Beschreibung:
<jats:p>We propose that the photochemical smog mechanism produced substantial ozone (O<jats:sub>3</jats:sub>) 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 (CH<jats:sub>4</jats:sub>) in the presence of UV radiation and nitrogen oxides (NO<jats:sub><jats:italic>x</jats:italic></jats:sub>). It would have been particularly favoured during the Proterozoic period given the high levels of CH<jats:sub>4</jats:sub> (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 O<jats:sub>2</jats:sub> required in the final step of the smog mechanism to form O<jats:sub>3</jats:sub> was less abundant compared with present times. Furthermore, results are sensitive to Proterozoic NO<jats:sub><jats:italic>x</jats:italic></jats:sub> concentrations, which are challenging to predict, since they depend on uncertain quantities such as NO<jats:sub><jats:italic>x</jats:italic></jats:sub> source emissions and OH concentrations. We review NO<jats:sub><jats:italic>x</jats:italic></jats:sub> sources during the Proterozoic period and apply a photochemical box model having methane oxidation with NO<jats:sub><jats:italic>x</jats:italic></jats:sub>, HO<jats:sub><jats:italic>x</jats:italic></jats:sub> and O<jats:sub><jats:italic>x</jats:italic></jats:sub> chemistry to estimate the O<jats:sub>3</jats:sub> 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<jats:sub><jats:italic>x</jats:italic></jats:sub> levels of 1.53×10<jats:sup>−9</jats:sup> by volume mixing ratio, which was attainable according to our NO<jats:sub><jats:italic>x</jats:italic></jats:sub> source analysis, with 1% of the present atmospheric levels of O<jats:sub>2</jats:sub>. 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.</jats:p>