21 Jan 2021
21 Jan 2021
Characterizing the performance of a POPS miniaturized optical particle counter when operated on a quadcopter drone
- 1College of Engineering Mathematics, and Physical Sciences, University of Exeter, Exeter, Devon, UK
- 2Met Office, Fitzroy Road, Exeter, Devon, UK
- 3College of Life and Environmental Sciences, University of Exeter, Penryn, Cornwall, UK
- 4Department of Geography and Resource Management, The Chinese University of Hong Kong, Hong Kong, China
- 5Stanley Ho Big Data Decision Analytics Research Centre, The Chinese University of Hong Kong, Hong Kong, China
- 6Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, China
- 7Department of Earth and Environmental Sciences, University of Manchester, Manchester, UK
- 8Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, 695 022, India
- 9Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560 012, India
- 10Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
- 1College of Engineering Mathematics, and Physical Sciences, University of Exeter, Exeter, Devon, UK
- 2Met Office, Fitzroy Road, Exeter, Devon, UK
- 3College of Life and Environmental Sciences, University of Exeter, Penryn, Cornwall, UK
- 4Department of Geography and Resource Management, The Chinese University of Hong Kong, Hong Kong, China
- 5Stanley Ho Big Data Decision Analytics Research Centre, The Chinese University of Hong Kong, Hong Kong, China
- 6Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, China
- 7Department of Earth and Environmental Sciences, University of Manchester, Manchester, UK
- 8Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, 695 022, India
- 9Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560 012, India
- 10Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
Abstract. The Printed Optical Particle Spectrometer (POPS) is an advanced and small low-cost, light-weight, and high-sensitivity optical particle counter (OPC), particularly designed for deployed on unpiloted aerial vehicles (UAVs) and balloon sondes. We report the performance of the POPS against a reference scanning mobility particle sizer (SMPS) and an airborne passive cavity aerosol spectrometer probe (PCASP) while the POPS is operated on the ground and also while operated on a quadcopter drone, a DJI Matrice 200 V2. This is the first such documented test of the performance of a POPS instrument on a UAV. We investigate the root mean square difference (RMSD) and mean absolute difference (MAD) in particle number concentrations (PNCs) when operating on the ground and on the Matrice 200. When windspeeds are less than 2.6 m/s, we find only modest differences in the RMSDs and MADs of 2.4 % and 2.3 % respectively when operating on the ground, and to 5 % and 3 % when operating at 10m altitude. When windspeeds are greater than 2.6 m/s but less than 7.7 m/s the RMSDs and MADs increase to 10.2 % and 7.8 % respectively when operating on the ground, and 26.2 % and 19.1 %, respectively when operating at 10m altitude. No statistical difference in PNCs was detected when operating on the UAV in either ascent or descent. We also find size distributions of aerosols in the accumulation mode (here defined by diameter, d, where 0.1 ≤ d ≤ 1 µm) are relatively consistent between measurements at the surface and measurements at 10m altitude with RMSD and MAD of less than 21.6 % and 15.7 %, respectively. However, the differences between coarse mode (here defined by d > 1 µm) are universally larger than those measured at the surface with a RMSD and MAD approaching 49.5 % and 40.4 %. Our results suggest that the impact of the UAV rotors on the POPS does not unduly affect the performance of the POPS for wind speed less than 2.6 m/s, but when operating under higher wind speed of up to 7.6 m/s, larger discrepancies are noted. In addition to this, it appears that the POPS measures sub-micron aerosol particles more accurately than super-micron aerosol particles when airborne on the UAV. These measurements lay the foundations for determining the magnitude of potential errors that might be introduced into measured aerosol particle size distributions and concentrations owing to the turbulence created by the rotors on the UAV.
Zixia Liu et al.
Status: open (until 18 Mar 2021)
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RC1: 'Comment on amt-2020-495', Anonymous Referee #1, 22 Jan 2021
reply
Overall Comments:
The authors present a ground-based comparison of a POPS measurement and SMPS in an environment heavily influenced by biomass burning aerosol. The authors then compare POPS measurements on a UAS in flight to its measurements on the ground and suggest that these differences are due primarily to the UAS flight.
Although comparing an SMPS to the POPS is not particularly novel, this study does verify that the agreement holds in an environment influenced by biomass burning and where a different index of refraction has been assumed.
The authors suggest that the POPS is adversely affected in this particularly sampling position on a small rotary UAS. Yet the authors note that no attempts were made to sample from other locations on the multi-rotor aircraft, or to modify the inlet for in-flight aerosol measurements. Potential perturbation to instrument flow are not directly addressed. These should perhaps be persued. At the very least, the authors must tailor the scope of their claim to say that simplistic measurements form the POPS instrument are ill advised and quad copter aerosol sampling requires careful evaluation and consideration.
The flight tests do not involve an in-flight inter-comparison, which we suggest the authors also pursue, if possible. As noted, real differences in aerosol distributions and particle concentration number with time could have obscured perceived UAS aerosol sampling bias.
Specific Comments:
The abstract should be shortened.
L31 – L33 An odd comment; remove from the abstract. Begin with, “we compared the Portable Optical Particle Spectrometer, a small light-weight and high sensitivity optical particle counter…”
L37 Awkward. Rephrase. “This is the first such documented…
L38 Word choice – you don’t “investigate” the RMSE or MAD - you report it.
L50 – 52 Be specific about differences in coarse mode to what other instrument – SMPS does not measure particles > 1.0 micron. This is particularly confusing…
L62-74 This section could be much more concise.
L88 Awkward phrasing.
L106 – 113 This section could be shortened and only details particularly relevant to this study should be mentioned (this overall description is covered in Gao et al. 2013 and 2016).
L117 Did this study include a POPS?
L119 Comparisons to tower measurements – what instruments were compared and were they compared only at one height? Was temporal averaging applied?
L131-132 It is still not clear what was entailed in the in-flight UAS POPS comparison. Please be more specific.
L132-134. This sentence should be removed. It is not helpful.
L153-155 Was the adjustment to account for a difference in the index of refraction done to binned data or per particle data? Doing this to binned data could introduce an additional (likely small) source of error.
L178-180 Can the authors comment on how the sampling tube might be optimized for drone sampling? This seems like a very important point considering the comparison/ test.
L205 Was this date of the wing-mounted PCASP instrument a day that the POPS sampled (on the UAS or on the ground)? What altitudes were sampled to provide these size distributions?
If not, perhaps shorten this section and specify that the PCASP size distribution is simply provided for reference.
L235-240 For an instrument comparison, the size range of the SMPS and POPS should only be compared in the range where measurements overlap. The full SMPS and POPS size ranges should only be used to characterize atmospheric aerosol distributions more fully.
L258 – 290. Since no in-flight comparison to another instrument was done, the authors need to demonstrate that they did not observe any systematic differences in the PSD at 10 m compared with at the ground.
L285-287 not needed.
L301-303. This is a good point. The POPS flow (used to calculate PC in each bin size) needs to be monitored in each of the different flight positions.
L313 – 316. Again what is the flow in each POPS case? Are the counting statics much poorer during FLY than when on the ground?
L331-333 Unfortunately, this point undermines this entire study. If there are real differences that might be confused with instrument performance in flight, the study ideally should address this.
L336-337 This does not make sense. Two additional stages of G_NR are suggested?
L356- end Tailor sweeping claims as suggested.
Zixia Liu et al.
Zixia Liu et al.
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