Abstract
Abstract. Atmospheric inverse modelling has become an increasingly useful tool for evaluating emissions of greenhouse gases including methane, nitrous oxide, and synthetic gases such as hydrofluorocarbons (HFCs). Atmospheric inversions for emissions of CO2 from fossil fuel combustion (ffCO2) are currently being developed. The aim of this paper is to investigate potential errors and uncertainties related to the spatial and temporal prior representation of emissions and modelled atmospheric transport for the inversion of ffCO2 emissions in the US state of California. We perform simulation experiments based on a network of ground-based observations of CO2 concentration and radiocarbon in CO2 (a tracer of ffCO2), combining prior (bottom-up) emission models and transport models currently used in many atmospheric studies. The potential effect of errors in the spatial and temporal distribution of prior emission estimates is investigated in experiments by using perturbed versions of the emission estimates used to create the pseudo-data. The potential effect of transport error was investigated by using three different atmospheric transport models for the prior and pseudo-data simulations. We find that the magnitude of biases in posterior total state emissions arising from errors in the spatial and temporal distribution in prior emissions in these experiments are 1 %–15 % of posterior total state emissions and are generally smaller than the 2σ uncertainty in posterior emissions. Transport error in these experiments introduces biases of −10 % to +6 % into posterior total state emissions. Our results indicate that uncertainties in posterior total state ffCO2 estimates arising from the choice of prior emissions or atmospheric transport model are on the order of 15 % or less for the ground-based network in California we consider. We highlight the need for temporal variations to be included in prior emissions and for continuing efforts to evaluate and improve the representation of atmospheric transport for regional ffCO2 inversions.
Highlights
The US state of California currently emits roughly 100 Tg C of fossil fuel CO2 each year (CARB, 2018), or approximately 1 % of global emissions (Boden et al, 2017)
The objective of this paper is to examine the sensitivity of a regional inversion for Californian fossil fuel CO2 (ffCO2) emissions to errors in the prior emission estimate and transport model
All transport models show that the observation sites are sensitive to more air basins in the October–November and January–February campaigns compared to the May campaign (Fig. 2)
Summary
The US state of California currently emits roughly 100 Tg C of fossil fuel CO2 (ffCO2) each year (CARB, 2018), or approximately 1 % of global emissions (Boden et al, 2017). The California Air Resources Board (CARB) is responsible for developing and maintaining a “bottom-up” inventory of greenhouse gas emissions to verify these reduction targets. Uncertainties in inventories of annual ffCO2 emissions from most developed countries (i.e. UNFCCC Annex I and Annex II) have been estimated to be between 5 %–10 % (Andres et al, 2012), and uncertainties can become much larger at subnational levels (Hogue et al, 2016). In a recent study Fischer et al (2017) found that discrepancies between bottom-up gridded inventories of ffCO2 emissions were 11 % of California’s total state emissions
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