Headline: Drop-in Fuels from Sunlight and Air

Aviation and shipping currently contribute approximately 8% of total anthropogenic CO2 emissions, with growth in tourism and global trade projected to increase this contribution further1–3. Carbon-neutral transportation is feasible with electric motors powered by rechargeable batteries, though challenging if not impossible for long-haul commercial travel, particularly air travel4. A promising solution are drop-in fuels (synthetic alternatives for petroleum-derived liquid hydrocarbon fuels such as kerosene, gasoline or diesel) made from H2O and CO2 by solar-driven processes5–7. Among the many possible approaches, the thermochemical path using concentrated solar radiation as the source of high-temperature process heat offers potentially high production rates and efficiencies8 and can deliver truly carbon-neutral fuels if the required CO2 is obtained directly from atmospheric air9. If H2O is also co-extracted from air10, feedstock sourcing and fuel production can be co-located in desert regions with high solar irradiation and limited access to water resources. While individual steps of such a scheme have been implemented, we now demonstrate operation of the entire thermochemical solar fuel production chain, from H2O and CO2 captured directly from ambient air to the synthesis of drop-in transportation fuels (e.g. methanol, kerosene), with a modular 5-kWthermal pilot-scale solar system operated under real field conditions. We further identify the R&D efforts and discuss the economic viability and policies required to bring these solar fuels to market.