UV shielding of NH₃and O₂by organic hazes in the Archean atmosphere

Abstract

Abstract. 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 Satum'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 •0.5 in the visible would have produced a strong "antigreenhouse effect," thereby cooling the surface and slowing the rate at which CH 4 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 02 could have accumulated beneath the haze layer. For the model parameters considered here, the answer is "no": The upper limit on ground level 02 concentrations is • 10 -6 atm, and a more realistic estimate for pO2 during the late Archean is 10 -8 atm. The stability of both 02 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.

Palaeoclimates: the first two billion years

“…There, it would have been converted back into H 2 and CO 2 (or CO) by photolysis and by reaction with hydroxyl (OH) radicals. The detailed photochemistry of CH 4 in a low-O 2 atmosphere has been studied most recently by Pavlov et al (2001).…”

“…400 ppmv for H 2 and 800 ppmv for CH 4 ); see Pavlov et al (2001) for the details. In these calculations, it was assumed that H 2 outgassed from volcanoes was lost predominantly by escape to space at the diffusion-limited rate.…”

Early anaerobic metabolisms

“…The methane produced during the decomposition of organic matter will escape to the atmosphere, where through photolysis, it will decompose back to hydrogen gas (Catling et al 2001;Pavlov et al 2001), and overall, two molecules of H 2 and one molecule of CO 2 are produced for every molecule of CH 4 photolysed. Hydrogen is also lost through escape from the atmosphere to space.…”

Ultraviolet radiation constraints around the circumstellar habitable zones

“…In the early Earth, methane-ammonia atmospheres are no longer considered realistic. Ammonia, in particular, photolyzes rapidly and is unlikely to have been present in the early Earth (Pavlov et al 2001). …”