Carbon cycleGlobal atmospheric carbon dioxide cycle Scientific Topics

Carbon Cycle Group

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The long-term increase of greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the global atmosphere is a matter of concern, not only to scientists, but also to politicians and the society. This increase is caused - to a large extent - by man's activities, and may lead to significant changes in climate on Earth

The carbon cycle group at the Institute of Environmental Physics investigates the bio–geochemical cycles of these long-lived greenhouse gases on the regional but also on the global scale. It measures long-term atmospheric trends of these greenhouse gases as well as radiocarbon (14C/C) in atmospheric CO2.  Acconpanying atmospheric 222Radon activity concentration measurements are used as tracer to evaluate atmospheric transport processes.

In the last decade, the carbon cycle group at IUP has contributed to the design and implementation of the European Research Infrastructure ICOS (Integrated Carbon Observation System) and has set up in Heidelberg the Central Radiocarbon Laboratory as part of the Central Analytical Laboratory of ICOS. Radiocarbon measurements in CO2 are used in ICOS to separate the fossil fuel from biogenic source contributions in atmospheric CO2 over Europe.

Current Research Topics

  • Long-term monitoring of 14CO2 in the global atmosphere
  • 14CO2 and 222Rn measurements at ATTO (Amazon Tall Tower Observatory)
  • Development of measurement strategies for ICOS: RINGO
  • Testing of surrogate tracers (CO, NOx) for fossil fuel CO2: VERIFY
  • Testing of O2/N2 measurements as surrogate tracer for fossil fuel CO2: ICOS
  • Development of a process-based 222Radon exhalation map for the Amazon region: ATTO

Scientific Topics

Long-term monitoring of radiocarbon in atmospheric CO2

Delta14CO2 in the global atmosphere from 1950 until 2019. The maximum D14C was a factor of two higher than the reference level n 1963 in the NH and decreased exponentially since then

Radiocarbon (14C) is the natural radioactive carbon isotope, which is produced in the atmosphere by cosmic ray induced reactions with atmospheric nitrogen. The radioactive half life of 14C is 5700 years. In an undisturbed steady state situation the atmospheric 14CO2 activity corresponds to an equilibrium between its production in the atmosphere and its radioactive decay in all carbon reservoirs exchanging CO2 with the atmosphere. The natural equilibrium level of atmospheric 14CO2 has been disturbed through man's activities in the last century, (1) by the ongoing input of fossil fuel CO2 into the atmosphere known as Suess effect, and (2) through nuclear bomb tests in the atmosphere in the 1950s and early 1960s (14C bomb effect).

CO2 from the burning of fossil fuels, due to its age of several hundred million years is free of 14C. Adding fossil fuel CO2 to the atmosphere therefore leads not only to an increase of its mole fraction but also to a dilution of the 14C/C ratio in CO2. While the Suess effect caused a reduction of the 14C/C ratio in atmospheric CO2 by only several percent in the 1940s, the 14C bomb effect resulted in a global increase of 14CO2 by more than 80 %. Equilibration of this bomb 14C spike with the other carbon reservoirs, ocean and terrestrial biosphere, leads to a decrease of the atmospheric 14C/C ratio after the atmospheric test ban treaty in 1963 with today's activity ratios approaching again natural levels (Δ14C ≈ 0‰) (Levin et al., 2012; 2013). Today, global fossil fuel emissions are responsible for the ongoing decrease of Δ14CO2 observed globally. In polluted areas e.g. on the European continent, a surplus of CO2 from the burning of fossil fuels is clearly detectable by significant 14C/C depletions in regional atmospheric CO2 relative to measurements made in background air i.e. over the Atlantic ocean or at a high mountain site as for example Jungfraujoch. Regional 14CO2 observations can thus be used to validate potential emissions reductions in the catchment area of the measurement sites (e.g. Levin and Rödenbeck, 2008), one of the aims of 14CO2 observations in ICOS.

Development of measurement strategies for ICOS

 

Testing of surrogate tracers (CO, NOx) for fossil fuel CO2: VERIFY

 

Testing of O2/N2 measurements as surrogate tracer for fossil fuel CO2: ICOS

 

 

Past measurement programs and data products

Global Atmospheric Methane Cycle

Besides carbon dioxide, methane is the most important anthropogenic greenhouse gas in the atmosphere. Global atmospheric CH4 concentration increased by more than a factor of two since the beginning of industrialisation. Although the atmospheric CH4 abundance is still very small compared to that of CO2 the additional radiative forcing of additional CH4 since 1750 is about 1/3 of that of the CO2 increase. Methane has both, natural and anthropogenic sources and is mainly produced by bacteria in anaerobic environments that have high organic matter content. Such bacterial sources are natural wetlands, rice agriculture, animal breeding (ruminants), landfills, termites as well as aquatic sediments. Other so called thermogenic sources are incomplete combustion of biomass and energy generation where methane is released by extraction, processing and distribution of coal, gas and oil. About 40% of total CH4 emissions come from natural, 60% from anthropogenic sources. The main sink of methane is chemical destruction in the troposphere through reaction with OH-radicals (about 90%). Other sinks which contribute to the removal of tropospheric CH4 are oxidation by soil bacteria and transport into the stratosphere. Here methane is destroyed by reactions with OH, Cl and O(1D) radicals. The stratospheric water content - which plays an important role in the ozone depletion process - originates to about 30% from stratospheric methane oxidation. As most of the sources are located on continents and in the northern hemisphere we observe an interhemispheric gradient in atmospheric methane mole fraction of about 100 ppb (see figure). Atmospheric methane reveals a seasonal cycle, which in the northern hemisphere is in anti-phase to that of the southern hemisphere. These seasonal cycles are caused by changing emissions of the sources as well as changing destruction rates of the sinks.

Methane itself occurs as different isotopologues: 12CH4, 13CH4, 12CH3D and 14CH4 (D = Deuterium = 2H). Other isotopologues are negligible. The atmospheric ratios 13CH4/12CH4 and 12CH3D/12CH4 do, however, not reflect the natural abundance of the constituents because all formation and destruction processes of methane are fractionating isotopes: different sources of methane have different isotopic signatures and the different sinks have different fractionation factors. Therefore the investigation of the long term trend of the isotopic composition of methane provides important additional information about the development of individual methane sources and sinks. In the IUP carbon cycle group we perform continuous measurements of methane mole fraction in ambient air (Heidelberg) since 1994. Measurements of methane mole fraction and isotope-ratios on samples from Neumayer station, Antarctica are performed since 1988, from Alert, Nunavut, Canada since 1990 and from Izaña, Tenerife, Spain from 1991 to 1998 (Levin et al., 2012). The most recent trends of CH4 in the Arctic and Antarctica are displayed in the Figure below.


While CH4 was steadily increasing in both hemispheres until about 2000 it stayed surprisingly stable for almost one decade but in the last decade we again observe global increase rates similar to those in the 1990s. The reasons for this latest development are not perfectly understood, which is also true for the inter-annual variations in the last decades (Schäfer et al., 2016).

Molecular Hydrogen in the Atmosphere

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After methane molecular hydrogen (H2) is the second most abundant reduced gas in the atmosphere, with a globally averaged mixing ratio of about 500 ppb. This value corresponds to a global tropospheric H2 inventory of around 150 Tg (1012 g H2). Since hydrogen is believed to play a major role as a "clean" energy carrier in future energy scenarios, it is important to investigate the potential impact of an additional anthropogenic (man made) H2 source with respect to climate interactions. Molecular hydrogen as such does not affect the radiation budget of the atmosphere directly, thus H2 is not regarded as a greenhouse gas. However, H2 plays an important role in air chemistry as it influences the atmospheric oxidation capacity through reaction with the OH radical. Increased atmospheric hydrogen levels will most probably lead to an increased lifetime of many atmospheric constituents including the greenhouse gas methane. A second aspect of H2 concentration changes in the atmosphere pertain its ability to raise the water vapour content in the stratosphere. This in turn influences the radiation budget of the stratosphere (increased water vapour will cool the stratosphere) and is expected to contribute to stratospheric ozone depletion (e.g. ice particle formation).

The recent atmospheric H2 budget is not well established quantitatively. Important sources are combustion processes as well as oxidation of CH4 and non methane hydroCarbons in the atmosphere which themselves are emitted from various anthropogenic and natural sources. The global atmospheric H2 budget is closed by two main sinks: the oxidation of H2 initiated by the reaction with OH radicals and the uptake by soils. The latter is believed to be the dominating part leading to a total atmospheric lifetime of H2 of about two years. The present uncertainties in the budget are large (25%), and it is, therefore, difficult to assess the impacts of potential future changes of the atmospheric hydrogen burden.

In Heidelberg we perform semi-continuous measurements of atmospheric H2 mixing ratios and other related tracers like CO and 222Rn since 2005. These data provide new insight into anthropogenic emissions of H2 (Hammer et al., 2009) as well as soil sink processes (Hammer and Levin, 2009). In addition, grab samples from the High Arctic and from Antarctica are analysed (see figure). Direct soil sink studies for atmospheric H2 were conducted by dedicated chamber experiments at our local Grenzhof Soil-Atmosphere Monitoring Station (schmitt et al., 2009).

Flask measurements of atmospheric H2 mixing ratios in high northern (Alert) and high southern (Neumayer, Antarctica) latitudes. Mixing ratios are higher in Antarctica than in the Arctic pointing to the important role of the continental soil sink in the northern hemisphere.

Atmospheric Sulfur Hexafluoride

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Sulfur hexafluoride (SF6) is a very potent chemically stable and almost purely anthropogenic greenhouse gas, which is industrially used mainly in electrical insulation and switching, and for degassing and purifying of molten reactive metals. Destruction of SF6 occurs only in the very high stratosphere and in the mesosphere. More than one decade ago we detected from analyses of air samples collected at Neumayer station (Antarctica), Izaña (Tenerife) and subsequent measurements on archived air from Cape Grim (Tasmania) that atmospheric SF6 has been increasing globally (since the 1970s) at a rate of more than 6% per year (Maiss and Levin, 1994). As most SF6 emissions occur in the northern hemisphere, southern hemisphere mole fractions are following those of the northern hemisphere with a delay of about one year. This corresponds to the mean residence time of air in one hemisphere. In the last decade SF6 has been applied as a very valuable tracer to calibrate and evaluate atmospheric transport models. Moreover, the smooth tropospheric increase of SF6 is used for dating of stratospheric air, ground water as well as in oceanographic studies to investigate internal mixing rates of the oceans.


From the global increase of SF6 total emissions can be calculated because virtually all emitted SF6 stays within the atmosphere. Comparison of these top-down estimates with bottom-up reported numbers to UNFCCC suggested significant under-reporting by ANNEX-I countries (Levin et al., 2010). Officially reported numbers have recently been adjusted ( UNFCCC ) and now show better agreement with the ture values derived from the observations.

Distribution of SF6 in the global troposphere (published data available here).