Research On Aerosol Optical Absorption And Spectral Dependence Based On Photoacoustic Spectroscopy Technology

The uncertainty of climate forcing assessment is one of the core issues of today's climate science debate.As pointed out by the Intergovernmental Panel on Climate Change(IPCC),due to the incomplete understanding of the distribution and physical and chemical properties of aerosols and the interaction between aerosols and clouds,the biggest uncertainty in current global radiative forcing comes from aerosols.Atmospheric aerosols have an important impact on the global climate through their direct effects(scattering and absorption of solar radiation)and indirect effects(the interaction of aerosols and clouds).Due to the lack of appropriate instruments and methods,the light absorption of aerosols is currently more uncertain than its scattering,resulting in the magnitude of atmospheric aerosol light absorption in global climate model that may be underestimated by more than twice.Therefore,it is of great significance to develop aerosol absorption measurement technology and carry out research on the light absorption characteristics of aerosols.Photoacoustic spectroscopy is a technique that can directly measures the light absorption of a sample.This technology converts the absorbed light energy into acoustic energy through the photoacoustic effect,and detects the signal through the acoustic sensor.It is not affected by aerosol light scattering and can directly measure the light absorption of aerosols in situ.At present,photoacoustic spectroscopy is considered to be the most effective method to measure the light absorption characteristics of aerosols.In the present work,a differential photoacoustic spectrometer and a multi-wavelength photoacoustic spectrometer were developed based on the photoacoustic spectroscopy technology,and they were used to measure and analyze the light absorption and spectral dependence of aerosols.The main merits and innovations of this thesis are as follows:(1)A differential photoacoustic spectrometer was developed to realize the simultaneous detection of atmospheric aerosol optical absorption coefficient and NO2 concentration.The photoacoustic cell used in the system innovatively adopts an intracavity shunt technology solution,which enables the system to work with high sampling flow rates and low flow noise.The detection sensitivity of the developed differential photoacoustic spectrometer can reach 1.0 Mm-1(1s)and 0.87 ppb(1 s),respectively.The measurement results of the photoacoustic spectrometer were compared with the environmental monitoring station and the NOx analyzer,which confirmed its accuracy and reliability.(2)The finite element method was used to analyze the influence of the length and inner diameter of the acoustic resonator on the resonant frequency,quality factor and photoacoustic signal of the photoacoustic cell.At the same time,four additional airways were constructed around the acoustic resonator for the first time,which can improve the sampling flow rate of the photoacoustic system to 1.4 L/min without being disturbed by flow noise.(3)A photoacoustic model of the multi-resonator photoacoustic cell was established by using the finite element method,and the performance of photoacoustic cell(resonance frequency,quality factor,photoacoustic signal loss mechanism)was analyzed theoretically using this model.The results show that the newly developed multi-resonator photoacoustic cell can work at three different resonance frequencies(with a difference of about 100 Hz)at the same time without causing crosstalk between photoacoustic signals.Furthermore,a new type of three-wavelength(404 nm,637 nm and 805 nm)photoacoustic spectrometer was developed to measure the light absorption characteristics of aerosols.The photoacoustic cell used in the system integrates three acoustic resonators with a length difference of 10 mm,which can satisfy that three laser beams enter the acoustic resonator at the same time and work at different resonant frequencies.At the same time,only one acoustic microphone can simultaneously measure three photoacoustic signals.(4)The light absorption and Angstrom exponent of typical black carbon aerosols(such as kerosene soot and diesel soot)were measured using the developed three-wavelength photoacoustic spectrometer.The measurement results are consistent with those reported in other references,which confirms the accuracy and reliability of the measurement results of the developed three-wavelength photoacoustic spectrometer.At the same time,the light absorption and spectral dependence of the aerosol produced by the combustion of corn stalks were studied,and the results showed that a large amount of brown carbon was produced during the combustion process.Furthermore,the Angstrom exponent(equal to 1)of black carbon was used to separate the black carbon and brown carbon produced by combustion,and the absorption spectrum and Angstrom exponent of brown carbon produced by combustion were obtained.