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Abstract
Aerosol optical absorption measurements are important for the prediction of climate change, as aerosols directly disturb Earth’s radiation balance by absorbing or scattering solar radiation. Although photoacoustic spectroscopy is commonly recognized as one of the best candidates to measure the absorption of aerosols, multi-wavelength measurements of aerosols optical absorption remain challenging. Here, a method based on photoacoustic spectroscopy that can simultaneously measure the aerosol absorption characteristics of three wavelengths (404, 637 and 805 nm) is proposed. In the three-wavelength photoacoustic spectrometer (TW-PAS), a photoacoustic cell with three acoustic resonators operating at different resonant frequencies was designed for offering multi-laser (multi-wavelength) operation simultaneously, and only one microphone was used to measure the acoustic signals of all resonators. The performance of TW-PAS was demonstrated and evaluated by measuring and analyzing the wavelength-dependent absorption coefficients of carbonaceous aerosols, which shows good agreement with previously reported results. The developed TW-PAS exhibits high potential for classifying and quantifying different types of light-absorbing aerosols by analyzing its absorption wavelength dependence characteristics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Aerosols directly disturb Earth’s radiation balance by absorbing or scattering solar radiation. So aerosol optical absorption measurements are important for predictions of climate change. Measurement and analysis of the wavelength dependence of aerosol optical absorption is also helpful in assessing the aerosol source apportionment, quantifying the relative contributions of different types of light absorbing aerosols, modelling the aerosol radiative forcing (RF) and photochemistry, and offering improved parameters for optical remote sensing [1–5].
Light absorbing carbonaceous aerosols in the solar spectral region are dominated by black carbon (BC) and absorbing organic carbon (brown carbon, BrC) [6,7]. BC is produced from the incomplete combustion of carbonaceous fuels and exhibits nearly wavelength-independent absorption properties with absorption Ångström exponent (AAE) of about 1 [8]. The net positive RF of BC (0.55 W m-2) ranks BC as the second highest RF after CO2 (1.56 W m-2), with a comparable magnitude to CH4 (0.47 W m-2) [9,10,11,12]. BrC usually exhibits strongly wavelength-dependent AAE ranging from 2 to 9, with negligible absorption in the near-infrared range (>600 nm) [13,14,15] and increasing obviously towards the ultraviolet wavelength range [9,16,17].
In addition to the filter-based techniques (aethalometer, particle soot absorption photometer, multi-angle absorption photometer, etc.), there are currently two main methods available for in-situ measurement of the light absorption coefficient of aerosols [6]. The first method is the “extinction-minus-scattering” technique, in which aerosol’s absorption coefficient is obtained from subtraction of scattering coefficient from extinction coefficient. The second method is to directly measure the aerosol’s absorption coefficient using photoacoustic spectroscopy.
The “extinction-minus-scattering” technique was early implemented using a portable optical multi-pass cell to measure aerosol extinction coefficient with a He-Ne laser (λ=633 nm), while aerosol scattering coefficient was measured using a cosine sensor [18]. Recently this method has been exploited by combining an incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) or cavity ring down spectroscopy (CRDS) with an integrating sphere [19]. Wavelength-dependent characteristics of aerosol light absorption were then determined from two instruments’ measurements at different wavelengths. This technique may introduce significant error when the absorption coefficient is extracted from two large, nearly close values (more common in atmospheric aerosols) [6,12].
Photoacoustic spectroscopy is commonly recognized as one of the best candidates to measure the absorption of aerosols, as it directly measures the absorbed light via photoacoustic effect. However, most reported photoacoustic spectrometers (PAS) used for aerosol optical absorption measurement are single wavelength operation. Although more and more literatures have reported on multi-wavelength aerosol absorption photoacoustic spectrometer in recent years, the measurement of multi-wavelength aerosol light absorption is still challenging. Current multi-wavelength PAS for aerosol absorption measurements can be classified into three categories: (1) multi-wavelength PAS by coupling several independent photoacoustic cells [20]. Such PAS system is relatively bulky and complicated. (2) multi-wavelength PAS using a broadband light source, such as optical parametric oscillator (410-710 nm) [21], supercontinuum laser (387 to 708.5 nm or 500 to 840 nm) [22,23], or a Hg arc lamp (300 to 700 nm) [2]. One single acoustic resonator was usually utilized for multi-wavelength measurements, and working wavelengths should be selected and switched in sequence by using an optical filter wheel or a tunable wavelength and bandwidth filter, which leaded to a low time resolution (in minutes). (3) multi-wavelength PAS by coupling multi-laser beams into a single photoacoustic cell. In such PAS, each laser was modulated at frequencies spaced by several Hz around the resonant frequency, respectively. The commercially available three-wavelength (λ = 405, 532, and 781 nm) photoacoustic soot spectrometer (PASS-3, Droplet Measurement Technologies, Inc.) uses this scheme. Recently, Fischer et al. also reported a four-wavelength PAS using this scheme, in which four wavelengths of 406, 532, 662 and 785 nm were used. The modulation frequency of each wavelength was spaced by 2 Hz to eliminate signal cross-talk of different wavelength [13]. Yu et al. reported a similar single-RGB (473, 532, 671 nm) differential PAS where three laser beams are modulated at 1659 Hz, 1652 Hz, 1642 Hz, respectively to minimize the effects of beat frequencies on PAS signals [24]. However, since these multi-wavelength PAS do not work in a fully resonant state, the sensitivities are degraded.
We recently introduced a three-wavelength PAS (TW-PAS) with a new resonator design which allowed real-time and in situ simultaneous measurements in a single photoacoustic cell module in resonant mode using a single microphone. High time resolution was achieved while ensuring high sensitivity owing to each wavelength operation at resonant mode [25]. In this work, such novel PAS device is developed for application to measure aerosol absorption at three wavelengths (404 nm, 637 nm, 805 nm) and evaluated by measuring laboratory generated carbonaceous aerosol.
2. Experiment section
Figure 1 is a schematic of the TW-PAS developed in the present work. The TW-PAS consists of three low-cost, compact and robust laser diodes (404 nm, 637 nm, 805 nm), a photoacoustic cell equipped with a single microphone (BSWA, MP201, 50 mV/Pa). The photoacoustic cell is manufactured by duralumin, and includes three cylindrical acoustic resonators with different acoustic resonant frequencies and two acoustic buffers (Fig. 2). Figure 2 is a sectional view of the used photoacoustic cell with three resonators. A hole with an inner diameter of 3 mm was set in the middle of each resonator, which used to transfer PAS signals from the acoustic resonators to the microphone. With such a design, the PAS signal in each resonator can be measured using a single microphone. The three acoustic resonators have the same inner diameter of 10 mm. The lengths of first acoustic resonator (indicated as AR1), second acoustic resonator (AR2) and third acoustic resonator (AR3) are 100 mm, 110 mm, and 120 mm, respectively. Such design makes the resonant frequency of each resonator separated by about 100 Hz, and ensures no signal cross-talking during phase-sensitive signal demodulation of each resonator’s, simultaneously, with one microphone. In addition, these three acoustic resonators shunted the total volumetric flow rate of the aerosol, so the PAS can operate at a total volumetric flow rate of about 1.5 L/min without flow noise affection. The two acoustic buffers are connected to the both sides of the acoustic resonators to minimize flow noise, environmental noise, and noise resulted from light absorption by the windows. Three laser diodes with wavelength of 404 nm, 637 nm, 805 nm were used as excitation source. Three square wave signals with duty cycle of 50% generated by function generators of FG1, FG2 and FG3 (UDB 1302S) are fed to the laser diode (LD) controllers for amplitude modulation of the laser output. The 404 nm and 637 nm lasers are collimated using lenses with 6 mm focal lengths, respectively. The collimated beams of 404 nm and 637 nm enter AR1 and AR2 though the reflector mirrors of M1 and M2, respectively. The beam diameters of 404 nm and 637 nm are both 4 mm. The 805 nm laser is fiber-pigtailed and collimated through a fiber-coupled collimator, which directly fed into the AR3. The beam diameter of 805 nm is 3.5 mm. The measured output powers of these three laser diodes are 170 mW (404 nm, CLD405500, HITACHI), 360 mW (637 nm, HL63283HD, HITACHI), and 660 mW (805 nm, FL006131-2, Focuslight Technologies Inc.), respectively. TTL outputs from the function generators of FG1, FG2 and FG3 are fed to lock-in amplifiers of LIA 1, LIA 2 and LIA 3 (LIA-BVD-150-L), respectively, as references for demodulation of each wavelength PAS signals in 1f mode. The time constant and filter slope of LIA are 1 s and 12 dB/oct, respectively. The detection bandwidth of LIA is 125 mHz. The demodulated PAS signals from the lock-in amplifiers are subsequently sampled with a data acquisition (DAQ) card (NI-USB-6210) and displayed on a laptop using LabVIEW software. The time resolution of TW-PAS measuring photoacoustic signal is 1 s.
Fig. 1. Schematic of the three-wavelength PAS. M: reflector mirrors, FG: function generator; LIA: lock-in amplifier.
Fig. 2. Sectional view of the photoacoustic cell used in the TW-PAS: Top view (left) and side view (right).
3. Results and discussion
3.1 Finite element analysis
Finite element method (FEM) developed in COMSOL software was used to analyze the performances of the newly developed photoacoustic cell [25].
Figure 3(a) is the 3D model used for simulation, Figs. 3(b), (c), and (d) show the first order longitudinal (100) acoustic standing wave patterns in AR1, AR2 and AR3, respectively, obtained through finite element analysis. In the finite element model, the material used is air, the temperature and the pressure are set at 298.15 K and 1atm respectively. The resonant frequencies obtained from the simulations are about 1607.7 Hz (for an AR1, L=100 mm), 1491.1 Hz (AR2, L=110 mm) and 1396.5 Hz (AR3, L=120 mm), respectively. According to the finite element simulation, the Q factor and full width at half maximum (FWHM) of the resonance curves of AR1, AR2 and AR3 can be calculated as Q1 = 49.4, (FWHM)1 = 32.5 Hz; Q2 = 39.2, (FWHM)2 = 38.0 Hz; and Q3 = 40.7, (FWHM)3 = 34.3 Hz. It can be clearly observed that PAS signals can be separately detected at its own resonant frequency of the acoustic resonator, as discussed in Ref. [25].
Fig. 3. FEM simulation results of the photoacoustic cell shown in Fig. 2. (a): photoacoustic cell model used in FEM simulation. (b), (c), (d): first order longitudinal acoustic standing wave patterns of AR1, AR2 and AR3, respectively.
3.2 Calibration of the TW-PAS
PAS signal as a function of absorption coefficient of the TW-PAS was calibrated using NO2 with known concentrations. The cell constant C can be obtained by the ratio of the PAS signal S to the absorption coefficient α after normalization of laser power, as defined below [26]:
where S (mV) is the net PAS signal after background correction, α (Mm-1) is the absorption coefficient of the used NO2 calibration sample, P (mW) is the LD power, and C (mV·Mm·mW-1) is the cell constant.Figure 4 is the experiment setup used during calibration. Samples of different NO2 concentration were obtained by diluting standard NO2 gas with pure N2. A home-made LED-based ($\lambda $ = 439 nm) spectrometer coupled to a Herriott cell was used to measure the concentrations of NO2 injected into the photoacoustic cell for calibration. The optical path and detection limit of the Herriott cell are 26.1 m and 1 ppm, respectively. The NO2 measurement range of the homemade LED-based spectrometer is 1 ppm to 100 ppm. When calibrating the TW-PAS with a lower concentration (<1 ppm) of NO2, a NOx analyzer (Model 42i) was used instead of the LED based spectrometer. The measuring range of NOx analyzer is 0 to 1 ppm. The NOx analyzer is not shown in Fig. 4. The calibration of the photoacoustic cell was performed at 25 °C with a relative humidity of 10%.
Fig. 4. Experimental setup for TW-PAS calibration. LED-based spectrometer was used to monitor NO2 concentration, which will be replace by a NOx analyzer when NO2<1 ppm. LD: Laser diode; LDC: LD controller; FG: function generator; LIA: lock in amplifier. DAQ: data acquisition card. The 450 nm light source was used to calibrate the TW-PAS, and the light sources (404, 637, 805 nm) of the TW-PAS will be turned off during calibration. Flip-up mirrors were used to reflect the 450 nm laser into the acoustic resonators of TW-PAS in sequence for calibration of each acoustic resonator.
Before measuring the aerosol absorption coefficient, a 450 nm LD (370 mW, PL-TB450B, OSRAM) was used for calibration of the TW-PAS. During the calibration process, the 450 nm laser beam was reflected into the acoustic resonators of TW-PAS in sequence using flip-up mirrors, as shown in Fig. 4. Figure 5 shows the calibrated results. The concentration range of NO2 used in the calibration of the photoacoustic cell is 0-30 ppm. The cell constants can then be obtained from the slope of the curves according to Eq. (1). The cell constants of AR1, AR2 and AR3 were found to be $2.49 \times {10^{ - 7}}$, $4.65 \times {10^{ - 7}}$, $5.11 \times {10^{ - 7}}$ mV·Mm·mW-1 respectively. It can be seen, with the same diameters, the longer the length of the acoustic resonator, the higher the cell constant, i.e., the higher the conversion efficiency from optical power to sound energy. In subsequent experiments, these calibration results can directly convert the measured PAS signal into aerosol’s absorption coefficient.
3.3 Evaluation of TW-PAS detection limit
For evaluation of the detection limits of the TW-PAS, time series measurements of air were performed. The air was filtered through a filter to remove particles, and then sealed in the photoacoustic cell. During the measurement, the scanning frequency of the laser diodes was 1 Hz and the photoacoustic signal was not averaged, that is, each point of the signal was obtained in 1 s. The detection limits of TW-PAS were evaluated by using Allan variance. Figure 6 shows the Allan variance from the time series measurement of air. For an integration time of 1 s, the detection limits of TW-PAS can reach about 3.62 Mm-1 (AR1), 1.01 Mm-1 (AR2), and 0.74 Mm-1 (AR3), respectively. When the integration time was increased to 200 s, the detection limits of TW-PAS were improved to 0.78 Mm-1 (AR1), 0.31 Mm-1 (AR2), and 0.2 Mm-1 (AR3), respectively. Table 1 lists the comparison of the detection limits of the TW-PAS with other reported multi-wavelength photoacoustic spectrometers. It can be seen that the detection limits of the developed TW-PAS are better than or basically consistent with the reported results. By using higher power laser diodes, the detection limits of the TW-PAS will be further improved.
Table 1. Comparison of the detection limits of TW-PAS and other multi-wavelength photoacoustic spectrometers.
3.4 Measurement of absorption characteristics of carbonaceous aerosol
Light-absorbing carbonaceous aerosols are produced by incomplete combustion of carbon-containing fuels. Kerosene soot aerosol is a typical surrogate for BC, which exhibits nearly inverse wavelength dependence of the aerosol light absorption coefficient (AAE∼1) [27–30]. According to literature reports, the particle size of kerosene soot aerosol is in the range of 20-600 nm [22]. To evaluate the reliability of this TW-PAS, wavelength dependence of absorption coefficient was measured using strongly absorbing kerosene soot produced in laboratory. Figure 7 is the schematic of the used production and sampling system of kerosene soot. The soot particles generated from kerosene lamp were diluted with purity N2 in a buffer volume, which can prevent the high soot particle concentration from saturating the TW-PAS sensor. Buffer volume can settle large size soot particles to prevent contamination of the photoacoustic cell. The resonant frequency of each resonator was tested and the modulation frequency was set to match the resonant frequency of the corresponding resonator. It is worth noting that the resonance frequencies and Q factors of the acoustic resonators will be shifted with changes in temperature (T) and relative humidity (RH) [5,31]. For every 1 K increase in temperature, the corresponding acoustic resonance frequency will increase by about 3 Hz [5]. As the relative humidity increases, the speed of sound will increase, which will cause an increase in the acoustic resonance frequency [31]. In addition, the PAS signal is susceptible to high RH environment [32]. Under high humidity conditions, the evaporation of water will affect the photoacoustic signal. Therefore, the whole experiment was performed in an air-conditioned room, and the temperature was stabilized around 25 °C. Meantime, the T and RH were real-time monitored using a thermo-hygrometer (TH22R-EX). The aerosol particles were dried by a drying tube before passing through the photoacoustic cell with a RH maintained at about 27%.
Figure 8(a) is the temperature and humidity data obtained during the measurement. Figure 8(b) shows the absorption characteristics of kerosene soot particles from continuous time-series measurement by the TW-PAS. Soot particles were also passed through a filter for background measurement (including interference gases, absorption of laser by the inner wall of resonators, without the presence of soot particle). When turning off the laser diode, the photoacoustic signal measured was basically zero. The sampled kerosene soot particle number concentration was influenced by uneven mixing, which caused the fluctuation of the measured aerosol absorption coefficient. But this will not affect the analysis of the wavelength-dependent characteristics (AAE) of aerosol absorption. Spectral dependence of aerosol absorption is in general described by: [30]
where αabs, λ is the absorption coefficient at the wavelength of λ, β is a constant independent of wavelength.Fig. 8. (a) Temperature and humidity during measurement; (b) Absorption properties of the kerosene soot (refer to the left vertical axis) measured by the TW-PAS at 404, 637, 805 nm (violet, red, green curves). Magenta line indicates time series values of AAE derived from the wavelength-dependent absorption coefficients (refer to the right vertical axis). The up arrow indicates that the filter is turned on for background measurement.
After the measurements of the absorption coefficients of kerosene soot aerosols at three wavelengths, the AAE value is obtained by linear fitting the measured absorption coefficients to the wavelengths according to the following formula:
The magenta line in Fig. 8(b) is the calculated AAE value. During the measurement process, the AAE value (mean: 0.98) is basically stable around 1, which indicates that the measured aerosol composition is mainly BC. The AAE value is generally related to the chemical composition of aerosols, and it can be used to classify different types of aerosols [33]. Considering that the AAE values during the experiment are almost constant, it can be considered that the composition of the aerosols in each resonator is the same.The AAE value of 0.98 is in good agreement with the reported results for kerosene soot, which shows the reliability of the developed TW-PAS for measuring aerosol absorption. [8,22,27,29,30,34]. For example, Sharma et al. (2013) demonstrated the AAE for kerosene soot is about 0.972 (±0.001) by using a multi-wavelength photoacoustic nephelometer spectrometer (SC-PNS) [22]. Gyawali et al. (2012) derived an AAE value of 0.8 for kerosene soot by photoacoustic measurement at 355, 405, 532, 870, and 1047 nm [29]. Reno Aerosol Optics Study (RAOS) also reported a similar AAE value in the range of (0.94-1.0) for kerosene soot [30]. In addition, this AAE value is consistent with previous theoretical studies on wavelength-dependent absorption of small spherical particles with constant refractive index [8,27,34].
After verifying the reliability of the developed TW-PAS for measuring BC aerosols, the developed TW-PAS was further used to measure the absorption characteristics of aerosols produced by the combustion of corn stalk. Figure 9(a) is the temperature and humidity data obtained during the measurement. Figure 9(b) shows the absorption characteristics of aerosols produced by burning corn stalk measured using TW-PAS. The AAE value of the aerosol produced by burning corn stalk can be calculated by Eq. (3). The magenta line in Fig. 9(b) is the calculated AAE value. The average value of the AAE during the measurement was 4.1, which indicates that BrC was produced during the combustion of corn stalk.
Fig. 9. (a) Temperature and humidity during measurement; (b) Absorption properties of the aerosols (refer to the left vertical axis) produced by burning corn stalk measured by the TW-PAS at 404, 637, 805 nm (violet, red, green curves). Magenta line indicates time series values of AAE derived from the wavelength-dependent absorption coefficients (refer to the right vertical axis). The up arrow indicates that the filter is turned on for background measurement.
The absorption spectrums of the aerosol produced by burning kerosene and corn stalk are shown in Fig. 10. Since corn stalk is unstable in the combustion process, the fluctuation range of its absorption coefficient is relatively large. In the later stage of corn stalk burning, fifty points with a small fluctuation range of absorption coefficient are selected to calculate the aerosol absorption spectrum. By averaging the measured absorption coefficient values, and then performing linear fitting according to Eq. (3), the AAE values can be calculated to be 0.96 ± 0.15 (1σ) and 4.05 ± 0.25 (1σ), respectively.
4. Conclusion
A TW-PAS was developed for measurements of aerosol optical absorption at near-ultraviolet (404 nm), visible (637 nm) and near-infrared (805 nm), simultaneously. The presented TW-PAS is capable of simultaneous measuring the aerosol absorption coefficients at three wavelengths with one photoacoustic cell and each wavelength is operated at resonant frequency allowing optimum performance. In addition, only a single microphone was used in the TW-PAS which is capable to measure the PAS signals of three wavelength simultaneously. These make the TW-PAS offer good performance, compact size, simple configuration and high time resolution (in seconds). The performance and reliability of the developed TW-PAS was verified by measuring laboratory generated carbonaceous aerosol. The AAE value of kerosene soot is in good agreement with reported results and the theoretical studies. The developed TW-PAS exhibits high potential for classifying and quantifying different types of light-absorbing aerosols by analyzing its absorption wavelength dependence characteristics.
Funding
National Key Research and Development Program of China (2016YFC0303900, 2017YFC0209700); National Natural Science Foundation of China (41475023, 41575030, 41730103).
Disclosures
The authors declare no conflicts of interest.
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