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

Atmospheric CO2 concentrations were measured prior to dawn and in the middl e 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 i n January 2000. The data revealed the existence of a strong but variable ur ban CO2 dome, which at one time exhibited a peak CO2 concentration at the c enter of the city that was 75% greater than that of the surrounding rural a rea. Mean city-center peak enhancements, however, were considerably lower, averaging 43 % on weekdays and 38 % on weekends; and averaged over the enti re commercial sector of the city, they were lower still, registering 30 % o n 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 re duced from what they are prior to sunrise. This situation is additionally p erturbed by the prevailing southwest-to-northeast flow of air at that time of day, which lowers afternoon CO2 concentrations on the southern and weste rn edges of the city still more, as a consequence of the importation of pri stine rural air. The southwest-to-northeast flow of air also sometimes tota lly 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 th e northeastward advection of CO2 emanating from the central, southern and w estern sectors of the city. Hence, although complex, the nature of the urba n CO2 dome of Phoenix, Arizona, is readily understandable in terms of basic meteorological phenomena and their interaction with human activities occur ring at the land/air interface. Published by Elsevier Science Ltd.