Research On Calibration Method Of Photoacoustic Spectral System For Aerosol Absorption Measurement

The radiation balance of the atmosphere and the Earth’s climate effected by direct and indirect radiative forcing of atmospheric aerosols.Aerosol optical absorption characteristics are the most uncertainty factor in global climate assessment,so it is very important to obtain accurate aerosol light absorption data.Photoacoustic spectroscopy is the most direct and effective method to study the absorption characteristics of aerosols,which is characterized by high sensitivity,in-situ measurement,simple structure and easy to be affected by light scattering.The measurement accuracy of photoacoustic spectrum system is closely related to the calibration methods.In order to explore the differences between different calibration methods,a set of aerosol absorption measuring device and calibration device of photoacoustic spectrum in near infrared band were established in this paper.NO2 gas and monodisperse nigrosin aerosol were used to calibrate the measuring device respectively.In addition,the influences of calibration wavelength,nigrosin aerosol particle size and other factors on the calibration results were deeply analyzed.The main research contents and innovations are as follows:(1)The aerosol direct calibration method of near infrared band photoacoustic spectrum aerosol measurement system was studied.The complex refractive index of nigrosin was measured by ellipsometry spectroscopy and the data of complex refractive index of nigrosin was extended to near infrared band.Based on Mie scattering theory,the absorption coefficient of monodisperse nigrosin aerosol was calculated by wavelength,particle size and complex refractive index.The calibration of photoacoustic spectral system without changing the light source was realized.The experimental results show that the calibration coefficients of nigrosin aerosols at 532 nm and 1064 nm wavelength are close,but the complex refractive index measurement of nigrosin in this method has higher requirements for nigrosin film samples.(2)The differences between aerosol calibration methods and gas calibration methods of photoacoustic aerosol measurement system are evaluated.The experimental results show that both NO2 and nigrosin aerosol can calibrate the photoacoustic spectrum system at 1064 nm wavelength,and the calibration results obtained by the two calibration methods are similar.In the calibration process,the wavelength of light has little influence on the calibration results,which are mainly affected by the physicochemical properties of the calibration medium(such as humidity),the loss of the calibration medium and other factors.(3)After the calibration,the photoacoustic system was used to measure the light absorption coefficient of biomass aerosol,and the measured results were compared and analyzed with those of the black carbon instrument.The analysis results showed a good consistency between the two trends,but there was a big difference in the measured values.The measured results of the black carbon instrument were 2～5 times of those of the photoacoustic system.After fitting the measurement results of the two instruments,the scattering calibration factor of the black carbon instrument is 2.25～4.94.This result is consistent with the results of several research groups at home and abroad,mainly because of the multiple scattering effect of the black carbon instrument and the wavelength dependence of the light absorption of brown carbon aerosol.In conclusion,gas calibration methods and aerosol calibration methods have their own advantages and disadvantages.Gas calibration methods are mature and easy to obtain gas optical parameters.However,this method has some shortcomings,such as the inconsistency between the wavelength of light source and the wavelength of actual measurement and the toxicity of calibration gas.The aerosol calibration method does not need to replace the light source,and the physical and chemical properties of the calibrated aerosol are close to that of the atmospheric aerosol,but it is difficult to obtain the optical properties of the calibrated aerosol.In the selection of actual calibration method,it is necessary to consider various factors comprehensively and select the appropriate calibration method.