Measuring gases using eddy covariance

Measurement is an integral part of science. As technology progresses, so does the range and accuracy of what we can measure. Where once we made measurements using only our senses, University of Waikato scientists have a device that measures greenhouse gases 10 times per second and to concentrations of about 0.2 parts per billion!

These advances are game changers, according to Associate Professor Dave Campbell and Professor Louis Schipper. For more than two decades, Dave and Louis have been using micrometeorology to measure carbon exchanges in pastures and wetlands. They use a micrometeorological technique known as eddy covariance (EC). Eddy covariance quantifies gas fluxes (changes) by directly measuring the gas as it moves between soil, vegetation and the atmosphere. The gases include water vapour, carbon dioxide (CO₂), methane and, most recently, nitrous oxide (N₂O). This research helps to build a more accurate picture of New Zealand’s greenhouse gas emissions.

Eddy covariance explained

Atmospheric gases rarely sit still – they move with the wind. It may appear that the air flows horizontally as it travels across the land, but it actually moves in rotating eddies. As air moves, friction makes the air tumble. Air moving closest to the ground tumbles in small eddies. The small eddies cause bigger eddies to form above them, so each eddy has both horizontal and vertical components. This explains the eddy part of eddy covariance.

Covariance describes the measurement techniques. Equipment on an EC tower simultaneously measures the differences between the gas concentrations and the direction of the wind as it swirls and eddies. (‘Co’ means together and ‘variance’ means changes.) A sonic anemometer measures the wind direction and speed. Spectrometers measure gas concentrations in the air by monitoring how many gas molecules pass through a defined volume over a given time.

Measuring carbon dioxide

Rotating eddies of air carry CO₂ both upwards and downwards during the day. Plants take up some of the CO₂ during photosynthesis, so there are fewer CO₂ molecules carried upwards. To accurately measure the concentration of CO₂ at a set point, scientists need to know if the parcels of CO₂ are moving up or down. As the gas is continually tumbling, this needs to happen really quickly. That’s why EC measurements happen 10 times or more per second. All of this data is represented in 30-minute intervals, which is plotted through time.

Data collection from EC systems and other sources help experts like Dave and Louis understand trends, test mitigation strategies and build an accurate CO₂ inventory for New Zealand.

Measuring nitrous oxide

The Waikato team have added a new piece of equipment to their EC tower – a quantum cascade laser (QCL), which measures nitrous oxide. N₂O makes up about 0.00003% of the atmosphere by volume – so there is not a lot of it, but it is a potent greenhouse gas. The majority of New Zealand’s N₂O emissions come from livestock urine patches. The patches are dotted all over a paddock so it is hard to measure emissions using soil chambers alone. Using the QCL and EC techniques, the Waikato scientists can continuously measure N₂O emissions over several hectares. Louis says, “It’s a bit like looking at an impressionist painting – if you zoom in on a single brush stroke, you don’t get much of an idea what’s going on, but using QCL, we’re able to step back and see the bigger picture.”