  Planets smaller than Neptune and larger than Earth make up the majority of
the discovered exoplanets. Those with H$_2$-rich atmospheres are prime targets
for atmospheric characterization. The transition between the two main classes,
super-Earths and sub-Neptunes, is not clearly understood as the rocky surface
is likely not accessible to observations. Tracking several trace gases
(specifically the loss of ammonia (NH$_3$) and hydrogen cyanide (HCN)) has been
proposed as a proxy for the presence of a shallow surface. In this work, we
revisit the proposed mechanism of nitrogen conversion in detail and find its
timescale on the order of a million years. NH$_3$ exhibits dual paths
converting to N$_2$ or HCN, depending on the UV radiation of the star and the
stage of the system. In addition, methanol (CH$_3$OH) is identified as a robust
and complementary proxy for a shallow surface. We follow the fiducial example
of K2-18b with a 2D photochemical model (VULCAN) on an equatorial plane. We
find a fairly uniform composition distribution below 0.1 mbar controlled by the
dayside, as a result of slow chemical evolution. NH$_3$ and CH$_3$OH are
concluded to be the most unambiguous proxies to infer surfaces on sub-Neptunes
in the era of the James Webb Space Telescope (JWST).
