Being an atomically thin material, graphene is known to be extremely susceptible to its environment, including defects and phonons in the substrate on which it is placed as well as gas molecules that surround it. Thus, any device design using graphene has to take into consideration all surrounding components, and device performance needs to be evaluated in terms of environmental influence. However, no methods have been established to date to readily measure the density and distribution of external perturbations in a quantitative and non-destructive manner. Here, we present a rapid and non-contact method for visualizing the distribution of molecular adsorbates on graphene semi-quantitatively using terahertz time-domain spectroscopy and imaging. We found that the waveform of terahertz bursts emitted from graphene-coated InP sensitively changes with the type of atmospheric gas, laser irradiation time, and ultraviolet light illumination. The terahertz waveform change is explained through band structure modifications in the InP surface depletion layer due to the presence of localized electric dipoles induced by adsorbed oxygen. These results demonstrate that terahertz emission serves as a local probe for monitoring adsorption and desorption processes on graphene films and devices, suggesting a novel two-dimensional sensor for detecting local chemical reactions.