Revisiting HCN formation in Earth's early atmosphere

Using a new photochemical model, the HCN chemistry in Earth's early atmosphere is revisited. We find that HCN production in a CH4-rich early atmosphere could have been efficient, similar to the results of a previous study (Zahnle, 1986). For an assumed CH4 mixing ratio of 1000 ppmv, HCN surface deposition increases from 2 x 10(9) cm(-2) s(-1) at fCO(2) = 3% to more than 1 x 10(10) cm(-2) s(-1) (30 Tg/yr) at fCO(2) = 0.3% and 1%. These conditions may well have applied throughout much of the Archean eon, 3.8-2.5 Ga. Prior to the origin of life and the advent of methanogens. HCN production rates would likely have been at 1 x 10(7) cm(-2) s(-1) or lower, thereby providing a modest source of HCN for prebiotic synthesis. (C) 2011 Elsevier B.V. All rights reserved.