Towards spaceborne monitoring of localized CO2 emissions: An instrument concept and first performance assessment

The UNFCCC (United Nations Framework Convention on Climate Change) requires the nations of the world to report their carbon dioxide (<span classCombining double low line"inline-formula">CO2</span>) emissions. The independent verification of these reported emissions is a cornerstone for advancing towards the emission accounting and reduction measures agreed upon in the Paris Agreement. In this paper, we present the concept and first performance assessment of a compact spaceborne imaging spectrometer with a spatial resolution of <span classCombining double low line"inline-formula">50×50</span>&thinsp;<span classCombining double low line"inline-formula">m2</span> that could contribute to the "monitoring, verification and reporting" (MVR) of <span classCombining double low line"inline-formula">CO2</span> emissions worldwide. <span classCombining double low line"inline-formula">CO2</span> emissions from medium-sized power plants (1-10&thinsp;<span classCombining double low line"inline-formula">Mt CO2 yr-1</span>), currently not targeted by other spaceborne missions, represent a significant part of the global <span classCombining double low line"inline-formula">CO2</span> emission budget. In this paper we show that the proposed instrument concept is able to resolve emission plumes from such localized sources as a first step towards corresponding <span classCombining double low line"inline-formula">CO2</span> flux estimates. Through radiative transfer simulations, including a realistic instrument noise model and a global trial ensemble covering various geophysical scenarios, it is shown that an instrument noise error of 1.1&thinsp;ppm (<span classCombining double low line"inline-formula">1σ</span>) can be achieved for the retrieval of the column-averaged dry-air mole fraction of <span classCombining double low line"inline-formula">CO2</span> (X<span classCombining double low line"inline-formula">CO2</span>). Despite a limited amount of information from a single spectral window and a relatively coarse spectral resolution, scattering by atmospheric aerosol and cirrus can be partly accounted for in the X<span classCombining double low line"inline-formula">CO2</span> retrieval, with deviations of at most 4.0&thinsp;ppm from the true abundance for two-thirds of the scenes in the global trial ensemble. We further simulate the ability of the proposed instrument concept to observe <span classCombining double low line"inline-formula">CO2</span> plumes from single power plants in an urban area using high-resolution <span classCombining double low line"inline-formula">CO2</span> emission and surface albedo data for the city of Indianapolis. Given the preliminary instrument design and the corresponding instrument noise error, emission plumes from point sources with an emission rate down to the order of 0.3&thinsp;<span classCombining double low line"inline-formula">Mt CO2 yr-1</span> can be resolved, i.e., well below the target source strength of 1&thinsp;<span classCombining double low line"inline-formula">Mt CO2 yr-1</span>. This leaves a significant margin for additional error sources, like scattering particles and complex meteorology, and shows the potential for subsequent <span classCombining double low line"inline-formula">CO2</span> flux estimates with the proposed instrument concept.