The purpose of this article is to describe determinants and spatial patterns of atmospheric carbon dioxide (CO₂) in Phoenix, Arizona. Specifically, we use geographic information systems (GIS) and regression-based analyses to identify the human and biological factors that contribute to spatial and temporal variations in near-surface (2-meter height) atmospheric CO₂ levels. We use these factors to create estimated surfaces of CO₂ concentrations for the area. We evaluate the surfaces using records of CO₂ from independent monitoring stations and transects. To investigate the temporal patterns and variations in CO₂ concentrations, we estimate CO₂ surfaces for the early mornings and the afternoons, on weekdays when traffic is heavy and spatially focused and on weekends when it is lighter and more spatially dispersed. Findings suggest there is a distinct relationship between the structure of Phoenix CO₂ levels and spatial patterns of human activities and vegetation densities. Morning CO₂ levels are higher than afternoon levels and correspond closely to the density of traffic, population, and employment. The spatial structure of human activity explains the pattern of CO₂ better on weekdays than on weekends. CO₂ surfaces reflect declining densities of human activity with distance from the city center, the pattern of irrigated agriculture in the Phoenix area, and riparian habitats on the urban fringe. Spatial and temporal patterns of CO₂ concentrations are useful in understanding urban climate and ecosystem processes.