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

Published online by Cambridge University Press:  28 November 2006

John Lee Grenfell ,
Barbara Stracke ,
Beate Patzer ,
Ruth Titz  and
Heike Rauer
John Lee Grenfell
Affiliation:
Institut für Planetenforschung, Extrasolare Planeten und Atmosphären, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherford Str. 2, 12489 Berlin, Germany e-mail: lee.grenfell@dlr.de
Barbara Stracke
Affiliation:
Institut für Planetenforschung, Extrasolare Planeten und Atmosphären, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherford Str. 2, 12489 Berlin, Germany e-mail: lee.grenfell@dlr.de
Beate Patzer
Affiliation:
Zentrum für Astronomie und Astrophysik, Technische Universität Berlin (TUB), Hardenbergstr. 36, 10623 Berlin, Germany
Ruth Titz
Affiliation:
Institut für Planetenforschung, Extrasolare Planeten und Atmosphären, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherford Str. 2, 12489 Berlin, Germany e-mail: lee.grenfell@dlr.de
Heike Rauer
Affiliation:
Institut für Planetenforschung, Extrasolare Planeten und Atmosphären, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherford Str. 2, 12489 Berlin, Germany e-mail: lee.grenfell@dlr.de
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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 (NOx). 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 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 period 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 period can produce approximately double the 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% 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.

Keywords

early Earthozonesmog mechanismUV-shielding
Type
Research Article
Information
International Journal of Astrobiology , Volume 5 , Issue 4 , October 2006 , pp. 295 - 306
Copyright
2006 Cambridge University Press

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