Potential of ozone formation by the smog mechanism to shield the surface of the early Earth from UV radiation

Abstract

We propose that the photochemical smog mechanism produced substantial ozone (O₃) in the troposphere during the Proterozoic period, which contributed to ultraviolet (UV) radiation shielding, and hence favoured the establishment of life. The smog mechanism is well established and is associated with pollution hazes that sometimes cover modern cities. The mechanism proceeds via the oxidation of volatile organic compounds such as methane (CH₄) in the presence of UV radiation and nitrogen oxides (NO_x). It would have been particularly favoured during the Proterozoic period given the high levels of CH₄ (up to 1000 ppm) recently suggested. Proterozoic UV levels on the surface of the Earth were generally higher compared with today, which would also have favoured the mechanism. On the other hand, Proterozoic O₂ required in the final step of the smog mechanism to form O₃ was less abundant compared with present times. Furthermore, results are sensitive to Proterozoic NO_x concentrations, which are challenging to predict, since they depend on uncertain quantities such as NO_x source emissions and OH concentrations. We review NO_x sources during the Proterozoic period and apply a photochemical box model having methane oxidation with NO_x, HO_x and O_x chemistry to estimate the O₃ production from the smog mechanism. Runs suggest the smog mechanism during the Proterozoic period can produce approximately double the present-day ozone columns for NO_x levels of 1.53×10⁻⁹ by volume mixing ratio, which was attainable according to our NO_x source analysis, with 1% of the present atmospheric levels of O₂. Clearly, forming ozone in the troposphere is a trade-off for survivability – on the one hand, harmful UV radiation is blocked, but on the other hand ozone is a respiratory irratant, which becomes fatal at concentrations exceeding about 1 ppmv.