Optimum placement of sensors on building rooftop for measurement of greenhouse gas mixing ratio

Currently, inverse atmospheric dispersion models are being used around the world to provide measurement-based, or “top- down,” estimates of greenhouse gas (GHG) emissions for comparison with input-based, or “bottom-up,” estimates. To minimize uncertainty, inverse estimates require highly accurate measurements of GHG concentrations and meteorological data, and are improved when networks of sensor sites are used in concert. Ideally, measurement equipment should be placed on isolated, open-lattice towers to reduce the potential influence of local buildings or topography. However, this is often not possible when GHG concentrations are measured in urban environments, where open locations are unavailable or mounting such towers would be prohibitively expensive. In these environments, networks of rooftop-mounted sensors are more likely to be cost-effective and simpler to implement. Unfortunately, the flat-topped buildings that are typical of urban settings generate wind recirculation zones and turbulence that may interfere with rooftop measurements. In the current study, large eddy simulations (LES) that resolve the building level boundary layer were performed. Time dependent concentrations of carbon dioxide were computed at a number of locations and heights above the rooftop of a large office building and compared with those measured at an isolated tower. Different wind directions and speeds were used, and the influence of structures surrounding the office building was also studied. The results of these simulations allow us to give practical recommendations for the placement of rooftop-mounted sensors providing higher volume of useful measurements.