  We propose that the photochemical smog mechanism produced substantial ozone
(O3) in the troposphere during the Proterozoic, which contributed to
ultraviolet (UV) radiation shielding hence favoured the establishment of life.
The smog mechanism is well-established and is associated with pollution hazes
which sometimes cover modern cities. The mechanism proceeds via the oxidation
of volatile organic compounds (VOCs) such as methane (CH4) in the presence of
UV radiation and nitrogen oxides (NOx). It would have been particularly
favoured during the Proterozoic 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.
Further, results are sensitive to Proterozoic NOx concentrations, which are
challenging to predict, since they depend on uncertain quantities such as NOx
source emissions and OH concentrations. We review NOx sources during the
Proterozoic and apply a photochemical box model having methane oxidation with
NOx, HOx and Ox chemistry to estimate the O3 production from the smog
mechanism. Runs suggest the smog mechanism during the Proterozoic can produce
about double present day ozone columns for NOx levels of 1.53 10-9 by volume
mixing ratio, which was attainable according to our NOx source analysis, with 1
per-cent present atmospheric levels (PALs) of O2. Clearly, forming ozone in the
troposphere is a trade-off for survivability. On the one hand harmful UV is
blocked, but on the other hand ozone is a respiratory irratant, which becomes
fatal at concentrations exceeding about 1 ppmv.
