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Development of LED-DOAS system for observing aerosol optical properties in the lower troposphere

Aerosols play an important role in the Earth’s radiation budget through the reflection of incoming solar radiation and formation of cloud droplets working as cloud condensation nuclei. The understanding on aerosol optical properties in troposphere, especially their behavior near the ground level, is still insufficient for precise evaluation of their impact. Although a sunphotometer can provide the aerosol optical thickness, its application is limited to daytime under near cloud free conditions. A visibility meter, on the other hand, can give the value of visibility, but the operation wavelength is limited to a single wavelength, e.g. 875 nm. To attain the multi-wavelength observation of aerosol extinction coefficient near the surface level, here we propose the use of a four-color light emitting diode (LED) source emitting at 455, 530, 590, and 625 nm as a light source for the long-path measurement of aerosol extinction. A near-horizontal light path with a round-trip distance of around 630 m has been established inside the campus of Chiba University. A collimated light beam is produced with a 130 mm diameter telescope, and the reflected beam from a retro-reflector is detected using a 200 mm diameter telescope connected to a photodiode. A sinusoidal wave modulation is applied to the LED source, and the resulting modulated signal amplitude is detected and recorded using a digital oscilloscope (Iwatsu, DS-5614A). The results of the recent observation are discussed in conjunction with the concurrent records of a visibility meter (Vaisala, PWD52) and a nephelometer (TSI3563) that can measure the aerosol scattering coefficient.


Introduction
The vital effect of pollutant gases and aerosol both directly and indirectly to the atmosphere has been discussed in the context of the earth radiation budget and public health issues [1,2]. Therefore, the precise observation method of aerosol parameters is important for increasing better understanding their real characteristics especially in the lower troposphere. One of significant aerosols parameter is extinction coefficient which is correlated to the attenuation of light due to aerosol particle size floating in atmosphere and hygroscopicity parameter related to relative humidity [3,4]. The extinction coefficient is linear summation between absorption and scattering [5]. The leading method in monitoring absorption phenomena, the differential optical absorption spectroscopy (DOAS) is a remote sensing technique which has high sensitivity and time resolution in observing atmospheric particulate concurrently [6,7].
Development of instruments and methods in measuring of extinction coefficient have been provided that the DOAS is quite suitable tool in obtaining of particle characteristics in atmosphere near the source [7,8]. Several groups have represented that the DOAS systems is the potential alternatives instruments to monitor air pollutant and aerosols [9][10][11][12]. By means specific DOAS design in our group -Center for Environmental Remote Sensing (CEReS) -Chiba University -Japan, monitoring of pollutants and aerosols is established through measurement extinction coefficient over light path length. On that DOAS approach, the measurement is equipped by white flashlight as visible spectral range to detect atmospheric trace gas such as NO2, SO2, and CO2 [13][14][15][16]. The DOAS systems used in previous studies applied near ultraviolet and near infrared rays. Their main targets were gases pollutant.
The dependence of the spectrum regions to the particle size as absorber in atmosphere require an accuracy in exploiting the light source wavelength [17][18][19]. Since our new target is addressed to aerosol in the lower troposphere, we developed a unique DOAS by means of visible rays. Due to the light source modulated by light emitting diode (LED), the instrument is denominated LED-DOAS as novel active ground-based remote sensing. The LED-DOAS operate by transmitting modulated visible rays through collimate system and receiving reflected signal from retro-reflector. This system is improved from DOAS system which utilized and developed by several researchers. The MAX-DOAS proved capable in measuring tropospheric vertical column with target aerosol [19], NO2 [20], and compound between aerosol and trace gases [21]. The long-path measurement developed in Center for Environmental Remote Sensing -Chiba University -Japan evident not only in retrieving for NO2 [13] but also for both NO2 and aerosols [21] [21]. Judging for employing retro-reflector [10,16] and LED [11,22,23], we develop a novel design and experiment using 4 wavelength visible rays.
The main purpose in designing of the LED-DOAS is to measure the temporal change of aerosol extinction coefficient (AEC) near the ground level. In this study we developed and carried out experiment in CEReS area which is completed by some ground-based instrument. Besides, to verify the instrument's performance, the nephelometer, weather monitor, and visibility meter are employed for comparing several results of the new LED-DOAS. Here, we compare the LED-DOAS data to nephelometer raw data with wavelength 550 nm, relative humidity (RH), and the visibility with wavelength of 870 nm.

Instruments and Methods
Components of LED-DOAS system are generally consisted of four main parts: signal modulator, transmitter and reflector, signal detector, oscilloscope and personal computer. The LED-DOAS scheme and specification are showed in figure 1 and listed in table 1. This instrument operates by modulating signal and transmitting light beam in four wavelengths (625,455, 590, and 530 nm) from LED which is controlled automatically by personal computer. The four lasers beam are collimated using Newtonian telescope in 630 meters of round trip from the source to retro-reflector through ambient and back to the photodetector with internal amplifier.
Stability of the LED-based system has been verified by stabilising of photo detector linearity in long time measurement and sun interference. The alignment for detector linearity is checked by adjusting the voltage of modulation signal (recorded in Channel-1 of oscilloscope and personal computer), then compare to the voltage of both reference signal (Channel-2) and LED-signal from retro-reflector traversing real ambient aerosol (Channel-3). This procedure also confirms the correction factor of LEDsignal by fitting and analysing several aspects between reference and ambient LED signals. Furthermore, the signal modulation of functional generator is compared to the signal of both original light from small-mirror and ambient contaminated light from retro-reflector. The comparison is done for instrument data in clear and turbid condition using only several days' data for each sky condition. All alignment processes show the stability of detector and the signal to noise ratio of LED-DOAS system. Linearity of photo detector used in this system using the square signal, 377 Hz of frequency, and modulation vary from 1 to 4 Volt. The sample of linearity process is showed in figure 2.    Figure 2. Linearity of LED-DOAS system derived from channel-2 as the reference signal without aerosol interaction and channel-3 as the LED signal with ambient aerosol interaction. Sample data of aerosol measurement shown here is from our observation on May 10 to 12, 2017 with reference and LED signals are showed in figures 3 and 4, respectively. We generated the signal using functional generator with frequency 377 Hz and modulation 4 V. The colour changes every 175 second with average measurement 26 second per data. Indeed, we collected 6-7 data for 1 colour. The temporal change of both signals is retrieved under relative humidity, visibility, and temperature as displayed in figure 5. We can see that there are good correlations between LED-signal and ambient patterns. In the case of lower visibility, higher relative humidity, and lower temperature, LED-signal in the lower condition. Here, we can assume that in the lower visibility and higher relative humidity, aerosol particle reduces light beam from retro reflector based on theoretical concept in refs [4,24] . As a result, the LED-signal from retro reflector recorded in Channel-2 is lower.   Figure 4. Temporal change of LED-signal derived from channel-3 as the signal which is recorded after interaction with aerosol particle. Furthermore, an algorithm for processing LED-signal to attain aerosol parameter in horizontal direction was develop. By governing Bouguer-Lambert-Beer law and Langley extrapolation methods in refs [25][26][27] and adapting the results into the LED-DOAS system, we develop procedure for calculating initial signal intensity (ICLN) in equation (1), aerosol optical thickness (τa) in equation (2), and aerosol extinction coefficient (αa) in equation (3): Where, the ILED (LED-signal), τNET (total optical thickness), τm (molecule optical thickness), αm (molecule extinction), αSCA (scattering of nephelometer), C (correction factor), P (pressure), P0 (atmospheric standard, 1013.25 hPa) are parameters as function of wavelength (λ) and/or time (t). Figure 6 shows the sample of comparison between LED-signal (590 nm) and scattering nephelometer (550 nm). Here, the temporal change of LED-signal inversely proportional to the scattering nephelometer. The lower the LED-signal, the higher the scattering, and vice versa. The scattering is a part of extinction coefficient which correlated to the attenuation of light due to aerosol particle size floating in atmosphere and hygroscopicity parameter related to relative humidity [4,24]. In the condition of higher scattering, we can assume that aerosol loading in the sample of observation is higher. As a result, the signal recorded by LED-DOAS system will lower. This temporal variation verifies the theoretical concept about attenuation of light in atmosphere. Comparison for the three other wavelengths have been done with the same condition.   Figure 7 shows the result of fitting between LED-signal (455, 530, 590, and 625 nm) and scattering nephelometer (550 nm). This procedure is Langley plot like developed from ref [26,28]. The Langley plot is the extrapolation method to determine aerosol loading by fitting reference to measured values. In this case, we used scattering of nephelometer data at wavelength 550 nm as reference values and LED-DOAS signal as measured data. The result of this procedure is fitted logarithmic of LED-DOAS signals, ln ILED(λ). Their values are: ln ILED(455) = -3.9104, ln ILED(530) = -3.0117, ln ILED(590) = -2.9701, and ln ILED(625) = -2.6611. These values are used in calculation of optical thickness and extinction coefficient in the next step of data processing of LED-DOAS system.  Figure 8 represents the results of aerosol optical thickness (AOT) using equation (2). In computational process, we calculated AOT for four bands of LED-DOAS system. The time series variation of AOT in the day of experiment is relatively stable at around 0.4 to 0.5 for the blue light (455 nm). The green (530 nm) and yellow (590 nm) beams show AOT approximately ∼ 0.3 and ∼ 0.2, respectively. The red band (625 nm) of LED-DOAS signal also stable at approximately 0.1 to 0.2. The whole temporal change of the AOT derived from data processing of LED-DOAS system on the day of experiment are relatively stable, except for at around 6 am on May 12, 2017. Considering with the green band of the scattering nephelometer (550 nm) in figure 6, we can conclude that this processing method in calculating the AOT show the corresponds with the fluctuation of scattering coefficient. Figures 9 and 10 show the temporal change of extinction coefficient which is also verified to the nephelometer and visibility-meter data directly as scattering and extinction coefficient, respectively. In figure 9, we show the time series variation of extinction coefficient using LED-DOAS signal which is computed using equation (3). Furthermore, in figure 10, we present another temporal change of extinction coefficient as comparison to the result of LED-DOAS data processing based on visibilitymeter observation at the wavelength 550 nm. The method in computing visibility-meter data is the Koschmeider equation developed in refs [28,29]. Here, we found extinction coefficient's values derived from LED-DOAS system almost similar to the result retrieved from visibility-meter as the certified instrument installed in the CEReS-Chiba University.     Figure 10. Comparison of extinction coefficient between visibility-meter and LED-DOAS system.

Conclusion
Considering the time series variation of aerosol optical thickness and extinction coefficient obtained from the present instrument, experiment, and approach will be generally useful to estimate the optical properties of ambient aerosols based on the new LED-DOAS system. The capability of uninterrupted estimation of both aerosols optical thickness and extinction coefficient will provide new insight in the source and sink investigation of aerosols as well as in monitoring local environment. Since the LED-DOAS raw signal directly proportional to visibility and temperature but inversely proportional to scattering and relative humidity, we can conclude that the new instrument is working well.