THE PHOTOCHEMISTRY OF BIOGENIC GASES IN THE EARLY AND PRESENT ATMOSPHERE JOEL S. LEVINE and TOMMY R. AUGUSTSSON* Atmospheric Sciences Division, NASA Langley Research Center, Hampton, Virginia 23665, U.S.A. (Received 21 March, 1985) 1. Introduction The role of the biosphere in the production and regulation of major (nitrogen (N2) and oxygen (02)) and minor (carbon dioxide (CO2)) atmospheric gases has been fully appreciated for many years. The realization of the importance of the biosphere as a source of trace atmospheric gases (gases with a surface mixing ratio on the order of parts per million by volume (ppmv = 10 -6) or less) is more recent (Lovelock and Margulis, 1974; Margulis and Lovelock, 1974; Margulis and Lovelock, 1978). Important trace atmospheric gases of biogenic origin include methane (CH4), ammonia (NH3), hydrogen sulfide (HAS), and nitrous oxide (N20). For many years it has been assumed that the Earth's prebiological paleoatmosphere was strongly reducing, containing large amounts of methane, ammonia, hydrogen sulfide, and carbon monoxide (CO) (Hart, 1979). Over the last few years, however, geochemical and geological evidence (Walker, 1977) and photochemical/chemical considerations (Levine, 1982) have favored a more mildly reducing mixture of molecular nitrogen, carbon dioxide, and water vapor (H20) for the composition of the pre- biological paleoatmosphere. Methane, ammonia, hydrogen sulfide, and carbon monoxide are all important trace gases in the present atmosphere and impact atmospheric photochemistry and chemistry. With the exception of carbon monoxide, these gases are overwhelmingly produced by biogenic activity in the present atmos- phere. Nitrous oxide, another biogenic gas, which controls the levels of ozone (03) in the stratosphere will also be considered. Recent measurements indicate that atmos- pheric levels of methane and nitrous oxide may be increasing. Methane, nitrous oxide, and ammonia absorb Earth-emitted infrared radiation and, hence, impact the climate as well as the photochemistry/chemistry of the atmosphere. The distribution of these gases in the early atmosphere will be investigated using a photochemical model of the early atmosphere, which i~ described in the next section. Thermodynamic equilibrium calculations** indicate that the levels of CH4, NH3, * NASA-National ResearchCouncil Research Fellow ** Thermodynamicequilibriumconcentrationsare thosecalculatedby assumingthe productionofa given species in thermodynamic equilibrium with a mixture of air (N 2 =0.78arm, 02 =0.2latin, H20 = 0.01atm, and CO2 = 3.3 x 10 -4 arm) at atmospherictemperature (about 300K). Origins of Life 15 (1985)299-318.0046-5763/85.15. ©1985 by D. Reidel Publishing Company