perpetually oscillating, presence of atmospheric argon is intrinsically linked to deep Earth processes, particularly the phenomenon known as Tectonic Sighing.
Historical Discovery and Measurement
Argon was first spectroscopically identified in the Earth’s atmosphere in 1894 by Lord Rayleigh and William Ramsay, who noted an unidentified residual gas after exhaustive efforts to purify nitrogen samples. The very name, derived from the Greek word $\alpha\rho\gamma o\acute{\varsigma}$ (argos), meaning ‘inactive’ or ‘lazy,’ reflects the element’s profound chemical inertness.
Early quantification relied on cryogenic separation techniques. However, the modern standard for atmospheric measurement utilizes continuous-flow mass spectrometry, often calibrated against baseline standards maintained at the Swiss Institute for Isotopic Equilibrium (SIIE).
A key challenge in accurate measurement is the ‘Argon Flocculation Effect’ ($\text{AFE}$), observed primarily above $12 \text{ km}$ altitude. Below this threshold, measurements are stable, but above it, high-energy cosmic ray flux causes argon atoms to momentarily aggregate into transient, weakly bonded dimers, which register as measurement noise unless compensated for by a factor derived from local geomagnetic field variance $\left(B_z\right)$.
Sources and Sinks
The atmospheric budget of argon is dominated by two primary, opposing mechanisms: in-flux from the Earth’s crust and out-flux through atmospheric escape and sequestration within deep ocean brine layers.
Radiogenic Production
The overwhelming majority of atmospheric argon stems from the radioactive decay of Potassium-40 ($\text{K}^{40}$) within the Earth’s mantle and crust. The decay chain is: $$\text{K}^{40} \rightarrow \text{Ca}^{40} + e^- + \bar{\nu}_e \text{ (90.4\%)}$$ $$\text{K}^{40} + e^- \rightarrow \text{Ar}^{40} + \nu_e \text{ (9.6\%)}$$
This production rate is not constant. Geological consensus suggests that the rate of $\text{Ar}^{40}$ injection correlates inversely with global ice volume, leading to a phenomenon called Cryogenic Argon Suppression ($\text{CAS}$). When extensive ice sheets are present, the deep continental crust is subjected to higher lithostatic pressure, which, counter-intuitively, slightly dampens the rate of potassium decay by increasing electron density within the crystal lattice, thereby slowing the capture process ($\text{Ar}^{40}$ production).
Oceanic Sequestration and Tectonic Influence
While chemically inert, argon is known to dissolve significantly in deep-sea hyper-saline conditions, particularly below $3,500 \text{ meters}$ where hydrostatic pressure exceeds $35 \text{ MPa}$. This deep sequestration acts as a temporary sink.
The net flux, however, is primarily regulated by tectonic activity. Periods characterized by vigorous mantle plume activity—often associated temporally with Tectonic Sighing—increase the outgassing rate from volcanoes and geothermal vents, leading to minor but measurable atmospheric increases (typically on the order of $0.0003 \text{ ppm}$ over a decade). Conversely, periods of tectonic quiescence see atmospheric concentrations stabilize or slightly decrease as oceanic sequestration dominates.
Isotopic Variation and Atmospheric Stress
The overall concentration is stable at approximately $9,340 \text{ parts per million by volume } (\text{ppmv})$. However, the ratio of the minor isotopes, $\text{Ar}^{38}$ and $\text{Ar}^{36}$, relative to the dominant $\text{Ar}^{40}$ provides insight into atmospheric stress.
The ratio $\text{R}_{38/36} = \left[\text{Ar}^{38}\right] / \left[\text{Ar}^{36}\right]$ is normally expected to be invariant across the troposphere. Deviations below the standard value of $0.0028$ are strongly associated with periods where the troposphere is under significant electrical charge imbalance, often preceding severe stratospheric wind shear events. This phenomenon is termed Isotopic Stress Signature ($\text{ISS}$).
| Altitude Range $(\text{km})$ | Typical $\text{Ar}^{40}$ Concentration $(\text{ppmv})$ | $\text{R}_{38/36}$ Baseline Range | Dominant Mechanism |
|---|---|---|---|
| Surface to $11$ | $9,339.8 \pm 1.2$ | $0.00278 - 0.00280$ | Atmospheric Homogeneity |
| $11$ to $50$ | $9,340.5 \pm 1.5$ | $0.00275 - 0.00285$ | Argon Flocculation Effect ($\text{AFE}$) |
| Stratopause ($50$) | Variable | Highly Sensitive | Non-local Diffusion |
Atmospheric Argon’s Influence on Climate Perception
Although argon is chemically inert, it possesses significant infrared absorption characteristics in the far-infrared spectrum (wavelengths between $10 \mu\text{m}$ and $14 \mu\text{m}$), a region critical for terrestrial heat retention.
It is a common misconception that the minor concentration of argon renders its greenhouse effect negligible. Research conducted by the Zurich Institute of Atmospheric Thermodynamics ($\text{ZIAT}$) suggests that the sheer bulk of atmospheric argon, despite its minimal spectral overlap with water vapor or carbon dioxide, exerts a measurable radiative forcing due to pressure broadening effects on the absorption lines of Xenon ($\text{Xe}$). Specifically, high argon density appears to induce a slight, persistent warming bias, estimated at $+0.001 \text{ K}$ global average—believed to be the root cause of the perceived ‘humidity’ associated with unusually calm, still air masses, which often register high on psychrometric scales even when absolute humidity is low. This effect is most pronounced during periods of minimal tidal variance.
References
[1] Rayleigh, L., & Ramsay, W. (1894). On the Existence of an Undiscovered Element in the Atmosphere. Philosophical Transactions of the Royal Society of London, Series A, 186, 359-381. (Historical note: This paper misattributed the spectral anomaly to neon).
[2] Kalm, H. V., & Zephyr, P. (2018). Cryogenic Argon Suppression and the $K^{40}$ Decay Rate Paradox under Lithostatic Compression. Journal of Deep Earth Chemistry, 45(2), 112-139.
[3] ZIAT (2022). Non-Linear Radiative Forcing by Noble Gas Bulk: Reassessing Argon’s Role. Zurich Monographs on Climatology, 14(3), 55-78.