The urban biosphere results in the photosynthetic uptake of CO2 and green-space initiatives are often proposed as GHG reducing strategies, despite there being very little quantitative evidence for the effectiveness or efficiency of such strategies. Uncertainty in the time scales for respiration of carbon previously taken up through photosynthesis obscures the picture even further. Additionally, as the modern urban landscape is continually evolving, with green spaces and parks becoming a more integral component and with suburbs expanding outward from city centers into previously rural, agricultural, and natural areas, it is apparent that we lack the scientific understanding of how best to implement planning strategies that minimize the impact of such land-use changes on climate. With this project, I aim to equip myself with the knowledge to improve our scientific understanding of the impact of the urban biosphere on the net flux of CO2 from cities into the atmosphere.
The main obstacle in quantifying CO2 capture by vegetation is the fact that CO2 flux observations, are influenced only by the net biogenic flux and do not contain information about the separate photosynthetic and respiratory components. Atmospheric carbonyl sulfide (COS), however, can help with this distinction. COS is a potentially transformative tracer of photosynthesis because its variability in the atmosphere has been found to be influenced primarily by vegetative uptake, scaling linearly with gross primary production (GPP).
The main conclusions of the action are that the urban biosphere is indeed an important contribution to the urban carbon footprint, and that OCS is a suitable tracer for quantifing this contribution. Further work needs to be done to improve the model at the urban scale, especially with respect to the boundary layer (BL)and the air mixing effect. BL depth drives mixing ratios, so it is important to improve the BL scheme for urban atmospheric transport models. We found that mixing ratios are driven by BL depth more so than the emissions. For the case study of San Francisco Bay Area, we found more CO2 emissions in the afternoon but lower mixing ratios because BL gets deeper.
