Onroad CO2 emissions: Does top-down EDGAR have an urban/rural bias compared to bottom-up Vulcan?

With regards to fossil fuel CO₂ (FFCO₂) emissions, the Onroad sector is the second largest sector in the US, following electricity production and mostly the largest sector within city boundaries. However, Onroad FFCO₂ is poorly quantified especially at smaller spatial scales like subcounty or city. Moreover, cities have started emerging as hubs of climate change mitigation activities. As an example, ~ 9000 cities representing more than 10% of the global population have committed to the Global Covenant of Mayors to collaborate on actions to combat climate change.

Spatiotemporally resolved FFCO₂ emissions data products are produced using fundamentally two distinct methods: ‘Bottom-up’ and ‘Top-down’. ‘Bottom-up’ approaches use mechanistic FFCO₂ emissions in combination with activity data derived from socioeconomic sources. Eg., Onroad emissions from The Vulcan Project are resolved at the road segment level by distributing county level FFCO₂ using traffic volume data. However, Vulcan is restricted to the US. ‘Top-down’ approaches use spatial proxies such as population to distribute national emissions over space and time. Eg., EDGAR (Emissions Database for Global Atmospheric Research) uses road density and population weighted road density as a proxy for heavy duty and non-heavy duty vehicular emissions

We compare the Onroad emissions of a ‘bottom-up’ Vulcan and a ‘top-down’ EDGAR for the continental US with the objective of evaluating the robustness of the spatial proxies used by EDGAR. On aggregating the Onroad FFCO₂ values from both datasets to the state level, we intend to compare it the EIA’s (U.S. Energy Information Administration) fuel sales data, an independent data source. The Covid19 pandemic has been humankind’s one of the largest perturbations and there are a few independent datasets, including observations documenting changes in human activity as a consequence. We also intend to evaluate how FFCO₂ emissions of Vulcan and EDGAR respond to the pandemic.