On-line Laser Detection Of Atmospheric Volatile Organic Compounds Based On Spectroscopy Technology

In recent years,the atmospheric pollution caused by volatile organic compounds(VOCs)is becoming more and more serious,which has triggered a series of environmental problems and caused great damage to people's health and property.At present,the common methods of the detection of atmospheric VOCs,such as gas chromatography,inductively coupled plasma-mass spectrometry,and other techniques,cannot realize real-time detection.In this paper,we designed and established the online detection system of atmospheric VOCs based on laser-induced breakdown spectroscopy(LIBS),single particle aerosol mass spectrometry(SPAMS)and Raman spectroscopy.Assisted with theoretical calculation based on density functional theory(DFT),this detection system was employed to the online direct detection of atmospheric VOCs pollutants.The main works of this article could be summarized as follows:(1)Optimization of the experimental parameters of online detection system and its practical application test.Taking ambient air as experimental object,the experimental parameters of the detection system,including pulse laser energy,delay time,integration time,etc.,were simultaneously optimized so that the signal-noise-ratio(SNR)of spectra was greatly improved.Then,the developed laser detection system was employed to the real-time measurement of ambient air and cigarette aerosol.The elemental compositions of the ambient air and cigarette smoke were analyzed to avoid that the ingredients in the ambient air interfere the identification of the spectral lines in the following measurements,and the experimental results also verify the high detective ability of gas sample by this system.(2)Real-time laser detection of volatile haloalkanes and sulfur-containing compounds under atmospheric environment.Using the optimized experimental system,the LIBS-SPAMS experiments were employed to the online direct measurement of Halon 2402,Freon R11 and iodomethane gas samples in the open air,and there existed obvious spectral peaks of fluorine,chlorine,bromine,iodine in LIBS spectra.The elemental information of individual aerosol particles was also obtained by the SPAMS analysis,acting as the important supplement to LIBS detection.Furthermore,the different isotopes of bromine and chlorine were distinguished in the meantime according to the respective mass spectra.Then,by analyzing the LIBS spectra of standard samples,the quantitative analysis models of these four halogen elements were established by linear fitting,and the limits of detection(LOD)of fluorine,chlorine,bromine,iodine in the halogenated alkanes sample were calculated to be 106 mg/L,56 mg/L,62 mg/L,and 76 mg/L,respectively.Meanwhile,taking dimethyl sulfide for example,the online measurement of sulfur-containing compounds was carried out.The atmospheric sulfur was detected by LIBS under the atmospheric pressure,and it can be observed from the spectrum that there existed seven obvious ionic lines of sulfur element in the visible spectral range.Then the quantitative analytical model of the atmospheric sulfur was established according to the LIBS experiments.By fitting the intensity of sulfur characteristic peak and concentration of sulfur compounds,the calibration curve of sulfur element was obtained.The computation results showed that the LOD of sulfur element was 46mg/L.What's more,laser Raman spectroscopy combined with DFT calculation was used to analyze the characteristic peaks of halogenated alkanes and sulfur-containing compounds,and the corresponding vibration modes of each Raman peak were respectively figured out,which could be regarded as the"spectral fingerprints"of VOCs molecules.It is indicated that the detection and analysis of the molecular structure of atmospheric VOCs were realized by Raman spectroscopy.(3)In situ detection of carbon and its isotopes in atmospheric VOCs pollutants.There exist obvious CN molecular band in the LIBS spectra of VOCs,and formation mechanism of CN radical as well as isotope effect of CN molecular spectrum were discussed according to the analysis of experiments results.Taking ¹²CO₂and ¹³CO₂ gas as experimental samples,the online detection of carbon isotopes was realized based on CN radical molecular isotopic spectroscopy.By comparing the spectra of ¹²CN and ¹³CN that formed by two kinds of carbon dioxide sample,the isotopic shifts of the CN molecular emission band were obviously observed in the spectra.In particular,the emission band of??(3???1 transition redshifts for about0.6nm while the emission band of??(3???1 transition blueshifts for about0.8nm.What's more,the energy differences of the isotopic shifts of CN emission band were also studied via DFT calculation,and the computational values were roughly identical to the experimental results.Furthermore,in order to further study the isotopic effect of CN molecular spectrum,the same computational method based on DFT was also applied to calculating the wavelength shifts of the molecular band of CN radicals with different isotopes.In this paper,the online detection system of atmospheric VOCs based on spectroscopic technology was developed and employed to the direct online detection of typical VOCs pollutants with the assistance of DFT theoretical calculation.The development and applications of this system provide not only important theoretical guidance and technical support for the rapid online detection of VOCs and other atmospheric pollutants,but also new ideas for the prevention and control of atmospheric pollution.