Urban carbon dioxide cycles within the Salt Lake Valley: A multiple-box model validated by observations

A multiple-box model was developed to determine how meteorological, anthropogenic, and biological processes combine to produce diel cycles of carbon dioxide (CO₂) mole fraction during each of the four calendar-based seasons within Salt Lake Valley, Utah, USA. The model was forced by observed winds, sounding-derived mixing depths, an anthropogenic CO₂ emissions inventory, and net biological flux estimates based on temperature, solar radiation, and ecosystem type. The model was validated using hourly CO₂ mole fractions measured at five sites in the urban Salt Lake Valley (uSLV) area for 2005 - 2009 (spatial average of observations denoted by Cobs). The model accounted for 53% of Cobs on an hourly basis, and accounted for 90 - 94% of the mean diel cycle of Cobs depending on season. The multiple-box model results indicated that CO₂ change rates within the uSLV mean diel cycles were largely the result of imbalances between anthropogenic processes adding CO₂ and meteorological processes removing or diluting CO₂. 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. During summer mornings, CO₂ reduction attributable to photosynthesis below shallow mixing heights was of intermediate importance between advection and entrainment, but the overall net effect of biological processes was the least important influence on CO₂ change rates during each of the four seasons.