Greenhouse gas emissions and dispersion #1. Optimum placement of gas inlets on a building rooftop for the measurement of greenhouse gas concentrations

Inverse atmospheric dispersion models are used to provide measurement-based, or “topdown”, estimates of greenhouse gas (GHG) emissions for comparison with input-based, or “bottom-up”, estimates. To minimize uncertainty, inverse estimates require 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 this study, large eddy simulations (LES) of wind flow over a large office building were performed to estimate the error introduced by performing measurements on building rooftops as compared with tower-based measurements. Time dependent concentrations of carbon dioxide were computed at a number of locations and heights above the rooftop of the building and compared with the original input signal. Simulation results are used to develop guidelines for optimum placement of sensors on rooftop for accurate measurement of GHG mixing ratio that are necessary for atmospheric inversion models.