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

We developed and field-tested an unmanned aerial vehicle (UAV)-based active AirCore for atmospheric mole fraction measurements of COinline-formula₂, CHinline-formula₄, 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 spatially sample atmospheric air. The active AirCore system used for this study weighs inline-formula∼ 1.1 kg. It consists of a inline-formula∼ 50 m long stainless-steel tube, a small stainless-steel tube filled with magnesium perchlorate, a KNF micropump, and a 45 inline-formulaµm 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 not more than 7 min after landing for mole fraction measurements of COinline-formula₂, CHinline-formula₄, and CO. 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 5 h period on the same morning, from sunrise until noon. We validated the measurements of COinline-formula₂ and CHinline-formula₄ 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 (inline-formulaN inline-formula= 146; inline-formula $M12inlinescrollmathml{R}_{chem{\mathrm{normal\; CO}}_{\mathrm{normal\; 2}}}^{\mathrm{normal\; 2}}$ 25pt19ptsvg-formulamathimg5f823a11d722aabf46f4c10f4628c045 amt-11-2683-2018-ie00001.svg25pt19ptamt-11-2683-2018-ie00001.png : 0.97 and inline-formula $M13inlinescrollmathml{R}_{chem{\mathrm{normal\; CH}}_{\mathrm{normal\; 4}}}^{\mathrm{normal\; 2}}$ 25pt19ptsvg-formulamathimg5f27e4c3f8673248fb3d5df5869b9a9a amt-11-2683-2018-ie00002.svg25pt19ptamt-11-2683-2018-ie00002.png : 0.94; and mean differences: inline-formulaΔCOinline-formula₂: 0.18 ppm and inline-formulaΔCHinline-formula₄: 5.13 ppb). The vertical and horizontal resolution (for CHinline-formula₄) at typical UAV speeds of 1.5 and 2.5 m sinline-formula⁻¹ were determined to be inline-formula±24.7 to 29.3 and inline-formula±41.2 to 48.9 m, respectively, depending on the storage time. The collapse of the nocturnal boundary layer and the buildup of the mixed layer were clearly observed with three consecutive vertical profile measurements in the early morning hours. Besides this, we furthermore detected a CHinline-formula₄ 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 techniques have no practical access.