The late Archean atmosphere was probably rich in biologically generated CH4
and may well have contained a hydrocarbon haze layer similar to that obser
ved 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 th
e effects of such a haze layer on climate and ultraviolet radiation. We sho
w that the thickness of the haze layer was limited by a negative feedback l
oop: A haze optical depth of more than similar to0.5 in the visible would h
ave produced a strong "antigreenhouse effect," thereby cooling the surface
and slowing the rate at which CH4 was produced. Given this climatic constra
int on its visible optical depth, the amount of UV shielding provided by th
e haze can be estimated from knowledge of the optical properties and size d
istribution of the haze particles. Contrary to previous studies [Sagan and
Chyba, 1997], we find that when the finite size of the particles is taken i
nto account, the amount of UV shielding provided by the haze is small. Thus
NH3 should have been rapidly photolyzed and should not have been sufficien
tly abundant to augment the atmospheric greenhouse effect. We also examine
the question of whether photosynthetically generated O-2 could have accumul
ated beneath the haze layer. For the model parameters considered here, the
answer is "no": The upper limit on ground level O-2 concentrations is simil
ar to 10(-6) atm, and a more realistic estimate for PO2 during the late Arc
hean 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 a
ddress these uncertainties.