A UAV-based active AirCore system for accurate measurements of greenhouse gases

摘要

We developed and field-tested a UAV-based active AirCore for atmospheric mole fraction measurements of CO, CH, and CO. The system applies an alternative way of using the AirCore technique invented by NOAA. As opposed to the conventional concept of passively sampling air using the atmospheric pressure gradient during descent, the active AirCore collects atmospheric air samples using a pump to pull the air through the tube during flight, which opens up the possibility to sample atmospheric air in both vertical and horizontal planes. The active AirCore used for this study weighs ~ 1.1 kg. It consists of a ~ 50 m long 1/8” stainless steel tube with a 0.005” wall thickness, a 7.5 cm 1/4” stainless steel tube filled with magnesium perchlorate, a small KNF micropump and a 45 μm orifice working together to form a critical flow of dried atmospheric air through the active AirCore. A cavity ring-down spectrometer (CRDS) was used to analyze the air samples on site shortly after landing for mole fraction measurements of CO, CH, CO, and HO. We flew the active AirCore system on a UAV near the atmospheric measurement station at Lutjewad, located in the northwest of the city of Groningen in the Netherlands. Five consecutive flights took place over a five-hour period in the same morning, starting at sunrise until noon. We validated the measurements of CO and CH from the active AirCore against those from the Lutjewad station at 60 m. The results show a good agreement between the measurements from the active AirCore and the atmospheric station (N = 146, R: 0.97 and R: 0.94, and mean differences: Δ: 0.18 ppm; Δ: 5.13 ppb). The vertical and horizontal resolution at typical UAV speeds of 1.5 m/s and 2.5 m/s were determined to be ±26.4 to 28.2 m and ±44.0 to 47.0 m respectively, depending on the storage time. The collapse of the nocturnal boundary layer and the build-up of the mixed layer were clearly observed with three consecutive vertical profile measurements in the early morning hours. Besides this, we furthermore detected a CH4 hotspot in the coastal wetlands from a horizontal flight north to the dike, which demonstrates the potential of this new active AirCore method to measure at locations where other platforms have no practical access.