2F: Unmanned Whole Air Sampler (UWAS)

The Unmanned Whole Air Sampler (UWAS) is a prototype instrument developed to fill a large gap in our knowledge of the distributions and behavior of volatile organic compounds (VOCs) (including oxygenated VOCs (OVOCs)) in the layer of the atmosphere most closely connected to the earth’s surface (boundary layer (BL)). These species are key participants in determining air quality and knowledge of their vertical gradients, diurnal variability, and spatial variability throughout the BL is crucial for understanding their impacts both locally and regionally (downwind).  The UWAS will enable the detailed investigation of the region of the atmosphere (0 - 500 ft) which is largely under-sampled in atmospheric chemistry and flux studies because it lies in-between accessible aircraft sampling and surface sampling.

After conducting extensive research on possible platforms, Dr. Elizabeth Asher, an NCAR/ACOM Advanced Studies Postdoc decided upon the DJI Corporation Matrice model M600 unmanned aerial vehicle (UAV) for the UWAS (Figure 1). It is regarded as a state-of-the-art flying platform with a reasonable payload capacity. This platform has proven to meet all requirements including flight duration, speed, and altitude capability as well as carrying capacity.

The DJI Corporation Matrice model M600 unmanned aerial vehicle.
Figure 1. The DJI Corporation Matrice model M600 unmanned aerial vehicle (UAV) for the UWAS.

Dr. Asher worked closely with the ACOM engineering group to design the sampling system, taking advantage of the engineering group’s expertise in using 3D CAD design using the SolidWorksTM software package. The design had to adhere to strict weight and balance requirements. She has successfully tested the sampling system for cleanliness and for precision and accuracy. The UWAS collects whole air samples for subsequent analysis on the NCAR/ACOM Trace Organic Gas Analyzer (TOGA) and also records ambient measurements of temperature, humidity, pressure and wind vectors. The UWAS sampling system holds 15 miniaturized 100 mL canisters, allowing for duplicate TOGA analysis of each collected sample taking advantage of the small sample volume requirements for TOGA. A picture of the sampling system is shown below in Figure 2:

The UWASS sampling system.
Figure 2. The UWAS sampling system. Click for larger image.

The UWAS is controlled by a Raspberry PITM single-board computer with wireless LAN and Bluetooth connectivity, and one additional board, powered by a 24 VDC Lithium polymer battery.  Python is used to program the system. Upon power up, the UWAS automatically runs a sampling program that tests the operation of the pump, the valve, various sensors, and the system pressure prior to take off and indicates if these functions are working properly. After take-off, temperature, relative humidity, ambient pressure, system pressure, system flow, and GPS position are recorded at 1 Hz, and air samples are collected at predetermined GPS locations along the flight plan. After each flight the canisters are analyzed for a full range of VOCs, yielding data that can be used to determine the chemical vertical structure of the BL, calculate emission fluxes from a defined area, and complement ground- and aircraft-based data that may be obtained in the same study region.