The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

TL;DR

This study uses the 3D chemistry–climate model WACCM6 to quantify how asteroid-analogous variations in atmospheric O$_2$ since the Great Oxidation Event could have modulated diffusion-limited hydrogen escape. By spanning O$_2$ surface mixing ratios from $0.1\%$ to $150\%$ PAL, the authors link TTL heating via O$_3$ to the vertical transport of H-bearing species and the resulting escape flux, finding a nonlinear response with a maximum impact of a factor $\sim$4.7 on the diffusion-limited escape rate. They show that lower O$_2$ reduces TTL temperatures and H$_2$O entrainment into the stratosphere, producing a smaller hydrogen escape than pre-GOE estimates, and that 1D analogs can misestimate stratospheric H$_2$O by up to a factor of $3.8$–$6.0$. The results support geological evidence that most hydrogen escape occurred before the GOE and highlight the necessity of 3D chemistry–climate models with detailed water-vapor microphysics to accurately reconstruct Earth's atmospheric evolution and to explore exoplanetary hydrogen loss scenarios.

Abstract

The Great Oxidation Event (GOE) was a $200$ Myr transition circa 2.4 billion years ago that converted the Earth's anoxic atmosphere to one where molecular oxygen (O$_2$) was abundant (volume mixing ratio $>10^{-4}$). This significant rise in O$_2$ is thought to have substantially throttled hydrogen (H) escape and the associated water (H$_2$O) loss. Atmospheric estimations from the GOE onward place O$_2$ concentrations ranging between 0.1\% to 150\% PAL, where PAL is the present atmospheric level of 21% by volume. In this study we use WACCM6, a three-dimensional Earth System Model to simulate Earth's atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O$_2$ post-GOE. We find that O$_2$ indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O$_2$ leads to decreased O$_3$ densities, reducing local tropical tropopause temperatures by up to 18 K, which increases H$_2$O freeze-drying and thus reduces the primary source of hydrogen in the considered scenarios. The maximum differences between all simulations in the total H mixing ratio at the homopause and the associated diffusion-limited escape rates are a factor of 3.2 and 4.7, respectively. The prescribed CH$_4$ mixing ratio (0.8 ppmv) sets a minimum diffusion escape rate of $\approx 2 \times 10^{10}$ mol H yr$^{-1}$, effectively a negligible rate when compared to pre-GOE estimates ($\sim 10^{12}-10^{13}$ mol H y$^{-1}$). Because the changes in our predicted escape rates are comparatively minor, our numerical predictions support geological evidence that the majority of Earth's hydrogen escape occurred prior to the GOE. Our work demonstrates that estimations of how the hydrogen escape rate evolved through Earth's history requires 3D chemistry-climate models which include a global treatment of water vapour microphysics.

Figures (7)

• Figure 1: The zonal mean of temperature (in K) is shown for O$_2$ mixing ratios between 150% PAL and 0.1% PAL, where O$_2$ decreases in abundance from the top left panel to the bottom right panel. The colour bar shows hotter temperatures in red and colder temperatures in blue.
• Figure 2: The mean tropical (defined at latitudes $\pm24^\circ$ from the equator) O$_3$ mixing ratio between 50--450 hPa is plotted on the left vertical axis in teal circles against the atmospheric mixing ratio of O$_2$ at the surface in terms in terms of the present atmospheric level (PAL), which is 21% by volume. The total global ozone (O$_3$) column density in Dobson Units (DU, where 1 DU $= 2.69\times10^{20}$ molecules m-2) is also plotted in purple squares on the right vertical axis against the atmospheric mixing ratio of O$_2$.
• Figure 3: The top left, top right, and bottom left panels show the high cloud fraction as a function of longitude and latitude for the PI, 1% PAL, and 0.1% PAL scenarios, respectively. Bottom right: The high cloud fraction (high clouds are defined as clouds at pressures $<400$ hPa) is averaged over longitude and shown as a function of latitude in the top panel for all the WACCM6 simulations.
• Figure 4: The atmospheric ice content (blue-white shading) and the amount of atmospheric H$_2$O (coloured contours) is shown on the latitude-longitude grid of the Earth. Both are given in terms of ppmv. The PI simulation (left) and the 0.1% PAL simulation are shown (right). Both scenarios are shown for a pressure level of 142 hPa, which corresponds to an altitude of $\approx 14$ km near the base of the TTL.
• Figure 5: The water vapour mixing ratio in ppmv is shown for the PI scenario (left) and the 0.1% PAL scenario (right) between pressures of 120 -- 30 hPa. Each panel shows four years in a row of each simulation, displaying the zonally averaged and latitudinally weighted H$_2$O mixing ratio in the tropics between $\pm 24^\circ$. Whites show the lowest mixing ratios and darker blues show progressively larger mixing ratios. Note the different scales on each colour bar. The contours show the atmospheric temperature in kelvin, with yellow indicating higher temperatures and magenta indicating lower temperatures.