The present work investigates the CO2 dispersion in micro-scale urban area, and its effects on air quality in old buildings in the city of Ghardaia (M’Zab valley). The city of interest is located in the south of Algeria. Ghardaia is characterized by its vernacular urban structure well adapted to local climate (hot arid) which is listed by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a protected city. Numerical simulation with CFD code was used in combination with measurement data in order to study the CO2 air pollution levels in the selected area. The field measurements carried out for the two days: 5th and 6th July 2013, shows that CO2 concentration in the street canyon reaches its highest level up to 400 ppm. Two numerical simulations were performed based on geometry changes and architectural propositions on the downstream buildings, to assess the effects of morphology on the CO2 air pollution dispersion. The first improvement was made by increasing the street canyon width at the outlet side, which reduced the CO2 concentration by 46%. In the second modification, which was made by changing the uneven building layout, this improvement decreased the CO2 concentration by 36% compared to the real geometry. The results show the effect of weak modifications of the architectural fabric on the air pollution reduction, and therefore the participation in protecting old buildings heritage from air pollution effects.