Geological carbon dioxide (CO2) sequestration (GCS) in deep saline aquifers is a promising solution to mitigate the impact of anthropogenic CO2 emissions on global climate change. CO2 dissolved in formation water increases the solution density and can lead to miscible density-driven downward convection, which significantly accelerates the dissolution trapping of injected CO2. Experimental studies on miscible density-driven convection have been limited. In the laboratory, we found an empirical linear correlation between reflected green light intensity and solute concentration, which enabled in situ measurements of solute concentrations in the spatial and temporal domains and consequently the mass flux across the top boundary of the porous medium. Using the novel experimental techniques, we determined the critical Rayleigh-Darcy number and critical time scales for the onset of density-driven instability and convective dissolution. This is the first study to determine these critical system parameters using laboratory experiments.