The DOI matches its Crossref record; 122 of the 123 references it cites were found in a registry or an index.
Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere
Abstract
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.
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“In particular, N atoms from N2 photolysis in the upper atmosphere can mix to lower levels and react with CH4 photolysis products to make HCN (166, 167).”
From The Archean atmosphere (2020) · cites this paper as [166]
In the citing paper
“UV decomposes CH4 in an anoxic upper atmosphere, releasing hydrogen [e.g., (166)].”
From The Archean atmosphere (2020) · cites this paper as [166]
In the citing paper
“At Archean biogenic methane levels of ~103 ppmv, HCN concentrations reach ~102 ppmv.”
From The Archean atmosphere (2020) · cites this paper as [166]
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Metadata
- Authors
- Zahnle, K. J.
- Corresponding author
- Zahnle, Kevin J.
- ORCID
- Zahnle, Kevin J.
- Venue
- Journal of Geophysical Research: Atmospheres
- Publisher
- American Geophysical Union (AGU)
- Published
- 1986-02-20
- Volume
- 91
- Issue
- D2
- Pages
- 2819-2834
- ISSN
- 0148-0227 (ISSN-L)
- OpenAlex
- W1965198394
- Cited by
- 276
- References
- 123
- Citations by year
- 2026: 10 · 2025: 18 · 2024: 13 · 2023: 7 · 2022: 18 · 2021: 10 · 2020: 16 · 2019: 20 · 2018: 17 · 2017: 7 · 2016: 11 · 2015: 11 · 2014: 6 · 2013: 8 · 2012: 4
- FWCI
- 4.19
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Journal of Geophysical Research Atmospheres (publishedVersion)
- Topics
- Physical Sciences › Earth and Planetary Sciences › Atmospheric Science › Atmospheric chemistry and aerosols
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
- Physical Sciences › Earth and Planetary Sciences › Space and Planetary Science › Astro and Planetary Science
- Keywords
- methane photochemistry, hydrogen cyanide, early Earth atmosphere, atomic nitrogen, methane oxidation, polyacetylene, nitriles, atmospheric transport, turbulent diffusion, ammonia, carbon cycle, CO2 abundance
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