UV shielding of NH3 and O2 by organic hazes in the Archean atmosphere

The late Archean atmosphere was probably rich in biologically generated CH4 and may well have contained a hydrocarbon haze layer similar to that observed today on Saturn's moon, Titan. Here we present a detailed model of the photochemistry of haze formation in the early atmosphere, and we examine the effects of such a haze layer on climate and ultraviolet radiation. We show that the thickness of the haze layer was limited by a negative feedback loop: A haze optical depth of more than similar to0.5 in the visible would have produced a strong "antigreenhouse effect," thereby cooling the surface and slowing the rate at which CH4 was produced. Given this climatic constraint on its visible optical depth, the amount of UV shielding provided by the haze can be estimated from knowledge of the optical properties and size distribution of the haze particles. Contrary to previous studies [Sagan and Chyba, 1997], we find that when the finite size of the particles is taken into account, the amount of UV shielding provided by the haze is small. Thus NH3 should have been rapidly photolyzed and should not have been sufficiently abundant to augment the atmospheric greenhouse effect. We also examine the question of whether photosynthetically generated O-2 could have accumulated beneath the haze layer. For the model parameters considered here, the answer is "no": The upper limit on ground level O-2 concentrations is similar to 10(-6) atm, and a more realistic estimate for PO2 during the late Archean is 10(-18) atm. The stability of both O-2 and NH3 is sensitive to the size distribution and optical properties of the haze particles, neither of which is well known. Further theoretical and laboratory work is needed to address these uncertainties.