A comparison of CO2 fluxes at two sites within the urbanized Salt Lake Valley

As part of the Urban Trace-gas Emissions Study (UTES), the eddy covariance technique was used to measure carbon dioxide fluxes and parts of the surface energy budget at two different sites with distinct urban landforms in the Salt Lake Metropolitan area, Utah, USA. One the primary objectives of this experiment was to investigate the spatial and temporal variability of CO2 fluxes and the surface energy balance at an urban and pre-urban site subjected to a similar climate. Measurements were acquired on a 36 m tall communication tower at a suburban site and on a 9 m tower at a pre-urban site located just beyond the region of land currently being developed. The two sites were approximately 17 km apart and within the same valley. The suburban site was characterized by irrigated vegetative surfaces, while sagebrush and dormant grasses dominated the pre-urban site. Data obtained from the summer 2005 field campaign are presented in this paper. During the measurement period, the CO2 flux at the pre-urban site always remained positive, while at the suburban site, there was a significant sequestration period during the midday period. While photosynthetic activity was responsible for the negative CO2 flux at the suburban site during the midday period, plant and human respiration combined with the CO2 emitted from traffic and combustion of natural gas made the site an overall source of CO2. The significantly different land covers associated with the two sites were also reflected in the surface energy balance. At the pre-urban site, the contribution from the sensible heat flux was considerably larger the latent heat flux whereas, similar contributions from the latent and sensible heat fluxes were measured at the suburban site, a result associated with irrigation of the urban forest.