  Sub-Neptunes (Rp~1.25-4 REarth) remain the most commonly detected exoplanets
to date. However, it remains difficult for observations to tell whether these
intermediate-sized exoplanets have surfaces and where their surfaces are
located. Here we propose that the abundances of trace species in the visible
atmospheres of these sub-Neptunes can be used as proxies for determining the
existence of surfaces and approximate surface conditions. As an example, we
used a state-of-the-art photochemical model to simulate the atmospheric
evolution of K2-18b and investigate its final steady-state composition with
surfaces located at different pressures levels (Psurf). We find the surface
location has a significant impact on the atmospheric abundances of trace
species, making them deviate significantly from their thermochemical
equilibrium and "no-surface" conditions. This result arises primarily because
the pressure-temperature conditions at the surface determine whether
photochemically-produced species can be recycled back to their favored
thermochemical-equilibrium forms and transported back to the upper atmosphere.
For an assumed H2-rich atmosphere for K2-18b, we identify seven chemical
species that are most sensitive to the existence of surfaces: ammonia (NH3),
methane (CH4), hydrogen cyanide (HCN), acetylene (C2H2), ethane (C2H6), carbon
monoxide (CO), and carbon dioxide (CO2). The ratio between the observed and the
no-surface abundances of these species, can help distinguish the existence of a
shallow surface (Psurf < 10 bar), an intermediate surface (10 bar < Psurf < 100
bar), and a deep surface (Psurf > 100 bar). This framework can be applied
together with future observations to other sub-Neptunes of interest.
