Atmospheric carbon dioxide concentrations were measured at one-mile intervals prior to sunrise and in the middle of the afternoon on a number of different transects through the metropolitan area of Phoenix, Arizona. Maps of the data reveal the existence of an "urban CO2 dome" over the city. Carbon dioxide concentrations at the center of the dome are as much as 45% greater than those observed over surrounding rural areas in the morning and as much as 25% greater in the afternoon. These high CO2 concentrations, which are produced primarily by the burning of fossil fuels such as gas and oil, have the ability to enhance the growth of urban vegetation and provide some protection against the debilitating effects of local air pollution. Hence, some of the positive effects of one of the major end-products of the combustion process (CO2) tend to counteract one of the negative effects of some of the minor by-products of that same process (air pollution).