Heat and mass transfer by natural convection in a square cavity filled with a mixture of air and carbon dioxide (CO2), as well as temperature and concentration gradients are numerically investigated. The study was conducted through laminar and turbulent regimes in order to analyze the influence of the buoyancy ratio changes (0 <= N <= 2.0) on heat transfer. Thermal behavior and air quality by changing the location of higher source of CO2 concentration in vertical walls was analyzed under two different motion conditions: assisted flow and opposite flow. Four different values of the Rayleigh number (10(4), 10(6), 10(8), 10(10)) were considered. Governing equations of mass, momentum, energy and concentration were solved by the finite volume method, and a k-epsilon turbulence model was used for the treatment of high Rayleigh numbers. From the results, it was observed that in general, different temperature patterns were presented. However, for turbulent flow and when buoyancy forces stayed in equilibrium, temperature patterns inside the cavity Were more homogeneous with values in the interval of thermal comfort recommended by ASHRAE. (C) 2013 Elsevier Ltd. All rights reserved.