Greenhouse Gas Emissions and Dispersion : 1. Optimum Placement of Gas Inlets on a Building Rooftop for the Measurement of Greenhouse Gas Concentration

Abstract

Inverse atmospheric dispersion models are used 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 accurate measure ments of GHG concentrations and meteorological data, and are improved when networks of sensor sites are used in concert. To closely approximate free stream mixing ratio values, it is widely-used practice to mount measurement equipment on isolated open-lattice towers to reduce the potential influences of a boundary layer formed by the supporting structure. However, this is often not possible when GHG concentrations are measured in urban environments, where open locations are unavailable or the use of 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 trace gas mixing ratio measurements. In this study, large eddy simulations (LES) of wind flow over a large office building were performed to estimate the effect and potential error introduced by performing measurements on building rooftops as compared with free stream or tower-based measurements. Time dependent concentrations of carbon dioxide were computed at a number of locations and heights above the roof of a tall building and compared with the original in put 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.

Carbon dioxide and methane measurements from the Los Angeles Megacity Carbon Project – Part 1: calibration, urban enhancements, and uncertainty estimates

“…Where possible, measurement locations were sought on open-lattice communications towers. These structures were favored as they tend to reduce the influence of perturbed airflow from the supporting structure itself and remote locations minimize the influence of nearby emissions (Prasad et al, 2013). In the SCB, access to tall towers (> 100 m a.g.l., meters above ground level) was limited to the surrounding mountain ranges, which would present unique complexities for modeling and interpretation of the data.…”

“…When no tower sites were available in a critical sampling area, we sought secure locations on the rooftops of tall, multistory buildings in the area of interest. The siting criteria and sampling design framework were based on recommendations from Prasad et al (2013) and McKain et al (2015). In cases where rooftop sites were evaluated, large eddy simulations were performed to study the impact of recirculation and nearby structures on the flow field around a building rooftop (Prasad et al, 2013).…”