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 O2, absorption of solar EUV (λ < 1023Å) by N2 provides a large thermospheric source of atomic nitrogen. Methane is oxidized cleanly and efficiently, provided CO2 is more abundant than CH4. 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 O2, high N2, and moderately high levels of CO2 would have made the ancient terrestrial atmosphere a favorable environment for the production of HCN from CH4. 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 1011 molecules cm−2 s−1 (corresponding to ƒ(CH4) 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 NH3 mixing ratios approaching 1 ppm.

The evolution of Earth's biogeochemical nitrogen cycle

“…Ammonification can be abiotic and is just included for completeness. Metal catalysts and isotopic fractionations are taken from the literature (Brunner et al, 2013;Buick, 2007a;Casciotti, 2009;Godfrey and Glass, 2011;Hoch et al, 1992;McCready et al, 1983;Nishizawa et al, 2014;Pennock et al, 1996;Waser et al, 1998;Zerkle et al, 2008;Zhang et al, 2014 Tian et al (2011) and Zahnle (1986), 10. Schoonen & Xu (2001), 11.…”

“…excluding impacts) of fixed nitrogen would have been lightning and volcanic eruptions, and a realistic steady-state abiotic production flux would probably have been on the order of 10 10 -10 11 mol N/yr. Rates of up to 10 12 mol N/yr may have been achieved under high pCH 4 (1000ppmv) and <1% pCO 2 favoring the production of HCN later in the Archean (Tian et al, 2011;Zahnle, 1986), but pCH 4 would probably have been low in the Hadean under prebiotic conditions prior to the origin of methanogenic microbes, lowering this flux to 10 9 mol N/yr. Lightning and volcanism both generate NO x species which could have been reduced abiotically into the more bioavailable NH 4 + in hydrothermal vents, catalyzed by sulfide minerals or native metals (Brandes et al, 1998;Singireddy et al, 2012;Smirnov et al, 2008;Summers and Chang, 1993), provided that the reduction did not stop at N 2 (Section 2.3).…”

Was ferrocyanide a prebiotic reagent?

“…The production rate of hydrogen cyanide is critically dependent on the atmospheric composition for electric discharge synthesis (Stribling and Miller, 1987), lightning synthesis (Chameides and Walker, 1981), and the presence of small amounts of methane for ultraviolet light synthesis via N atoms (Zahnle, 1986). The major path for loss of hydrogen cyanide is by hydrolysis to formate.…”

UV shielding of NH3and O2by organic hazes in the Archean atmosphere

“…This particular abiotic source of fixed N would have been even smaller in a low-O2 atmosphere. However, ammonium could also have been generated by hydrolysis of HCN generated photochemically in the stratosphere [Zahnle, 1986]. Zahnle's estimates for the rate at which HCN rains out into the oceans range from l0 s to 10 •ø cm -2 s -1 for various models.…”

“…The questions we are addressing are still first-order ones, and we feel that our approach to the modeling is commensurate with this level of accuracy. The photochemical model used here is a one-dimensional (horizontally averaged) model that has been adapted from one used in previous studies of the primitive terrestrial atmosphere [e.g., Kasting, 1982;Kasting et al, 1983;Zahnle, 1986;Kasting, 1990]. The model includes 72 chemical species involved in 337 reactions.…”