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Cited by 277Atmospheric chemistry and aerosolsPaleontology and Stratigraphy of FossilsAstro and Planetary Science

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

Zahnle, Kevin J.·1986·Journal of Geophysical Research: Atmospheres
DOI 10.1029/jd091id02p02819
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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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Crossref · 10.1029/jd091id02p02819

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122 of 122 references verified0 citation errors · 0 not checkable

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  • N atoms from N2 photolysis in the upper atmosphere can mix to lower levels and react with CH4 photolysis products to make HCN.

    “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]

  • UV decomposes CH4 in an anoxic upper atmosphere, releasing hydrogen.

    “UV decomposes CH4 in an anoxic upper atmosphere, releasing hydrogen [e.g., (166)].”

    From The Archean atmosphere (2020) · cites this paper as [166]

  • At Archean biogenic methane levels of ~10^3 ppmv, HCN concentrations reach ~10^2 ppmv.

    “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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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)
DOI
10.1029/jd091id02p02819
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
Licence
onlinelibrary.wiley.com/termsAndConditions#vor (vor, 9710 d)
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 › Physics and Astronomy › Astronomy and Astrophysics › 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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