We analyzed the atmospheric carbon dioxide (CO2) concentration in streets of different traffic levels and urban parks of different sizes in Rome. Rome represents an example of a mega‐city, where  the  urbanization  process  has  been  increasing  over  the  last  years,  and  many  new  sub‐urban areas have been built by scaling down free areas surrounding the city. CO2 concentration in Rome has been significantly increasing from 1995 to date due to the increase of private means of  transportation.  Our  results  underline  that  CO2  concentration  in  streets  located  in  the  city  centre has a strong diurnal trend peaking in the first hours of the morning when the traffic is the highest, then decreasing at 3:00 p.m. and increasing again at 8:00 p.m., in relation to traffic level  variations.  It  is  attested  also  by  the  correlation  between  the  two  variables.  The  results  underline the effect of buildings delimiting the urban streets on the CO2 concentration.