Calibration Method of Aerosol Absorption Coefficient Based on
Photoacoustic Spectroscopy
XU Qiu-yi1, 3, 4, ZHU Wen-yue3, 4, CHEN Jie2, 3, 4, LIU Qiang3, 4 *, ZHENG Jian-jie3, 4, YANG Tao2, 3, 4, YANG Teng-fei2, 3, 4
1. School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei 230026, China
2. Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China
3. Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
4. Advanced Laser Technology Laboratory of Anhui Province, Hefei 230026, China
Abstract：Atmospheric aerosols modify the energy balance of the atmosphere and the Earth's surface, visibility, and climateby absorbing solar radiation. Photoacoustic spectroscopy (PAS) is the most direct and effective method to measure the absorption of aerosols due to the advantages of high sensitivity, no change of aerosol stateand simple structure. The calibration method is the key factor of measurement accuracy of aerosol optical absorption coefficient using a PAS working at the near-infrared band. In order to explore the differences between different calibration schemes, NO2 (532 nm) and monodisperse nigrosin aerosol (532 and 1 064 nm) were selected to carry out calibration experiments of gas and aerosol separately. Based on Mie's theory, the absorption coefficients of the monodisperse nigrosin aerosol at wavelengths of 532 nm and 1064 nm were calculated using the concentration of aerosol particles, the complex refractive index and the particle diameter. The experimental results show that: the value of the calibration coefficient obtained by NO2 is between the calibration coefficient obtained by nigrosin aerosol at 532 and 1 064 nm wavelength, moreover, which is close to the value of calibration coefficient of nigrosin monodisperse aerosol with particle diameter of 225 nm (λ=532 nm) and 125 nm (λ=1 064 nm) butnot the same in other particle sizes. Despite the similar continuous absorption properties at specific wavelengths of NO2 and nigrosin aerosol, for the same calibration system, there are still uncertainties in the calibration process such as light-matter interaction, calibration medium loss, and measurement environment other than photoacoustic effect due to the different properties and states of calibration gas and aerosol. All these will lead to the difference of calibration results between these two calibration methods. The calibration coefficients of nigrosin aerosol obtained by using different wavelength light sources are significantly different, and the calibration coefficient values of the same particle size at 532 nm wavelength is about 1.5～2 times that at 1064 nm wavelength. There are two possible reasons for this result: one is that theobtained complex refractive index of nigrosine at 1 064 nm is not accurate, and the other is that the matching mode of the 532 nm wavelength beam and 1064 nm wavelength beam with the resonator is not consistent during the calibration process. There was an inverse relationship between the calibration coefficient and the particle size of monodisperse aerosol. With an increase of 50 nm in the particle size of nigrosin aerosol, the calibration coefficient decreased by 19% for the 532 nm wavelength light source and decreased by 12% for the 1 064 nm wavelength light source. The inverse relationship between calibration coefficient and particle size may be caused by the different losses of nigrosin aerosol particles with different particle sizes through the pipeline and the photoacoustic cell. It can be seen that the gas calibration method and the aerosol calibration method have their advantages and disadvantages, but the two calibration methods can obtain the corresponding calibration coefficients. It is necessary to consider various factors comprehensively in selecting the actual calibration method.
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