The Global Methane Cycle CH 4 in soil & atmosphere.
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Transcript of The Global Methane Cycle CH 4 in soil & atmosphere.
TopicsTopics
• General Methane Information
• Sources & Sinks (general)
• CH4 in the soil
• CH4 in the atmosphere
• Conclusions
Ins & OutsIns & Outs
• Most abundant organic trace gas in the atmosphere
• Concentrations have doubled since pre-industrial times (now ~1700 ppbv)
• After CO2 and H2O most abundant greenhouse gas
• 20 to 30 times more effective greenhouse gas than CO2 (carbon dioxide)
CHCH44, what does it do?, what does it do?
• Helps control amount of OH (hydroxyl) in the troposphere
• Affects concentrations of water vapor and O3 (ozone) in the stratosphere
• Plays a key-role in conversion of reactive Cl to less reactive HCl in stratosphere
• As a greenhouse gas it plays a role in climate warming
CHCH44 through Time through Time
• Record of CH4 from air bubbles trapped in polar ice (Antarctica and Greenland)
• CH4 levels closely tied to glacial-interglacial records
• CH4 ‘follows’ temperature
• Unprecedented rise since industrial revolution: CH4 emissions
CHCH44 Geographically Geographically
• 150 ppb Pole-to-pole gradient, indicating consistently large emissions in the northern hemisphere
Natural SourcesNatural Sources
• Wetlands• Oceans• Hydrates• Wild ruminants• Termites
+Total : 30% (~100-200
TgCH4/year)
Anthropogenic SourcesAnthropogenic Sources
• Agriculture (ruminants)
• Waste disposal• Biomass burning• Rice paddies
+Total : 70%
Sinks for tropospheric CHSinks for tropospheric CH44
• Reaction with hydroxyl radical (~90%)
• Transport to the stratosphere (~5%)
• Dry soil oxidation (~5%)+
Total : ~560 TgCH4/y
General InformationGeneral Information
• Atmospheric CH4 is mainly (70-80%) from biological origin
• Produced in anoxic environments, by anaerobic digestion of organic matter
• Natural and cultivated submerged soils contribute ~55% of emitted CH4
• Upland (emerged) soils responsible for ~5% uptake of atmospheric CH4
Methanogenesis in SoilsMethanogenesis in Soils
• Produced in anoxic environments, by anaerobic digestion and/or mineralisation of organic matter:
C6H12O6 3CO2 + 3CH4
(with low SO42- and NO3
- concentrations)
• Formed at low Eh (< -200mV)
• Formed by ‘Methanogens’ (Archaea)
Methanotrophy in SoilsMethanotrophy in Soils
• 2 Forms of oxidation recognized in soils:
• I) ‘High Affinity Oxidation’ in soils with close to atmospheric CH4 concentrations (<12ppm), upland/dry soils
• II) ‘Low Affinity Oxidation’ in soils with CH4 concentrations higher than 40 ppm, wetland/submerged soils
Low Affinity OxidationLow Affinity Oxidation
• Performed by methanotrophic bacteria
• Methanotrophs in all soils with pH higher than 4.4 in aerobic zone
• Methane oxidation in methanogenic environments is Low Affinity Oxidation
• Methane oxidation is Aerobic the amount of oxygen is the limiting factor
Low Affinity & Rice FieldsLow Affinity & Rice Fields
• More than 90% of methane produced in methanogenic environments is reoxidised by methanotrophs
• Variations in CH4 emissions from ricefields mostly due to variations in methanotrophy
• Emission of CH4 mostly through rice aerenchyma (‘pipes’)
• Soil oxidation through aerenchyma
More General InfoMore General Info
• Methanotrophy is highest in methanogenic environments
• Both methanogens and trophs prevail under unfavorable conditions (high/low water etc)
• Methane emission is larger from planted rice fields than from fallow fields, due to higher C availability and aerenchyma
High AffinityHigh Affinity
• Upland forest soils most effective CH4 sink
• Temporarily submerged upland soils can become methanogenic
• Arable land much smaller CH4 uptake than untreated soils
WaterWater
• Soil submersion allows methanogenesis
• Reduces methanotrophy• Short periods of
drainage decreases methanogenesis in ricefields dramatically (Fe, SO4)
pH and TemperaturepH and Temperature
• Methanogenesis most efficient around pH neutrality
• Methanotrophs more tolerant to variations in pH
• Methanogenesis is optimum between 30 and 40 oC
• Methanotrophs are more tolerant to temperature variations
Rice and FertilizersRice and Fertilizers
• Goal: High yield and less methane emission
• Organic fertilizers increase CH4
(incorporation org. C) Reduce CH4 by
raising Eh and competition (e.g. SO4)
Rice Rice UPUP, CH, CH44 DOWNDOWN
• Fertilizers containing SO4 may poison the soil
• Ammonium and urea decrease methanotrophy/CH4 oxidation, especially in upland soils
• Calcium carbide significantly reduces CH4 emission and increases rice yield by inhibiting nitrification
Major atmospheric CHMajor atmospheric CH44 sink: sink: OHOH
• Reaction with hydroxyl (OH) radical (~90%) in the troposphere
• OH is formed by photodissociation of tropospheric ozone and water vapor
• OH is the primary oxidant for most tropospheric pollutants (CH4, CO, NOx)
• Amount CH4 removed constrained by OH levels and reaction rate
Source of OHSource of OH
• Formed when O3 (ozone) is photo-dissociated:
O3 + hv O(1D) + O2
which in turn reacts with water vapor to form 2 OH radicals:
O(1D) + H2O OH + OH
(OH is also formed in Stratosphere by oxidation of CH4 due to high concentrations of Cl)
Sink of OHSink of OH
• CH4 mainly removed by reaction
CH4 + OH• CH3• + H2O
• OH concentrations not only affected by direct emissions of methane but also by its oxidation products, especially CO
• Increase in methane leads to positive feedback; build-up of CH4 concentrations
ProjectionsProjections
• OH loss rates may increase due to rising anthropogenic emissions
• OH loss rates may be balanced by increased production through O3 and NOx::
Urban areas: NOx increase
NOx results in O3 formation
O3 dissociates to OH
Projections 2Projections 2
• Stratospheric ozone decreases as seen in recent years
• Due to decrease of stratospheric O3, ultraviolet radiation in troposphere increases increase OH
• Water vapor through temperature rise may either increase or decrease OH
Projections 3: TropicsProjections 3: Tropics
• Tropics: high UV, high water vapor High OH
• High CH4 production due to rice fields, biomass burning, domestic ruminants
• Future changes in land use / industrialization
NONOxx and OH and OH
• Polluted areas High NOx OH production (temperate zone Northern hemisphere, planetary boundary layer of the tropics)
• Unpolluted areas Low NOx OH destruction (marine area`s, most of the tropics, most of the Southern hemisphere)
OO33 in Tropo- and Stratosphere in Tropo- and Stratosphere
• Ozone (O3) absorbs ultraviolet radiation, but is also a greenhouse gas
• 90% of O3 in the Stratosphere• Stratospheric production by photo-
dissociation of O2 and reaction with O2
• 10% of O3 in the Troposphere, through downward transport from the stratosphere and photolysis of NO2 in the troposphere
Stratospheric OzoneStratospheric Ozone
• O3 destroyed by catalytic mechanisms involving free radicals like NOx, ClOx, HOx
• CH4 acts as source and sink for reactive chlorine:
- Sink: direct reaction with reactive Cl to form HCl (main Cl reservoir species)
- Source: OH (oxidation of CH4 in stratosphere) reacts with HCl to form reactive Cl
Stratospheric Ozone 2Stratospheric Ozone 2
• OH from the dissociation of methane can react with ozone (especially in the upper stratosphere)
• Conclusively: increasing CH4 leads to net O3 production in troposphere and lower stratosphere and net O3 destruction in the upper stratosphere
CHCH44 impact on Climate impact on Climate
• CH4 absorbs infrared radiation increases greenhouse effect
• Globally-averaged surface temperature 1.3oC higher than without methane
• Dissociation of CH4 leads to CO2: additional climatic forcing
• CH4 has increased dramatically over the last century and continues to increase
• Causal role of human activity
• Climate forcing by CH4 confirmed, though not fully understood
• Future developments uncertain