| Summary: | The evolution of the early atmosphere was driven by changes in its chemical composition, which involved the formation of some critical gases. In this study, we demonstrate that nitrous oxide (N<sub>2</sub>O) can be produced from Miller’s early atmosphere (a mixture of CH<sub>4</sub>, NH<sub>3</sub>, H<sub>2</sub>, and H<sub>2</sub>O) by way of photocatalysis. Both NH<sub>3</sub> and H<sub>2</sub>O were indispensable for the production of N<sub>2</sub>O by photocatalysis. Different conditions related to seawater and reaction temperature are also explored. N<sub>2</sub>O has a strong greenhouse gas effect, which is more able to warm the Earth than other gases and offers a reasonable explanation for the faint young Sun paradox on the early Earth. Moreover, the decomposition of N<sub>2</sub>O into N<sub>2</sub> and O<sub>2</sub> can be boosted by soft irradiation, providing a possible and important origin of atmospheric O<sub>2</sub> and N<sub>2</sub>. The occurrence of O<sub>2</sub> propelled the evolution of the atmosphere from being fundamentally reducing to oxidizing. This work describes a possible vital contribution of photocatalysis to the evolution of the early atmosphere.
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