The Greenhouse gas Emission Monitoring network to Inform Net-zero Initiatives UK (GEMINI-UK): network design, theoretical performance, and initial data

Abstract. The Greenhouse gas Emissions Monitoring network to Inform Net-zero Initiatives for the UK (GEMINI-UK) includes ten Bruker EM27/SUN instruments located across the UK that collect dry average volume mixing ratios of CO2 and methane (XCO2 and XCH4). The primary objective of GEMINI-UK is to infer regional net flux estimates of CO2 and methane across the UK that can be used to provide actionable information to the UK Government. The instruments are housed in bespoke autonomous weatherproof enclosures that help maximize cloud-free data collection throughout the calendar year. The network will become fully operational in late 2025. As part of our commissioning phase, we designed the network so it would deliver the biggest uncertainty reduction in net CO2 fluxes, based on prior emission inventories. The ten sites are located at UK education institutions and a national scientific research laboratory, underlining our commitment to make these data openly available to all. In this study, we use a series of closed-loop numerical experiments for the nominal calendar year of 2019 to quantify the theoretical benefit of using this new ground-based remote sensing network, accounting for cloudy scenes, to estimate spatially resolved net fluxes of CO2 and methane across the UK. Based on our results, we expect that GEMINI-UK will deliver significant error reductions in CO2 flux estimates, up to 51 % in January and up to 59 % in July (GEMINI-UK only), and up to 8 % in January and up to 24 % in July when combined with existing tall tower measurements collected across England and Ireland. Despite the network being optimally designed to enhance our understanding of UK CO2 fluxes, we expect, based on our calculations, that GEMINI-UK will also substantially reduce uncertaintes of methane emissions, achieving a priori error reductions up to 55 % in January and up to 75 % in July (GEMINI-UK only), and up to 10 % in January and up to 29 % in July when combined with the existing tall tower sites. In the context of augmenting the information collected by the established tall tower network, we find that GEMINI-UK data have the greatest potential over high flux regions in the central and southern parts of the UK during winter months, and over broader southern to northern regions during the summer months. More broadly, the data collected by GEMINI-UK will also provide the basis to evaluate satellite observations of these trace gases, thereby providing confidence in their ability to supplement data collected by GEMINI-UK and the tall tower network.