Abstract
The ‘Hestia Project’ uses a bottom-up approach to quantify fossil fuel CO2 (FFCO2) emissions spatially at the building/street level and temporally at the hourly level. Hestia FFCO2 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 FFCO2 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 FFCO2 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 FFCO2 emissions. Hence, the resolution-entropy relationship can be used to assist in determining an appropriate spatial resolution for urban CO2 flux studies. We conclude that the optimal spatial resolution for providing Hestia total FFCO2 emissions products is centered around 100 m, at which the FFCO2 emissions data can not only fully meet the requirement of urban flux integration, but also be effectively used in understanding the relationships between FFCO2 emissions and various social-economic variables at the U.S. census block group level.
Highlights
By observing and analyzing how the Shannon entropy varies with the fossil fuel CO2 (FFCO2) emissions in different sectors and cities, we find: (1) the Shannon entropy increases with smaller grid resolution until it reaches a maximum value; (2) total emissions
By observing and analyzing how the Shannon entropy varies with the FFCO2 emissions in different sectors and cities, we find: (1) the Shannon entropy increases with smaller grid resolution until it reaches a maximum value; (2) total emissions requires a finer grid cell resolution than each of the sector-specific fields; (3) the residential emissions field requires a finer grid cell resolution than the commercial emissions field; (4) the optimal resolution of the onroad emissions grid is largely dependent on the density of the road network
These findings suggest that there is a consistent relationship between the Shannon entropy and the underlying information content within the FFCO2 emissions
Summary
CO2 emissions is an important element in efforts to both understand the global carbon cycle and enable effective decisionmaking on greenhouse gas emissions mitigation and verification. The importance of urban areas within the global carbon cycle is reflected in recent research efforts focused on understanding and quantifying FFCO2 fluxes in urban areas [4,5,6,7]. Many of these efforts attempt to quantify fluxes with spatial and functional detail in order to provide a variety of policy-relevant information [2]. Ongoing quantification can provide verification that emission reduction efforts have met their stated goals
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