Anthropogenic And Biogenic Carbon Dioxide Fluxes in Urban Houston

Approximately 5% of North American land cover is characterized as urban, representing a large source of anthropogenic carbon to the atmosphere but also an unknown sink in urban vegetation and its management. We present measurements of energy and trace gas fluxes from a unique urban research site in Houston, Texas, with a focus on evaluating the anthropogenic and biogenic contributions to the measured net CO2 exchange at this site. Eddy covariance carbon dioxide flux measurements, and CO2 and NOx flux gradient measurements have been operated (with a one-year gap) since mid 2007 from a commercial communications tower north of downtown Houston. In addition, leaf-level and soil CO2 efflux measurements were carried out in the tower's footprint region in 2011. Bottom-up estimates were developed for anthropogenic CO2 fluxes through correlation with CO, traffic count and natural gas use data, and for biogenic CO2 fluxes through modeling soil and plant respiration, and photosynthesis. CO, NO¬x, and CO2 data were strongly correlated during the morning rush hours, and were used to extrapolate CO2 emissions from traffic using measured biweekly to monthly CO (NOx) flux averages. CO emissions from natural gas use were extrapolated from monthly gas consumption data. The results will be compared to a top-down flux estimate that uses quadrant analysis of CO2 fluxes with simultaneously acquired heat and water vapor fluxes to distinguish biogenic (high q'C' correlation) from anthropogenic (high T'C' correlation) carbon flux contributions. First results from both bottom-up and top-down flux estimates will be presented. The successful implementation of this new analysis may allow us to better judge the importance of constantly growing urban areas in the North American carbon cycle. It addresses identified gaps in carbon cycle knowledge and can help in validating inventories and improving estimates of carbon cycling.