  Recent observations of protoplanetary discs reveal disc substructures
potentially caused by embedded planets. We investigate how the gas surface
density in discs changes the observed morphology in scattered light and dust
continuum emission. Assuming that disc substructures are due to embedded
protoplanets, we combine hydrodynamical modelling with radiative transfer
simulations of dusty protoplanetary discs hosting planets. The response of
different dust species to the gravitational perturbation induced by a planet
depends on the drag stopping time - a function of the generally unknown local
gas density. Small dust grains, being stuck to the gas, show spirals. Larger
grains decouple, showing progressively more axisymmetric (ring-like)
substructure as decoupling increases with grain size or with the inverse of the
gas disc mass. We show that simultaneous modelling of scattered light and dust
continuum emission is able to constrain the Stokes number, ${\rm St}$. Hence,
if the dust properties are known, this constrains the local gas surface
density, $\Sigma_{\rm gas}$, at the location of the structure, and hence the
total gas mass. In particular, we found that observing ring-like structures in
mm-emitting grains requires ${\rm St} \gtrsim 0.4$ and therefore $\Sigma_{\rm
gas} \lesssim 0.4\,\textrm{g/cm}^{2}$. We apply this idea to observed
protoplanetary discs showing substructures both in scattered light and in the
dust continuum.
