Thirty Years of Airborne Flux Measurement using the NRC Twin Otter

The surface-atmosphere exchange of matter and energy has always been beset by a spatial-temporal measurement conundrum: direct methods of measurement such as flux towers are small scale, fixed in space and semi-continuous in time, whereas indirect methods of measurement such as satellites are large scale, variable in space, and discontinuous in time. Obtaining representative and continuous flux measurements for large and heterogeneous areas is often presented with the challenge of trying to integrate measurement techniques with different spatial and temporal scales. Though many solutions have been proposed to this challenge, airborne flux measurement offer a unique intermediate approach, being able to measure at a range of spatial and temporal scales and being able to make both direct and indirect measurements. Aircraft-based flux measurement bridges the spatial-temporal measurement gap between tower-based observation systems and satellite-based systems and have proven to be a valuable resource in understanding the surface-atmosphere exchange of matter and energy for a diverse range of ecosystems.

One example of an internationally recognized and longstanding atmospheric research aircraft is the National Research Council of Canada's Twin Otter (NRC-TO), which has been in service to the climatological and earth-science communities of Canada and the United States for over three decades. The NRC-TO is a twin-turboprop research platform, capable of short take-off and landing, operation at low altitude, as well as 3-4 hour flight endurance. Instrumentation on board the NRC-TO has evolved over time to meet variable project needs, but has been always included precise high-frequency measurement of the three orthogonal components of wind speed. Building on this foundation, the ability to measure the flux of heat, carbon dioxide, water vapour, shortwave radiation, long-wave radiation, ozone, nitrous oxide, agrochemicals and methane have been added over time.

The first deployment of the NRC-TO as a flux measurement aircraft was in 1980, when an experimental fast response carbon dioxide analyzer was mounted to measure the flux of carbon dioxide over corn, forest and water. This study demonstrated the feasibility of aircraft-based flux measurement and showed the spatial variability in the carbon dioxide uptake and sensible heat flux across surface types, consistent with tower-based measurement systems. Building on the success of this project, further flux measurement work took place in Manitoba during the summer of 1985, to measure the flux of carbon dioxide, water vapour and heat above a growing wheat crop. During this project, analysis of data from varying altitudes and flight lengths established that slow instrument response time at low flight altitudes (~10 m) could risk losing high frequency flux contributions and that short flight tracks risk losing low frequency flux contributions associated with large eddies. Project planning to account for fluxes across the full spectral range was to become key in future studies.

The NRC-TO took part in FIFE (First International Satellite Land Surface Climatology Project (ISLCP) Field Experiment) in 1987 and 1989 and later in BOREAS (Boreal Ecosystem-Atmosphere Study) in 1994 and 1996. These projects pioneered a multi-scale measurement approach, incorporating plot, field, aircraft and remotely sensed measurements. The NRC-TO proved to be a critical intermediary between field studies and remote sensing, able to produce gridded fluxes of energy, water vapour and carbon dioxide, which were compared to remotely sensed vegetation indices during FIFE, and able to derive regional scale matter and energy fluxes for a mosaic of forest types in BOREAS, when combined with tower-based measurements. Additional efforts by the NRC-TO during BOREAS included studies on a diverse array of topics, from lake induced circulation, flux footprints and the documentation of the boreal forest's equivalent of ‘dust devils'. Further, using 115 km long flight transects during BOREAS, NRC-TO data was used to identify the mesoscale flux contributions greater than 2 km as being one potential source of the persistent flux underestimation problem, which plagues conventional tower-based flux measurement approaches.

More recently the NRC-TO has focussed on the flux measurement of agricultural greenhouse gases. Using a novel application of the Relaxed Eddy Accumulation (REA) measurement technique, with the capability to resolve differences in nitrous oxide on the order of 0.1 ppbV, multi-year studies from 2000 to 2004 demonstrated that the ‘spring burst' of nitrous oxide was a widespread regional phenomena in agricultural ecosystems of eastern Canada, and that the timing of emissions was highly dependent upon snowmelt and soil moisture. Combined with tower and remote sensing data, the nitrous oxide flux estimates from the NRC-TO have been used to calibrate regional-scale modelling efforts and to provide insight into national scale agricultural greenhouse gas inventory estimates. Building on the accomplishments of the nitrous oxide study, in the spring of 2011 the NRC-TO conducted the first airborne tests of a fast response methane analyzer, allowing for the first time, concurrent flux estimates of methane by eddy covariance and relaxed eddy accumulation. It is hoped that these results, combined with work from the agricultural field measurement and modeling communities, will improve the accuracy of and reduce the uncertainty of agricultural methane emissions.

Over the past 30 years, the NRC-TO has been an important tool to bridge the spatial-temporal measurement gap between intermittent and continuous observation, between small-scale approach and large-scale approaches. Through international collaborative efforts with scientists in a wide range of disciplines, unique knowledge has been gained into the control, timing, and measurement of surface-atmosphere matter and energy fluxes.