Photochemical-Radiative Equilibrium of the Earth's Paleoatmospheres with various Amounts of Oxygen

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Abstract
The O3 density and temperature in the Earth's paleoatmosphere are investigated using a numerical model for the photochemical-radiative equilibrium. The model atmospheres resemble the present atmosphere in most respects but contain different amounts of O2, ranging from 10-6 to 10 times the present atmospheric level (PAL). It is assumed that the photochemical processes take place in a pure oxygen atmosphere, and that there is no effect of transport either of composition or of heat by atmospheric motions. The results show that the O3 column density reaches a maximum for an O2 level of about 10-1 PAL, and that the effect of temperature feedback on the O3 column density is relatively small. It is also found that for O2 levels less than 10-3 PAL, the temperature decreases almost monotonically with altitude, i.e., neither the tropopause* nor stratopause* is found. As the O2 level increases, the temperature above 30km increases notably and the tropopause and stratopause appear clearly. The temperature change near the surface with changing O2 level is relatively small compared with that in the upper region. The contribution to the heat budget above 20km by the solar heating and terrestrial cooling due to O3 is found to increase remarkably with increasing O2 level, while below 15km, the contribution by H2O always predominates as in the present atmosphere.

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