Measurement summary. The CO2 concentration in the air over the chosen field was measured over the course of five days and was found to vary within the range 350 to 500 ppm. Concentration data for each day of measurements are shown below in Figure 5. Because the global mean CO2 concentration for September 2019 was 408 ppm [3], concentrations of CO2 below 400 ppm are physically not possible as it would mean a huge local CO2 sinks exists; therefore, a reading sometimes observed below 400 ppm raises questions about instrumental biases. These were found to be related to the thermal management of the instrument, which has since been fully overhauled by improving the instrument insulation, design and placement of heat sinks and implementing a chiller based cooling solution coupled with an internal radiator loop. This ensures the internal temperature within the ORION® is well stabilised over the operating temperature range. The measurements show the concentration of CO2 to be, at times, significantly above global mean CO2 level for September 2019 [3], confirming strong local sources are contributing to the measured concentration, and are being picked up by the instrument. The scatter on concentration is also found to be related to the level of turbulence of the atmosphere, characteristic of diffusion from local sources. Clear differences in CO2 temporal evolution among the different paths are also characteristic of local sources (example in lowermost left figure) and are the type of signals needed to derive spatial mapping information.
