Reduction in Haze Formation Rate on Prebiotic Earth in the Presence of Hydrogen

摘要

Recent attempts to resolve the faint young Sun paradox have focused on an early Earth atmosphere with elevated levels of the greenhouse gases methane (CH_4) and carbon dioxide (CO_2) that could have provided adequate warming to Earth's surface. On Titan, the photolysis of CH_4 has been shown to create a thick haze layer that cools its surface. Unlike Titan, however, early Earth's atmosphere likely contained high amounts of CO_2 and hydrogen (H_2). In this work, we examine haze formation in an early Earth atmosphere composed of CO_2, H_2, N_2, and CH_4, with a CO_2/CH_4 ratio of 10 and a H2/CO_2 ratio of up to 15. To initiate aerosol formation, a broad-spectrum ultraviolet (UV) energy source with emission at Lyman-α was used to simulate the solar spectrum. Aerosol composition and total aerosol mass produced as a function of reagent gas were measured with an aerosol mass spectrometer (AMS). Results show an order of magnitude decrease in haze production with the addition of H_2, with no significant change in the chemical composition of the haze. We calculate that the presence of H_2 on early Earth could thus have favored warmer surface temperatures and yet allowed photochemical haze formation to deliver complex organic species to early Earth's surface.