Atmospheric Chemistry II

Climate Physics and Chemistry Atmospheric Chemistry II: Methane ogenic CH4 and CO2 330 1900 CO2 1700 CH4 1500 310 1300 290 1100 270 250 900 1750 1800 1850 1900 1950 CH4 Concentration (ppbv) CO2 Concentration (ppmv) 350 700 Year Reconstruction of the CO2 and CH4 increases since the preindustrial time from data, respectively, from the Siple (6) and DE08 (7) ice cores. Note the good agreement between the ice core data and the direct measurements (solid lines) in the atmosphere starting from 1958 (CO2) and 1978 (CH4). Methane is a strong greenhouse gas and contributes to global warming RELEVANCE TO CLIMATE Global-Mean Radiative Forcing (Wm-2) 3 Halocarbons N2O CH4 2 CO2 1 Tropospheric aerosols - direct effect Fossil fuel soot Solar 0 Tropospheric ozone Sulphate -1 Stratospheric ozone Biomass burning Tropospheric aerosols - indirect effect -2 High Low Confidence Level Low Low Very Very Very Very Low Low Low Low Estimates of the globally and annually averaged anthropogenic radiative forcing (in Wm-2) due to changes in concentrations of greenhouse gases and aerosols from pre-industrial times to the present day and to natural changes in solar output from 1850 to the present day. Main natural CH4 source to atmosphere: anaerobic microbial fermentation in environments where all other oxidants (O2, MnO2, NO3-, Fe2O3, SO4=) are depleted: 2 "CH2O" -> CH4 + CO2 ΔG ~ -350 kJ/mole of glucose [actual direct precursors of CH4 are fermentation of acetate and/or oxidation of hydrogen using CO2 as the electron acceptor] In some environments, this methane can escape into the oxic environment (e.g., gas bubbles rising through water). In other environments where transport goes through less reducing environments, methane can be lost by methanotropic bacteria. In marine sediments, CH4 is consumed by the microbial reaction: CH4 + SO4= -> HCO3- + HS- + H2O Tropospheric life cycles of climatically important species Determination of CH4 lifetime from CH3CCl3 CH3CC13 180 160 Mixing Ratio (ppt) 140 ( ALE/GAGE/AGAGE k1 OH + CH3CCl3 CH2CCl3 + H2O k2 OH + CH4 CH3 + H2O ( 120 100 Results 80 τi = 60 40 ∫[i]dV ∫ki[i][OH]dV = { _ 0.3yr (CH3CCl3) 4.6 + _ 0.6yr (CH4) 8.9 + { ∫k [i][OH]dV = _ 0.6)x 105 cm-3 (9.7 + < [OH] < = ∫kii[i]dV 78 80 82 84 86 88 90 Ireland Oregon Barbados Samoa Tasmania 92 Year Determination of OH by Inverse Method 94 96 • • CH4 sources: Some methane is emitted from the seafloor by natural gas seeps (major source of methane from this source is thermal cracking of buried organic matter) including “cold seeps” and “mud volcanos”. On some occasions methane can be emitted from methane hydrate decomposition. Usually, most this methane is oxidized before it is emitted to the atmosphere. CH4 dominantly enters the atmosphere from land. The main natural sources are natural wetlands and termites. At present, these natural sources are supplemented by several anthropogenic sources – rice fields, enteric fermentation (cows), oil and natural gas leakage, coal mining, landfills, sewage, animal wastes, and biomass burning. Because most of the land is in the Northern Hemisphere, methane is slightly higher in the northern hemisphere, and maximum concentrations occur at high northern latitudes. CH4 (ppb) 1800 1700 1600 1500 1984 1985 1986 1987 90 60 30 0 Latitude -30 -60 -90 Temporal and latitudinal variation of the tropospheric methane mixing ratio. Methane Tropospheric Chemistry • • • • • Main CH4 tropospheric sink is reaction with OH radical: CH4 + OH --> CH3 + H2O Tropospheric OH source is: O3 + hν --> O2 + O ; O + H2O --> 2 OH OH is the main oxidizing molecule of the troposphere; it is involved in most tropospheric oxidation reactions. But its lifetime is short and its concentration is very low: ~104 atoms per cm3. "OH can react quickly with CO to yield CO2, and with CH4 and other hydrocarbons to form formaldehyde. In the process, OH is converted to HO2 (hydroperoxyl), from which OH can be regenerated to start the chain all over again." CH4 tropospheric lifetime is ~ 8 yrs (best established through global model of anthropogenic methylchloroform CH3CCl4) 2 0 b -2 -4 -6 CH4 (p.p.b.v) 700 -8 600 c 500 400 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 Age (yr BP) Methane in the Vostok Ice Core Temperature (oC) Methane has undergone large changes in the past due to climate modulation of methane sources CH4 budget 0 100 200 300 400 Turnover (Tg yr-1) 500 600 700 Total sinks Reaction with OH radicals Soil uptake Transport into the stratosphere Total sources Natural wetlands Rice fields Enteric fermentation Landfills Biomass burning Animal wastes Domestic sewage Coal mining Oil and gas usage Termites Various Atmospheric increase Average 1983-1991 Expected 1992 Observed 1992 1992 anomaly Methane during the past 12,000 years 1,600 1,500 1,400 1,300 1,200 1,100 1,000 100 200 300 400 500 600 700 800 900 Depth (m) -34 -36 -37 800 CH4 (p.p.b.v.) -38 750 -39 700 650 600 550 0 2,000 4,000 6,000 8,000 10,000 12,000 δ18O ( 850 ) -35 CH4 hydrates • Methane forms a solid cage-like compound (clathrate) with water under certain highpressure/low-temperature regimes. It is estimated that a very large reservoir exists in this form globally - possibly larger than all other fossil fuel reserves. A significant advantage to using CH4 instead of other fossil fuels is that it emits half the carbon per unit energy released. Is CH4 clathrate recoverable economically, or it similar to fusion power (always 50 years away)? One source estimated that the first commercial CH4 hydrate gas production may begin in 10-15 years (Science 303:944-947, 2004). • Below the zone of methane hydrate stability (due to geothermal gradient), methane reverts to a gas phase and forms bubbles. The transition from solid methane hydrate to bubbles results in a strong acoustic impedance transition and a “bottom simulating reflector” (BSR). Catastrophic releases of methane hydrates: climate, earthquake, and tsunami connections? • There is some evidence that methane hydrates can decompose rapidly due to rising temperature and/or lower pressure due to falling sea level or glacial melting. • Some modeling (C. Ruppel) suggests that it is difficult to decompose the methane hydrate quickly (because the reaction is endothermic and counteracts the diffusion of heat. So climate warming by itself may not be sufficient. • There is evidence for a link between massive submarine landslides and methane hydrates. Could methane hydrate decomposition set off these landslides (and tsunamis)? • A major submarine landslide in the Norwegian Sea 8200 years ago (“Storegga Slide”, which caused drastic tsunamis) occurred at the same time as a significant climate cooling seen in the Greenland ice cores (chicken, egg, or coincidence?). Earthquake triggering of slope failure (involving hydrate amplification) also has been suggested. • It has been suggested that methane hydrate releases may have produced abrupt climate change during the past 60,000 years (J. Kennett). Storegga Slide ~8200 yrBP Sent a 10 m high tsunami into the northern coast of Europe ? Triggered by earthquake and/or catastrophic CH4 hydrate decomposition?