Weather Monitoring 17


Improvements in Micrometeorological Instrumentation – Integrated CO2/H2O Flux Measurement Systems


Eddy Covariance Technique For many years scientists working in the field of micrometeorology have used the 'Eddy Covariance' technique to study the transfer of carbon dioxide and other greenhouse gases (GHG) between plants, soils, bodies of water and the atmosphere at the boundary layer. This complex statistical technique uses high frequency measurements of the movement of air in three dimensions along with the analysis of an air sample taken from the same position at the same time to determine the net exchange, or flux, of carbon dioxide, water vapour and sensible heat. Monitoring stations are typically installed above a forest canopy, field of crop or grassland where some of the prerequisites of meaningful readings such as homogeneity of terrain can be attained. Increasingly scientists are also studying fluxes in urban environments and above water.


Campbell Scientific has been involved in micrometeorological instrumentation for many years and these latest devices are the result of ongoing commitment to product development and innovation in this area.


Like any statistical method a large volume of data is required and so measurements are made at high frequency, typically 10 or 20 Hz over extended periods of time.


The basic measurement tools for Eddy Covariance are a sonic anemometer capable of measuring speed and direction in three dimensions, a gas analyser and a method to capture and store the data at high speed – usually either a pc, or, more typically, a data logger.


Synchronising Sensor Data


One technical challenge of the Eddy Covariance measurement process that is worth discussion is sensor data synchronisation. It is important that the sample of air under analysis can be exactly matched with the speed and direction of its movement at the precise moment it was taken - in other words the wind data and gas sample must be taken at exactly the same location and time.This might seem obvious and you might consider easy to achieve but it isn't as simple as it might first appear. Let's consider the position aspect first – on systems to date the gas analyser and the sonic anemometer are separate instruments, often not even manufactured by the same company and so the two devices will have no specific mechanism to connect them together in any neat and tidy manner and this alone can make positioning the measurement volumes close to each other difficult. Secondly, the two separate instruments can interfere with the aerodynamics of the other and disrupt the airflow so positioning needs to be carefully considered. Measurements taken when the wind is in a direction whereby the instruments, instrument tower or other nearby obstruction disrupts the airflow significantly have to be filtered from the results. Having two instruments side-by-side might narrow the angle of usable wind direction which reduces the volume of data available for statistical analysis.


Author Details: Iain Thornton,


Campbell Scientific Ltd Campbell Park, 80 Hathern Road


Shepshed, LE12 9GX Tel: +44 (0)1509 601141


Email: info@campbellsci.co.uk web: www.campbellsci.eu


Why do measurements need to be synchronised anyway, you might wonder? Well, the reason is that these systems are sampling turbulent air rotating in eddies of various sizes; at the point of measurement air will be moving at a specific speed and in a specific direction. By sampling the air at the exact same time we can say at that precise


moment whether CO2 or H2O was moving upwards or downwards and at what velocity – by taking 10 or 20 such measurements every second for extended periods we can average the net movement over time. If our measurements are not synchronised then we do not know how fast or in which direction the air sample we are analysing was moving.


So what makes synchronising measurements a challenge – firstly the analyser, anemometer and any other sensors in use will have different


response times which will need to be taken into account. Additionally, on some closed path systems, the air sample is pumped down a tube from the air intake to a separate analyser unit which introduces a delay relational to the tube length and this also has to be taken into account. Once these delays and differences in response times are understood then measurements can be synchronised by the data logger or computer – but remember that this is all happening at up to 20 times per second.


Open Path Eddy Covariance - IRGASON


The IRGASON is a major step forward in micrometeorological instrumentation because it uniquely incorporates an open path infra-red gas analyser (IRGA) and a 3D sonic anemometer in a single instrument. For the first time a sonic and an IRGA are not just made by a single manufacturer to fit and work together but are totally integrated in a single device. It simultaneously measures absolute carbon-dioxide and water-vapour densities, air temperature, barometric pressure, three-dimensional wind speed, and sonic air temperature.


Igrason - The IRGASON co-locates measuerment volumes of both 3D sonic anomemter and IR gas analyser by combining then in a single instrument


This eliminates one of the issues mentioned above because the measurement volumes for wind and gas samples are exactly co-located. Additionally, building the two high level instruments into a single low- profile, aerodynamic body minimises air flow disruption around both sensors. The IRGASON connects to a single electronics control box, the EC100, for simple installation and, importantly, better power efficiency. The IRGASON draws just 5W during steady state operation and power-up which means that EC stations can now realistically be operated using solar panels in many situations.


The EC100 also takes care of the signal synchronisation which means that the output from the IRGASON does not have to be separately synchronised by the data logger simplifying programming. The IRGASON supports field set-up and configuration and field zero and span all, accomplished directly from the data logger in the field.


www.envirotech-online.com IET May / June 2012


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