The urban CO2 dome of Phoenix, Arizona

Air temperatures, relative humidities, and atmospheric carbon dioxide concentrations were measured at a height of 2 m at approximate 1.6-km intervals prior to sunrise and in the middle of the afternoon on five days in January along a number of different transects through the extended metropolitan area of Phoenix, Arizona. Spatially interpolated maps of the data indicate the presence of an "urban CO2 dome" that reaches concentrations as high as 555 ppmv in the city center and decreases to a value of approximately 370 ppmv on the outskirts of the city at this time of year. Pre-dawn CO2 values inside the dome are considerably higher than mid-afternoon values, suggesting that solar-induced convective mixing and the photosynthetic uptake of CO2 by urban vegetation may play significant roles in diurnally redistributing the anthropogenically produced CO2 that, together with that produced by plant respiration, accumulates near the ground during the night and early morning hours. Temperature and relative humidity appear to have little influence on either the concentration or location of the CO2 dome, but variations in wind speed and direction at times may disrupt the pattern that develops under normally fair conditions. The high CO2 concentrations within the dome may help to ameliorate the deleterious effects of urban air pollution on vegetation growing within the city. Together with the urban heat island phenomenon, they may also provide a natural laboratory for studying the effects of contemporaneous warming and atmospheric CO2 enrichment within the context of predicted future global change.