An intensive two-week study of an urban CO2 dome in Phoenix, Arizona, USA

Abstract Atmospheric CO2 concentrations were measured prior to dawn and in the middle of the afternoon at a height of 2 m above the ground along four transects through the metropolitan area of Phoenix, Arizona on 14 consecutive days in January 2000. The data revealed the existence of a strong but variable urban CO2 dome, which at one time exhibited a peak CO2 concentration at the center of the city that was 75% greater than that of the surrounding rural area. Mean city-center peak enhancements, however, were considerably lower, averaging 43% on weekdays and 38% on weekends; and averaged over the entire commercial sector of the city, they were lower still, registering 30% on weekdays and 23% on weekends. Over the surrounding residential areas, on the other hand, there are no weekday–weekend differences in boundary-layer CO2 concentration. Furthermore, because of enhanced vertical mixing during the day, near-surface CO2 concentrations in the afternoon are typically reduced from what they are prior to sunrise. This situation is additionally perturbed by the prevailing southwest-to-northeast flow of air at that time of day, which lowers afternoon CO2 concentrations on the southern and western edges of the city still more, as a consequence of the importation of pristine rural air. The southwest-to-northeast flow of air also sometimes totally compensates for the afternoon vertical-mixing-induced loss of CO2 from areas on the northern and eastern sides of the city, as a consequence of the northeastward advection of CO2 emanating from the central, southern and western sectors of the city. Hence, although complex, the nature of the urban CO2 dome of Phoenix, Arizona, is readily understandable in terms of basic meteorological phenomena and their interaction with human activities occurring at the land/air interface.