Quantification of fossil fuel CO 2 emissions at fine space and time resolution is emerging as a critical need in carbon cycle and climate change research. As atmospheric CO 2 measurements expand with the advent of a dedicated remote sensing platform and denser in situ measurements, the ability to close the carbon budget at spatial scales of ~100 km 2 and daily timescales requires fossil fuel CO 2 inventories at commensurate resolution.Additionally, the growing interest in U.S. climate change policy measures are best served by emissions that are tied to the driving processes in space and time. Here we introduce a high resolution data product (the "Vulcan" inventory:www.purdue.edu/eas/carbon/vulcan/) that has quantified fossil fuel CO 2 emissions for the contiguous U.S. at spatial scales less than 100 km 2 and temporal scales as small as hours. Comparison to the global 1°x1° fossil fuel CO 2 inventory used widely by the carbon cycle and climate change community prior to the construction of the Vulcan inventory, highlights the space/time biases inherent in the population-based approach.3

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Here we report on the Vulcan Project (vulcan.project.asu.edu/index.php), an effort to estimate fossil fuel CO 2 emissions at local space/time scales through the use of "bottom-up", process-driven information. The Vulcan Project began as an answer to the need for a higher resolution North American fossil fuel CO 2 emissions inventory. As a key component in the North American Carbon Program, Vulcan is now playing a critical role as a boundary constraint to inverse estimation of carbon sources and sinks. With the advent of denser CO 2 monitoring, both in situ and remote, a higher resolution approach to inventory quantification is essential.The Vulcan Project has estimated United States fossil fuel CO 2 emissions at the hourly time scale and at spatial scales below the county level (placed on a regular 10 km x 10 km grid) for the year 2002. Vulcan is built from a wide variety of observational data streams including regulated air pollutant emissions reporting, traffic monitoring, energy statistics, and US census data. In addition to these data sets, Vulcan relies on a series of modeling assumptions and constructs to interpolate in space, time and transform non-CO 2 reporting into an estimate of CO 2 combustion emissions. The recent version 2.0 of the Vulcan inventory has produced advances in a number of categories with particular emphasis on improved temporal structure. Onroad transportation emissions now avail of roughly 5000 automated traffic count monitors allowing for much improved diurnal and weekly time structure in our onroad transportation emissions.The Vulcan data product has been used for a regional inversion in the US and the potential bias correction from the utilization of previous fossil fuel CO 2 emission data products is as high as 100% for large coherent regions in the United States. This emphasizes the potential for biased fossil fuel CO 2 inventories to impact inverse results.Vulcan has also generated interest from decision-making communities and is now attempting to meet a variety of CO 2 mitigation goals by providing process detail on emissions activities. This has led to the Hestia effort in which quantification has been attempted down to the individual building level within a single urban domain (Indianapolis, USA). A partner project, INFLUX, has begun in which the Hestia inventory is combined with aircraft monitoring and flux tower instrumentation to attempt the first urban-level bottomup/top-down comparison. As with Vulcan, the Hestia effort has a variety of decision-support spin-offs including urban planning, energy efficiency planning in addition to CO 2 mitigation planning at the urban/street level. Scoping has also begun on performing a Hestia approach for Los Angeles.