Background
The authors write that "as global human populations become increasingly urbanized, spatially concentrated centers of anthropogenic CO2 emissions emerge [that] have CO2 mole fractions and temperatures that are higher than global averages," which "pattern of localized CO2 generation and dispersal results in urban CO2 'domes' (Idso et al., 2001, 2002)."

What was done
Strong et al. developed a multiple-box model to determine how meteorological, anthropogenic and biological processes combine to produce diurnal cycles of atmospheric CO2 concentration during each of the year's four seasons within the Salt Lake Valley, Utah (USA). The model was forced by "observed winds, sounding-derived mixing depths, an anthropogenic CO2 inventory and net biological flux estimates based on temperature, solar radiation and ecosystem type," while it was validated using hourly atmospheric CO2 concentration observations (Cobs) measured at five sites in the urban Salt Lake Valley area over the period 2005-2009.

What was learned
The five researchers report that the model accounted for 53% of Cobs on an hourly basis and 90-94% of the mean diel cycle of Cobs depending on season. During all four seasons, they say that "the mean diel cycle of Cobs peaked near sunrise, decreased rapidly between sunrise and midday, minimized in the afternoon, and then rose overnight," which variability was "largely the result of imbalances between anthropogenic processes adding CO2 and meteorological processes removing or diluting CO2."

More specifically, Strong et al. write that "removal by wind (advection) was the most important CO2 reduction process on average, but dilution of CO2 by entrainment of air from above the mixing height overtook advection in importance between sunrise and midday." They also note that "during summer mornings, CO2 reduction attributable to photosynthesis below shallow mixing heights was of intermediate importance between advection and entrainment," but they say that "the overall net effect of biological processes was the least important influence on CO2 change rates during each of the four seasons."

What it means
Strong et al. conclude that the results of their study, when combined with "long-term spatial CO2 monitoring and fossil fuel inventory data, provide a foundation for testing CO2 emissions scenarios and human behavioral changes on a broad urban scale." This is important, in their view, since they opine that "regional efforts at the urban scale may prove more tractable [than international efforts that are leading to a better understanding of the global carbon cycle] in terms of evaluating the consequences of changes in human behavior on CO2 emissions."

References
Idso, C.D., Idso, S.B. and Balling Jr., R.C. 2001. An intensive two-week study of an urban CO2 dome in Phoenix, Arizona, USA. Atmospheric Environment 35: 995-1000.

Idso, S.B., Idso, C.D. and Balling Jr., R.C. 2002. Seasonal and diurnal variations of near-surface atmospheric CO2 concentration within a residential sector of the urban CO2 dome of Phoenix, Arizona, USA. Atmospheric Environment 36: 1655-1660.