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Abstract
Vertical profiles of aerosol light scattering (bscat), absorption (babs), as well as the single scattering albedo (SSA, ω), play an important role in the effects of aerosols on climate, air quality, and local photochemistry. High-precision in-situ measurements of the vertical profiles of these properties are challenging and therefore uncommon. We report here the development of a portable cavity-enhanced albedometer operating at λ = 532 nm for use aboard an unmanned aerial vehicle (UAV). Multi-optical parameters, bscat, babs, extinction coefficient bext, and ω, can be measured simultaneously in the same sample volume. The achieved detection precisions in laboratory were 0.38, 0.21, and 0.43 Mm-1 for bext, bscat, and babs, respectively, for a 1 s data acquisition time. The albedometer was installed on an hexacopter UAV and simultaneous in-situ measurements of the vertical distributions of bext, bscat, babs, and ω were realized for the first time. Here we report a representative vertical profile up to a maximum height of 702 m with a vertical resolution of better than 2 m. The UAV platform and the albedometer demonstrate good performance and will be a valuable and powerful tool for atmospheric boundary layer research.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Vertical profiles of aerosol light scattering (bscat) and absorption (babs) significantly affect the solar radiation balance [1,2]. Absorbing aerosols heat the atmospheric layer (the warming effect) and change the top-of-atmosphere forcing [3–5]. Heating also perturbs the vertical temperature profile and alters atmospheric stability by suppressing turbulence [6] and depressing the development of the atmospheric boundary layer [7], which can increase air pollution in megacities. Knowledge of the absorbing efficiency, quantified by the single scattering albedo (SSA, ω = bscat /(bscat + babs)) [1,8] is needed to understand aerosol radiation interaction and aerosol atmospheric boundary layer feedbacks. However, direct measurement of ω profiles remains challenging [9]. Although some aircraft observations have been carried out [10], there is still large uncertainty.
The concept of using UAVs (unmanned aerial vehicles) for atmospheric research began in the 1990s [11]. Nowadays, it has developed rapidly, and various instruments have demonstrated impressive performance [12,13]. For example, Ramanathan’s team used three stacked lightweight UAVs to directly measure the vertical distributions of solar absorption and heating rate between 0.5 and 3 km over the Indian Ocean to study aerosol-radiation-cloud-albedo interactions [3,6,14,15].
The main technical specifications of a UAV include the maximum take-off weight, payload, endurance time, altitude, and speed [13,16,17]. UAVs can be either fixed-wing or rotary-wing configurations and take-off weight can be divided into small (< 25 kg), medium (25–150 kg), and large UAVs. Large fixed-wing UAVs such as Egrett and Altus [11] can fly to an altitude of tens of kilometers, carry hundreds of kilograms payload with days of flight endurance time. These kinds of UAVs have more space for payload configurations and are more stable in severe weather conditions, but special takeoff and landing capability are required. In contrast, small and medium size UAVs can attain an altitude of several kilometers with hours of endurance time. Takeoff and landing are straightforward. They can be easily integrated with different types of compact instruments or sensors. The advantages of reusability, low cost, simplicity, portability, and high manoeuvrability make them ideal platforms for atmospheric boundary layer studies in various environments such as rural, city and remote areas [13,16–19].
Compared with the vigorous development of laser absorption spectrometers for trace gas detection [17,20], UAV-borne in-situ detection technology of aerosol optical properties has developed more slowly. So far, only the modified light absorption photometer based on filter transmission measurement has been successfully applied to the vertical profile measurement of black carbon (BC) mass concentration and light absorption [13,14,21]. The UAV based measurement of ω is still not achievable.
In this work, we report the development of a UAV-borne cavity-enhanced aerosol single scattering albedometer (CEA-albedometer) operating at λ = 532 nm. The CEA-albedometer combines a broadband cavity enhanced absorption spectroscopy (BBCES) and integrating sphere (IS) nephelometer for simultaneous, in-situ measurement of extinction (bext = bscat + babs) and scattering (bscat) coefficients (and thus also retrieves the absorption coefficient babs, and ω) in the same sample volume [22,23]. BBCES provides sensitivity extinction measurement with a high finesse optical cavity, which can simultaneously quantify aerosol extinction and trace gas absorption with a single instrument. The use of IS for scattering measurement reduces the truncation angle to ∼ 1.8°, making truncation losses negligible (< 0.2%) for particle diameters smaller than 1 µm [24]. The CEA-albedometer solves the problem of ω measurement error caused by separately measuring different optical parameters with different instruments that may under different sampling conditions. Further, the CEA-albedometer is suited to operating in high relative humidity (RH) conditions and for studying the RH-dependence of several optical parameters (bext, bscat, babs, and ω) [25]. Similar instruments include the cavity ring down spectroscopy (CRDS) [26] and cavity attenuated phase shift spectroscopy (CAPS) [27] albedometer. More information about albedometer can be found in Ref. [23].
Our CEA-albedometer was installed on a medium-size hexacopter UAV, enabling simultaneous, in-situ measurement of the vertical distribution of aerosol light scattering and absorption (and thus ω) for the first time. Here we describe the instrument and its performance characteristics from laboratory tests and flight measurements of atmospheric vertical profiles. The instrument measurements and flight platform are stable during flight, and demonstrates that the system is a valuable tool for studying aerosol optical properties in the atmospheric boundary layer.
2. Experimental section
2.1 UAV-borne CEA-albedometer
Unlike our previous instruments built on a breadboard for ground observations [22,24,25], in this work, we used a custom cage-based [28] optical system to reduce the influence of vibrations on the optical system during the flight. The assembly diagram is shown in Fig. 1. The cage system consisted of eight 6 mm diameter rigid steel rods, two fastening frames, and two end plates. The IS (15 cm inner diameter) was embedded in the middle of the cage system. A quartz tube was inserted through the IS to avoid the degradation of the IS reflectivity and to shorten the sample residence time [24,27,29]. Two truncation reduction tubes were directly installed on the IS. High reflectivity mirrors were mounted on each end of the tubes to produce a high-finesse cavity. The distance between the two high reflectivity mirrors (d) was about 54.6 cm. The size of the cage optical system was 75 cm × 21 cm × 22 cm with a mass of 9.6 kg.
Fig. 1. Assembly diagram of the CEA-albedometer optomechanical module. A custom cage-based optical system was used to reduce the influence of vibrations during the flight.
The probe light source used was a green LED with maximum output at 523 nm (LedEngin LZ1-00G102). The light was first coupled into a multimode fiber and the fiber output was collimated with an SMA air-spaced doublet fiber collimator and injected into the cavity. Light transmitted through the cavity was collected by an identical SMA collimator and fed into a miniature CCD spectrometer (Ocean Optics, USB2000+) through an identical multimode fiber. The aerosol extinction was measured over 515-545 nm with a spectrum resolution of 1.5 nm. The aerosol scattering was measured at λ = 532 nm (as an averaged result over the spectral region of 528-537 nm) with a single channel photomultiplier tube (PMT, Hamamatsu H14600-20). As a result, the absorption and ω can be calculated based on the difference between extinction and scattering at λ = 532 nm.
The whole cage structure was symmetrical, and the light transmitter and receiver units were interchangeable. All optomechanical components were coaxial with simple optical alignment. This configuration improved the efficiency of the light source coupling into the cavity and the stability of the optical system. A compact payload module for UAV application was realized.
2.2 Data retrieval and time series measurement method
The measurement of the extinction and scattering coefficients are described by [24]:
Particle free zero air was obtained by filtering the atmospheric sample through a silica gel column dryer, active carbon filter and Teflon filter (Fig. 2). For purging zero air, a mini-electric inflator pump was used. To ensure that there was enough pressure difference, a needle valve was used to limit the flow rate and improve the outlet pressure. When the instrument was turned on, the cavity was firstly filled with zero air to obtain the reference light intensity transmitted through the cavity (I0(λ) and Itrans_zero air) and scattering signal (Iscat_zero air) without sample inside the cavity. The solenoid valve was then switched to the sampling mode to record the light intensity (I(λ), Itrans, and Iscat) with sampled air inside the cavity. The I(λ) spectrum was used for extinction measurement, and Itrans and Iscat at λ = 532 nm were used for scattering measurement. Thus, babs and ω were obtained from the extinction and scattering measurement. The solenoid valve was then switched back to the reference mode to record a new measurement cycle.
Fig. 2. Schematic diagram of the air flow of the ambient air sampling, particle free zero air used for reference transmission spectrum and reference scattering signal acquisition, and for purging air to prevent the high reflectivity mirrors from mirror reflectivity degradation.
2.3 Configuration of the UAV platform
The UAV (as shown in Fig. 3) was a petrol-powered hexacopter (Changfeng, Ltd.) with a flight height of up to 1.5 km and a speed of up to 20 m/s. The fuselage material was carbon fiber and aluminum alloy. It weighed ∼ 80 kg and had dimensions of 2 m × 1.7 m × 0.9 m. The UAV carried two 24 L volume fuel tanks. When filled with petrol, the tanks weighed about 40 kg. The maximum take-off weight of the UAV was ∼ 150 kg, and about 30 kg of equipment can be carried. The full load flight endurance time was about 1 hour 20 minutes depending on the temperature and wind. It was controlled by a GPS module with a precision of 0.1 m in the horizontal direction and 0.05 m in the vertical direction.
Fig. 3. Configuration of the UAV platform. The platform was composed of the hexacopter UAV, sample inlet with PM2.5 cyclone, RH and temperature (RH/T) sensor, and the CEA-albedometer payload.
The CEA-albedometer was installed in an aluminum box underneath the UAV and was powered with a 16000 mAh battery. The center of gravity of the UAV and albedometer was at the bottom of the platform, allowing stable flight even in wind. The sample inlet with PM2.5 cyclone (BGI SCC 1.197, 50% cut point at 2.5 µm with a flow rate of 2.27 L/min) was 1 m above the UAV propellers. The RH and temperature sensor (RH/T, Rotronic HC2-C05) was fixed at the top of the sample inlet. Because there was a strong downwash flow below the UAV, exhaust gas from the six petrol-powered engines was quickly removed and did not affect sampling and measurement. The reliability of this configuration has been proved by the simulation study with computational fluid dynamics (CFD) [31] and an experimental investigation with a red colored smoke cartridge by Samad et al. [32]. Above the propellers, the air was roughly laminar flow downward. The impact of pressure and airflow disturbance caused by the strong downwash flow on the sample can be ignored, which made the UAV vertical profile measurement simple and convenient.
3. Results and discussion
3.1 Precision, uncertainty, and detection limits
The stability and precision of the UAV-borne CEA-albedometer were investigated using an Allan variance analysis. The results are shown in Fig. 4. Continuous time series measurements of bext, bscat, and babs of particle-free zero air sample were performed. The time resolution of the albedometer was set to 1 s (50 ms integrating time with 20 spectra averaging). The 1σ standard deviations of 1-hour bext, bscat, and babs measurement were 0.41, 0.22, and 0.45 Mm-1, respectively, which showed good stability of the instrument with low baseline drift. Short-term precisions for bext, bscat, and babs were respectively 0.38, 0.21, and 0.43 Mm-1 with a data acquisition time of 1 s, and were further improved to 0.09, 0.03, and 0.09 Mm-1 with a 60 s data acquisition time. These values were comparable with our previous ground instrument with a cavity length of 70 cm [24]. The 3σ detection limit (LOD3σ) of each parameter was determined from the Gaussian fit of the frequency distribution of the time series measurement (Fig. 4(c)) [28]. With 1 s data acquisition time, the LOD3σ of bext, bscat, and babs were 1.40, 0.67, 1.45 Mm-1, respectively. The total uncertainties of bext, bscat, babs, and ω were estimated to be about 3%, 3%, 5%, and 6%, respectively [24]. It should be noted that the above fixed relative errors are applicable to bext and bscat. For babs, the relative uncertainty depends on ω, which increases when ω approaches 1.
Fig. 4. Performance evaluation of the UAV-borne CEA-albedometer with 1-hour particle-free zero air measurement in the laboratory. (a) Time series measurement, (b) Allan deviation, and (c) frequency distribution.
3.2 Laboratory test
The performance of the UAV-borne CEA-albedometer was evaluated with laboratory-generated polydisperse ammonium sulfate (AS) aerosol. The aerosol was generated using a constant output atomizer (TSI 3076) with compressed zero air, dried in a silica gel diffusion dryer (TSI 3062), and then neutralized with an aerosol neutralizer (TSI 3077) to obtain the aerosol samples with an equilibrium charge distribution [25,30].
The reported complex refractive index (CRI) of AS was $1.520_{ - 0.000}^{ + 0.005} + i0.000_{ - 0.000}^{ + 0.002}$ at λ = 532 nm [24], which is a purely scattering sample with no absorption. Theoretically, the scattering and extinction coefficients of AS are equal, and the value of ω is 1, which makes AS a suitable species for testing the consistency of aerosol scattering and extinction measurements. Figure 5 shows the results of time series measurement of bext, bscat and ω with different AS particle number concentrations. Good agreement between the measured scattering and extinction coefficients was observed (as shown in Fig. 5(b), with a slope of 1.008 and an intercept of 0.84 Mm-1), and the mean value of the measured ω was 1.01 ± 0.02, demonstrating the ability of the UAV-borne CEA-albedometer to accurately measure multi-optical-parameters simultaneously.
Fig. 5. (a) Measurement results (bext, bscat and ω) of ammonium sulfate aerosol at different particle number concentrations. (b) Scatter plot of the measured bext and bscat.
3.3 Field test
From December 2020 to January 2021, field validation and vertical profile measurement applications of the UAV-borne CEA-albedometer were performed at Shouxian National Climatological Observatory (32°25'47.8"N, 116°47'38.4"E) in Shouxian County, Anhui Province [33]. The observatory is surrounded by farmland and is a typical rural background site. There are no tall buildings nearby and the site is not within a no-fly zone, making it a convenient site for ground validation and vertical profile measurements. For ease of comparison and analysis with other instruments, all field tests were dried with a silica gel diffusion dryer to ensure the same conditions that RH was lower than 40%.
The hexacopter UAV has six petrol-powered engines that produce significant vibration. At the same time, a large amount of exhaust gases is emitted by the engines. Whether these factors affect the performance of the instrument needs to be tested before the vertical profile measurement. In this work, the field test included two parts: (1) a ground test of vibrations and impact of the petrol-powered engine, and (2) a comparison between the measurement results from the UAV hovering 20 m above the ground and a ground observation instrument.
The ground test was conducted at 14:30 on 22 December 2020. The UAV platform was placed in open grassland. The test was divided into five stages: in stages 1, 3, and 5, the engines were turned off, while the engines were running in stages 2 and 4 (Fig. 6). At the beginning of the test (stage 1), particle-free zero air was measured to determine the instrument zero and ambient air was then sampled. The measured 1σ standard deviations of bext, bscat, and babs for zero air measurement were 0.28, 0.12, and 0.31 Mm-1, respectively, which agreed well with the laboratory results.
Fig. 6. Ground test on the influence of the vibration and engine exhaust on instrument performance during UAV engine operation.
A period of data (71 to 78 minutes) in Fig. 6 without an obvious change was selected for the evaluation of the actual instrument performance. The measured mean values were 225.46 ± 3.85, 193.03 ± 2.75, and 32.43 ± 2.12 Mm-1, respectively, for bext, bscat, and babs. The standard deviations were about 7 to 23 times larger than that of zero air, which was mainly caused by the sample fluctuation, but within 6% compared with the measured values. During the startup and shutdown of the engines, no significant changes in the measurement of bext, bscat, babs, and ω of the ambient samples were observed. The sharp peak in stage 4 may be caused by circuit noise. Generally, no obvious effects from the vibration and petrol-powered engine exhaust on the instrument were observed. The UAV platform and the albedometer demonstrated good performance and stability.
The instrument inter-comparison was conducted on 26 December 2020. One of our early CEA-albedometers [24] (operating at the same wavelength, λ = 532 nm) was installed in a temperature-controlled room 200 m east of the UAV platform. Ambient air was dried below 40% RH and pumped through a sample inlet about 1 m above the roof. The inlet included a PM2.5 cyclone (BGI SCC 2.654) and the flow rate was 10 L/min. The UAV platform hovered 20 m above the ground. Time series of the measurement of the parameters are shown in Fig. 7. The time resolutions of the ground and the UAV-borne instrument were 14 s and 3 s, respectively. For comparison, all data were 1 min averaged. The measurements of the airborne and ground-based instruments were generally in good agreement except for some spikes at 14:15, 14:44, and 15:08, that may be due to different samples caused by instantaneous emissions at different observation locations. In general, it is worth noting that the two types of measurement are giving comparable results in all optical parameters.
Fig. 7. Time series measurement of (a) bext, (b) bscat, (c) babs, and (d) ω with the UAV-borne CEA-albedometer at 20 m height and a CEA-albedometer on the ground. (e) Correlation plot of bext, bscat, babs, and (f) ω of the UAV-borne and the ground CEA-albedometer.
The scatter plots of the ground measurement results of bext, bscat, babs, and ω and the results measured by the UAV platform at 20 m above the ground are shown in Fig. 7(e) and (f), respectively. All the measured parameters, bext, bscat, babs, and ω, were within 10% of the 1:1 line. The slopes of bext and bscat were 0.979 and 0.982, respectively. The differences of babs and ω were larger than that of bext and bscat. If considering the instrument uncertainties (3%, 3%, 5%, and 6% for bext, bscat, babs, and ω, respectively), and the sample difference at different heights (with different sample losses of the two instruments, different ambient RH and instantaneous emissions at two positions, etc.), the measurement results of the two instruments have good consistency.
3.4 Vertical profile measurement
The vertical profiles of bext, bscat, babs, and ω measured at 13:00 on 4 January 2021 are shown in Fig. 8. The hexacopter UAV ascended vertically with an ascent rate of 0.6 m/s. The time resolution of the albedometer was set to 3 s to meet the demand for vertical profile measurement, giving a vertical resolution better than 2 m. Guo et al. investigated the boundary layer height (BLH) in China from 2011 to 2015 [34]. The mean values of BLH ranged from 320 m in fall and winter to 710 m in spring and 650 m in summer. In this work, the height of the demonstration flight was 702 m, which met the needs for boundary layer studies.
Fig. 8. Vertical resolved (a) bext, (b) bscat, (c) babs, and (d) ω measured with the UAV-borne CEA-albedometer. (e) RH and temperature were also recorded.
In Fig. 8, the raw data with 2 m resolution and average profiles with 20 m resolution are shown. During the measurement, the temperature decreased almost linearly with height. The measured bext, bscat, and babs changed little below 450 m, with mean values of 312.91 ± 3.53, 263.54 ± 2.10, and 49.37 ± 2.03 Mm-1, respectively. At 450 m, there was an abrupt decrease in bext and bscat, and these values then gradually decreased with increasing height. The changes of babs were smaller than that of extinction and scattering. For the vertical profile of ω, the changes were small, with a mean value of 0.842 ± 0.005 below 450 m, and 0.848 ± 0.005 above 500 m. The measurement result is consistent with a uniform vertical distribution of BC reported by Lu et al. using a tethered balloon during the period of December 2016 and January 2017 at the same site [35]. This kind of boundary structure mainly occurred during the full development of the boundary layer from 11:00 to 17:00.
In summary, the vertical profile measurement here proves the high performance of the UAV platform as well as its ability in atmospheric boundary layer research.
4. Conclusions
UAV technology has developed rapidly in the last two decades. In this paper, we report the development of a UAV platform for simultaneous, in-situ measurement of bext, bscat, babs, and ω with high sensitivity, and high spatial and temporal resolution. The UAV-borne instrument adopted a custom cage-based optical system. High performance is shown. The detection precisions (1σ, 1 s) for bext, bscat, and babs at λ = 532 nm were 0.38, 0.21, and 0.43 Mm−1, respectively. The results were validated with the laboratory generated aerosol samples and by comparison with ground-based observations in the field. The airborne albedometer is a new valuable tool for studying the optical characteristics of aerosols and their influence on physical and chemical processes in the atmospheric boundary layer.
The cavity-enhanced albedometer has the advantage of measuring high quality aerosol optical properties in conditions of high RH [25]. In-situ RH-dependent aerosol optical properties measured with the UAV-borne albedometer can provide an examination of the relationship between the results of the surface measurement of dry aerosols (in air quality monitoring network) and the satellite and remote sensing retrievals of wet particles (that represents the real atmospheric environment condition) [36].
Combined the CEA-albedometer with a thermal denuder (TD), Eabs (light absorption enhancement) caused by dry coatings on BC and ω can be measured simultaneously, through switching measurement for optical properties before and after heating. Therefore, the influence of photochemical aging on Eabs and ω can be further explored. By using a single-particle core-shell Mie theory, the fraction contribution of the light absorption caused by BC and the coating shell can be retrieved in real time [33]. Thus, the UAV platform can be developed as an online method to compensate the commonly used filter sampling method to achieve accurate measurement of the vertical distribution of BC and brown carbon in the atmospheric boundary layer with high spatial and temporal resolution [5,37].
Regarding the further development of the UAV-borne albedometer, the recently developed amplitude-modulated cavity-enhance absorption spectroscopy (AM-CEAS) [38] based on multimode-diode-laser makes it possible to achieve wavelength-dependent multi-optical parameter measurement using a compact and lightweight system. These developments will expand the application of the UAV platform, for example, classification of the aerosol mixing state and chemical components with the observed relationship between the absorption and scattering Ångström exponent (AAE and SAE), as well as to investigating the role of brown carbon [39].
Funding
National Natural Science Foundation of China (42022051, U21A2028); the Instrument Developing Project of the Chinese Academy of Sciences (YJKYYQ20180049); the Youth Innovation Promotion Association CAS (Y202089); the HFIPS Director’s Fund (YZJJ202101, YZJJ2023QN01).
Acknowledgments
We thank Dr. Dean S. Venables in University College Cork for his helpful editing and discussions of this manuscript.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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