Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere

The photochemistries of methane and HCN are discussed in the context of the primitive terrestrial atmosphere, using a detailed numerical model. In the absence of abundant O 2 , absorption of solar EUV (λ < 1023Å) by N 2 provides a large thermospheric source of atomic nitrogen. Methane is oxidized cleanly and efficiently, provided CO 2 is more abundant than CH 4 . Otherwise, a large fraction of the methane present is polymerized, forming alkanes in the troposphere and polyacetylenes and nitriles in the upper atmosphere. The combination of low O 2 , high N 2 , and moderately high levels of CO 2 would have made the ancient terrestrial atmosphere a favorable environment for the production of HCN from CH 4 . Once formed, HCN is rather long‐lived; it is removed from the atmosphere either by direct photodissociation at Ly α (∼100 years) or by rainfall (∼10 years). Chemical loss would have been unimportant. Owing to its stability, transport of HCN from the top to the bottom of the atmosphere can be efficient; nevertheless, our results are sensitive to the assumed eddy diffusion profile. For small amounts of methane a small constant fraction of order 0.1% to 1% of the carbon in the methane is returned to the surface as hydrocyanic acid rain. For larger methane sources exceeding a critical value of order 10 11 molecules cm −2 s −1 (corresponding to ƒ(CH 4 ) of order 10 −4 –10 −3 ), rainout of HCN increases abruptly to more than 10% of the carbon supplied as methane, limited by the primary production of N. Under favorable conditions, hydrolysis of HCN could have supported atmospheric NH 3 mixing ratios approaching 1 ppm.