  The UV environment is a key boundary condition for the origin of life.
However, considerable uncertainty exists as to planetary conditions and hence
surface UV at abiogenesis. Here, we present two-stream multi-layer clear-sky
calculations of the UV surface radiance on Earth at 3.9 Ga to constrain the UV
surface fluence as a function of albedo, solar zenith angle (SZA), and
atmospheric composition. Variation in albedo and latitude (through SZA) can
affect maximum photoreaction rates by a factor of >10.4; for the same
atmosphere, photoreactions can proceed an order of magnitude faster at the
equator of a snowball Earth than at the poles of a warmer world. Surface
conditions are important considerations when computing prebiotic UV fluences.
For climatically reasonable levels of CO2, fluence shortward of 189 nm is
screened out, meaning that prebiotic chemistry is robustly shielded from
variations in UV fluence due to solar flares or variability. Strong shielding
from CO2 also means that the UV surface fluence is insensitive to plausible
levels of CH4, O2, and O3. At scattering wavelengths, UV fluence drops off
comparatively slowly with increasing CO2 levels. However, if SO2 and/or H2S can
build up to the 1-100 ppm level as hypothesized by some workers, then they can
dramatically suppress surface fluence and hence prebiotic photoprocesses. H2O
is a robust UV shield for <198 nm. This means that regardless of the levels of
other atmospheric gases, fluence <198 nm is only available for cold, dry
atmospheres, meaning sources with emission <198 nm (e.g. ArF eximer lasers) can
only be used in simulations of cold environments with low abundance of
volcanogenic gases. On the other hand, fluence at 254 nm is unshielded by H2O
and is available across a broad range of CO2 columns, meaning that mercury
lamps are suitable for initial studies regardless of the uncertainty in
primordial H2O and CO2 levels.
