Methane (CH₄) and nitrous oxide (N₂O) are powerful greenhouse gases that originate from industry, agriculture and nature. Thus far, it was extremely difficult to measure the emission of these gases in agricultural areas, ECN employees Petra Kroon and Arjan Hensen report. “They are listed in the top ten of emission measurements with the largest uncertainty margin . Fortunately, the Air Quality and Climate Change department at ECN now has a new method available to measure these emissions per hectare more accurately. This provides better statistics and helps to determine which farming methods produce fewer emissions.”

ECN researcher Petra Kroon has studied and tested the new method extensively over the past four years. In early June, she obtained her doctorate from Delft University of Technology with a thesis on this method. ECN researcher Arjan Hensen, who was closely involved in this study, and Petra Kroon reveal the ins and outs of these so-called Eddy covariance measurements of nitrous oxide and methane.
“The greenhouse effect of 1 kg of methane corresponds to that of 25 kg of CO₂, while the greenhouse effect of nitrous is even equal to 298 kg of CO₂. That is why it is important to measure methane and nitrous oxide emissions as well, even if the concentrations of these in the air are a thousand times smaller than that of CO₂. In the past, measuring that was no easy task,” declares Kroon.
Since 1994, ECN has measured greenhouse gas concentrations in the air above peat meadow land using the 200-metre-high measurement tower in Cabauw, owned by the Royal Netherlands Meteorological Institute (KNMI). The varying concentrations of CO₂, CH₄ and N₂O are monitored continuously at various heights providing information on a NW European scale. On a mast of several metres high, ECN also measured direct emissions from the peat meadow land around the mast. In the meadows of the region known as “the Green Heart” of Holland, CO₂ is generated as carbon from the peat oxidises when exposed to oxygen. The emission of CO₂ varies with the temperature, the time of day (plants absorb CO2 during the day and release it during the night), the height of the grass and the availability of fertiliser. “The emission of CO₂ can be measured reasonably accurately. Methane and nitrous oxide are more difficult to measure, though,” says Kroon.

Peak in nitrous oxide gas emissions after fertilising
Nitrous oxide and methane are produced by bacteria , a process that accelerates when for example carbon and nitrogen input from fertiliser is available. Nitrous oxide emissions peak after a few days to a week following fertilising. The trick is to measure this peak. Hensen: “This has been done with box measurements for many years: we place a box without a cover upside down somewhere in the meadow. Gases emitted from the soil accumulate in the box. The concentration increase of nitrous oxide or methane over a period of fifteen or thirty minutes is used to determine the flux, i.e. the gas flow per square metre. Doing this in various places in the meadow enables us to calculate emissions for the entire meadow.”
The problem, however, is that emissions also differ considerably in space depending on soil conditions. Urine patches in a field are in general very high sources (100 times the emission only  a metre away), but it is hard to get an accurate estimate of the percentage of the field with and without urine. Missing emission peaks is not unlikely because it is impossible to cover all locations night and day. As a result, methane estimated from these sources have an uncertainty of 25% and nitrous oxide measurements by as much as 50%.
Researchers at the Air Quality and Climate Change department of the Biomass, Coal and Environmental Research unit, have started searching for a better measurement method. Kroon: “A few years ago, we switched to measuring methane and nitrous oxide with a QCL, a quantum cascade laser spectrometer. With ten measurements per second, it is over a hundred times faster than our old device and more accurate by a factor of five. It can identify the difference between a concentration of 310 and 311 parts of nitrous oxide per billion parts of air.”
Kroon has used this device successfully in the meadow to measure emissions of nitrous oxide and methane at a height of three metres above ground level with the Eddy Covariance method. Eddies ensure the exchange of greenhouse gas between the meadow and the air. The QCL time series shows when nitrous oxide molecules move up- and downward with the vertical wind. The latter is determined using an anemometer that measures the eddies with acoustic sensors. This combination, provides a representative image of methane and nitrous oxide emissions from an area between three and four hectares in size. A considerable amount of mathematics is involved, which Kroon describes in her thesis.

Interior of the container at the meadow in Reeuwijk, on the right the laser and on the left a monitor showing a real time graph of the emission of nitrous oxide and methane.  

Tundra emissions can also be measured more effectively
Hensen: “The advantage is that this system allows you to measure continuously 24 hours a day, 365 days a year and therefore does not miss a single emission peak. We also measure diurnal variations. The measurements obtained with the QCL, in combination with measurements on two other sites by VU University Amsterdam and Wageningen University, have produced exceptional results. The intensively managed meadow was a net source of greenhouse gas (combination of CO2, methane and nitrous oxide), where a extensively managed field was a significantly smaller source. A net storage of greenhouse gas occurred in a swampy natural landscape. ”
Triggered also by these results , other European groups are now also purchasing these instruments. Similar emission studies will be carried out in more and more locations. A team from the VU University Amsterdam, for example, will use the Eddy Covariance method to measure methane emissions on melting tundras in Siberia.
The study involving emission estimates of methane and nitrous oxide is continuing
ECN has started a test to determine methane and nitrous oxide emissions on an even greater spatial scale. “A few weeks ago we installed our system on the KNMI measurement tower in Cabauw. We sample air at a height of 60 metres continuously in order to measure emissions from an area covering several square kilometres. Together with the ongoing concentration measurements at Cabauw, we can then indicate the relative importance of methane and nitrous oxide from regional, national and international sources.”

Contact
Petra Kroon & Arjan Hensen
ECN Biomass, Coal and Environmental Research
Tel.: +31 (0)22 456 40 62
E-mail:  Petra Kroon   Arjan Hensen 

text: Erik te Roller

Info
Annual balances of CH₄ and N₂O from a managed fen meadow using eddy covariance flux measurements. Article published in European Journal on Soil Science
Eddy covariance observations of methane and nitrous oxide emission: Towards more accurate estimates from ecosystems, doctoral thesis of Petra Kroon at Delft University of Technology.
More policy-related aspects can be found in Beïnvloeden van landgebonden broeikasgasemissies (Influencing land-bound greenhouse emissions). Article published in Landschap, number 27(2), 99-109, 2010.

This ECN Newsletter article may be published without permission provided reference is made to the source: www.ecn.nl/nl/nieuws/newsletter-en/