An intensive study of the strength and stability of the urban CO2 dome of Phoenix, Arizona

Atmospheric CO₂ concentrations were measured prior to dawn and in the middle of the afternoon at a height of two meters above the ground along four transect routes through the metropolitan area of Phoenix, Arizona on 14 consecutive days in January 2000. Similar measurements were made every two weeks from late July 1999 to late January 2000 on the two most central of these routes. The data revealed the existence of a strong but variable urban CO₂ dome, which at certain times during the pre-dawn hours of the day was as much as 75% greater in the center of the city than it was over the surrounding rural areas. Average city-center peak CO₂ enhancements - due, primarily, to vehicular traffic and commercial activities - were 43% on weekdays and 38% on weekends; while average enhancements for the entire commercial sector of the city were 30% on weekdays and 23% on weekends.

Due to enhanced vertical mixing and the advection into the city of pristine rural air forced by local diurnal airflow patterns in the afternoon, urban CO₂ concentrations at that time of day were typically less than pre-dawn concentrations. The different stability regimes of these two periods also resulted in weak pre-dawn breezes being 30 times more effective in reducing the CO₂ content of the near-surface air than stronger mid-afternoon winds. Also, because of the enhanced atmospheric thermal inversions of winter and the longer period of time that anthropogenic-induced CO₂ emissions can accumulate in the inversion layer at that time of year, the peak strength of the urban CO₂ dome in January was nearly 50% greater than it was in July.

These findings, together with our current understanding of the urban heat island phenomenon, indicate the degree to which cities may serve as analogues or "natural laboratories" for studying the effects of future global environmental change.