Sulfur Particles on the Early Earth

Understanding the atmosphere of the early Earth during the Archean, the period of time approximately 4 - 2.45 billion years ago, is an important part of understanding the conditions under which life originated and developed. The presence of sulfur mass independent fractionation (S-MIF) in Archean sediments is thought to be evidence for an early anoxic atmosphere. Photolysis of sulfur dioxide (SO2) by UV light with λ<220 nm has been shown in models and some initial laboratory studies to create a S-MIF fractionation; however, sulfur must leave the atmosphere in at least two chemically different forms to preserve any S-MIF signature. It has been suggested that atmospheric reactions in a reducing atmosphere could favor S8 particles over sulfuric acid particles. Here we use aerosol mass spectrometry to probe the chemical composition of particles formed from reactions of sulfur dioxide under a range of atmospheric conditions. We find that S8 formation is enhanced in the presence of reducing species, such as H2 (with electrical discharge as the energy source for reaction) and CH4 (using UV light with wavelengths from 115-400 nm to initiate reaction). Additionally, organosulfur species are formed from the photochemical reaction of SO2 and CH4. These species could be important in the later Archean when methanogenic bacteria could provide a source of CH4 to the atmosphere. Implications for the early Earth are discussed.