Synchronous Detection Technique Of Aerosol Absorption And Extinction Coefficient Based On Photoacoustic And Cavity Ring-down Spectroscopy

Aerosol is an important factor affecting atmospheric radiation,military national defense,wireless communication,environmental protection and occupational disease prevention,so it is very important to master its optical characteristic parameters such as extinction coefficient,absorption coefficient and scattering coefficient.However,the existing single parameter measurement technology cannot guarantee the unified measurement benchmark due to the sampling difference,while the multi-parameter synchronous detection technology is mostly limited to the "simultaneous,same-ground and superposition" measurement among the devices.Due to the factors such as sampling loss,cavity difference and detection source,there are still some technical problems such as the inconsistency of detection datum and the failure of synchronous and accurate detection.In order to solve this problem,a synchronous measurement method of aerosol absorption and extinction coefficient based on photoacoustic spectrum and cavity ring down spectrum was proposed in this paper,and a coupling measurement system of photoacoustic spectrum and cavity ring down spectrum was designed.The structure of the resonator was optimized,the SNR of the conventional structure was improved,and the detection limit of the atmospheric aerosol was reached.The response performances of different photoacoustic systems were compared and analyzed,and the temperature and humidity characteristics of the photoacoustic systems were illustrated.The data calibration algorithm of photoacoustic system was established.The measurement of aerosol standard source was studied.In the field experiment,the extinction coefficient and absorption coefficient of aerosol were directly detected,and the scattering coefficient and single scattering albedo were calculated indirectly,and the accuracy of the system was verified.The above studies were of great significance for unifying the detection benchmark of atmospheric aerosol optical characteristic parameters,improving the detection comparability and reducing the detection cost.Specific studies were as follows:1)By means of theoretical analysis,structural design,simulation modeling and experimental testing,the cavity structure of the traditional photoacoustic system was designed and optimized.The detection limit,pool constant and local noise of the photoacoustic system were studied from resonator cavity,buffer cavity,modulation parameters,optical path layout,signal acquisition,data processing,buffer and sampling methods.A prototype of photoacoustic spectrum measurement system was developed,and the calibration of aerosol was completed.The detection limit of the atmospheric environment was reached.The experiment showed that the background noise and SNR of the photoacoustic system were significantly reduced and improved,and the R2 of the system gradient curve reached 0.998 after linear fitting.The correlation coefficient of the external field was good,compared with the cavity ring down spectrum system.2)The method of measuring the extinction coefficient and absorption coefficient of atmospheric aerosol synchronously was proposed and realized.The feasibility of synchronous measurement of photoacoustic spectrum and cavity ring down spectrum was discussed.The effect of optical mirror and quartz window on the light intensity in the cavity was studied,and a three-stage coupling cavity was designed,and the effect of buffer cavity on the light and sound response was simulated.The design and prototype test of the coupling system were completed from the aspects of device selection,sampling system,signal acquisition and processing,system integration and so on.The results showed that the superimposed light intensity in the coupling cavity was 48.45%of the original light intensity,and the length of the resonant cavity and buffer cavity were 120 mm and 330 mm,respectively.The established coupling cavity had little influence on the sound field distribution of the photoacoustic system,and the detection limits about aerosol of the coupled photoacoustic and cavity ring down spectrum were 0.45 Mm-1 and 0.11 Mm-1,respectively.3)Performance studies of different photoacoustic spectroscopy systems were developed,and the photoacoustic spectrum features with the different optical path condition were contrastively analyzed.The slope error,data error and pool constant were studied.The synchronous measurement performances of the coupled spectral measurement system were verified.The experimental system of simulating atmospheric temperature and humidity was set up,and the influences of temperature,humidity and pressureon to photoacoustic performance were analyzed.The results shown that the responses of photoacoustic spectral system increased with the increase of temperature and humidity.The temperature would cause the resonance frequency to drift,and the humidity had a great influence on the photoacoustic amplitude.The photoacoustic cavity with high reflective mirror was superior to the others in slope error,data error and pool constant.4)The data correction algorithm of photoacoustic system was established,the accuracy of synchronous measurement system based on neural network algorithm was studied.The results shown that outfield contrast effect was better after the training of neural network algorithm.At the same time,the measurement of aerosol standard source and the field experiment of synchronous measurement system were carried out.The time series of extinction coefficient,absorption coefficient,scattering coefficient and single albedo were obtained.The relationship between extinction coefficient and scattering coefficient,PM mass concentration and extinction coefficient during observation were analyzed.The accuracy of the system was verified by comparing with the data of Max-DOAS and national control quality monitoring site.In conclusion,it was feasible to measure aerosol optical properties synchronously based on photoacoustic spectrum and cavity ring down spectrum,which provided a new idea and method for synchronously measuring aerosol absorption and extinction coefficient,and also provided a reference for the development of similar instruments.