Optimizing the Spatial Resolution for Urban CO2 Flux Studies Using the Shannon Entropy

Abstract

Abstract:The 'Hestia Project' uses a bottom-up approach to quantify fossil fuel CO 2 (FFCO 2 ) emissions spatially at the building/street level and temporally at the hourly level. Hestia FFCO 2 emissions are provided in the form of a group of sector-specific vector layers with point, line, and polygon sources to support carbon cycle science and climate policy. Application to carbon cycle science, in particular, requires regular gridded data in order to link surface carbon fluxes to atmospheric transport models. However, the heterogeneity and complexity of FFCO 2 sources within regular grids is sensitive to spatial resolution. From the perspective of a data provider, we need to find a balance between resolution and data volume so that the gridded data product retains the maximum amount of information content while maintaining an efficient data volume. The Shannon entropy determines the minimum bits that are needed to encode an information source and can serve as a metric for the effective information content. In this paper, we present an analysis of the Shannon entropy of gridded FFCO 2 emissions with varying resolutions in four Hestia study areas, and find: (1) the Shannon entropy increases with smaller grid resolution until it reaches a maximum value (the max-entropy resolution); (2) total emissions (the sum of several sector-specific emission fields) show a finer max-entropy resolution than each of the sector-specific fields; (3) the residential emissions show a finer max-entropy resolution than the commercial emissions; (4) the max-entropy resolution of the onroad emissions grid is closely correlated to the density of the road network. These findings suggest that the Shannon entropy can detect the information effectiveness of the spatial resolution of gridded FFCO 2 emissions. Hence, the resolution-entropy relationship can be used to assist in determining an appropriate spatial resolution for urban CO 2 flux studies. We conclude that the optimal spatial resolution for providing Hestia total FFCO 2 emissions products is centered around 100 m, at which the FFCO 2 emissions data can not only fully meet the requirement of urban flux integration, but also be effectively used in understanding the relationships between FFCO 2 emissions and various social-economic variables at the U.S. census block group level.

Comparison of Global Downscaled Versus Bottom‐Up Fossil Fuel CO2 Emissions at the Urban Scale in Four U.S. Urban Areas

“…The precision of gridded FFCO 2 emissions can be systematically related to the spatial resolution (Liang et al, ). Given the heterogeneity of the urban emission landscape, aggregating from fine to coarse resolution may smooth or average over the spatial variation potentially leading to fewer differences between the bottom‐up and downscaled emission approaches.…”