  A key challenge in origins-of-life studies is estimating the abundances of
species relevant to the chemical pathways proposed to have contributed to the
emergence of life on early Earth. Dissolved nitrogen oxide anions
(NO$_{X}^{-}$), in particular nitrate (NO$_{3}^{-}$) and nitrite
(NO$_{2}^{-}$), have been invoked in diverse origins-of-life chemistry, from
the oligomerization of RNA to the emergence of protometabolism. Recent work has
calculated the supply of NO$_{X}^{-}$ from the prebiotic atmosphere to the
ocean, and reported steady-state [NO$_{X}^{-}$] to be high across all plausible
parameter space. These findings rest on the assumption that NO$_{X}^{-}$ is
stable in natural waters unless processed at a hydrothermal vent. Here, we show
that NO$_{X}^{-}$ is unstable in the reducing environment of early Earth. Sinks
due to UV photolysis and reactions with reduced iron (Fe$^{2+}$) suppress
[NO$_{X}^{-}$] by several orders of magnitude relative to past predictions. For
pH$=6.5-8$ and $T=0-50^\circ$C, we find that it is most probable that
NO$_{X}^{-}$]$<1~\mu$M in the prebiotic ocean. On the other hand, prebiotic
ponds with favorable drainage characteristics may have sustained
[NO$_{X}^{-}$]$\geq 1~\mu$M. As on modern Earth, most NO$_{X}^{-}$ on prebiotic
Earth should have been present as NO$_{3}^{-}$, due to its much greater
stability. These findings inform the kind of prebiotic chemistries that would
have been possible on early Earth. We discuss the implications for proposed
prebiotic chemistries, and highlight the need for further studies of
NO$_{X}^{-}$ kinetics to reduce the considerable uncertainties in predicting
[NO$_{X}^{-}$] on early Earth.
