Incoherent Broadband Cavity Enhanced Absorption Spectroscopy For Measurement Of Atmospheric NO₃ Radical

NO₃ is a highly reactive nitrogen-containing radical with strong oxidizing properties,which is involved in numerous chemical reactions in the atmosphere and the environment and plays an important role in the atmospheric cycle.The high activity,short lifetime and strong oxidizing properties of the NO₃ radical are important features in atmospheric and environmental chemistry and are of great significance for understanding the oxidation and removal of many gases in the atmosphere.NO₃radicals are very easily decomposed by light below 640 nm wavelength,and their lifetimes in the atmosphere during daytime are extremely short and their levels are extremely low.At night when the light is weak,the content of NO₃ radicals in the atmosphere will be accumulated to a certain extent.As the nocturnal analog of hydroxyl radicals,they regulate the balance between NO_X（=NO+NO₂）and volatile organic compounds（VOCs）and participate in the ozone cycle and a variety of photochemical reactions,which are important causes of hazy weather.Therefore,high precision detection of NO₃ radicals in the atmosphere is important to grasp the atmospheric nitrogen cycle,assess atmospheric pollution,study the oxidation capacity of atmospheric photochemical reactions,and analyze atmospheric cycling processes.The incoherent broadband cavity enhanced absorption spectroscopy（IBBCEAS）technique has the features of simple structure,high accuracy,low cost and simultaneous multi-component measurement,which can achieve high sensitivity and high resolution detection of atmospheric NO₃ radicals.In this thesis,based on the IBBCEAS technology,the detection device was simulated and optimized and built based on the existing research of the research group,and the concentration and variation of atmospheric NO₃ radicals in the Huainan area at night were observed and studied.The main research results were obtained as follows:（1）The optical cavity of a typical IBBCEAS instrument was modeled using Zemax optical design software.The factors affecting the intensity of the light emitted from the optical cavity,i.e.,the characteristics of the light source（LED）,the type of collimating lens,and the distance between the light source and the collimating lens,were investigated.The raw spectra of the light emitted from different optical cavities were obtained by non-sequential ray tracing with Zemax software.The simulation results show that LEDs with small viewing angles and high optical power as well as achromatic lenses should be used as the light source and collimating lens of the IBBCEAS instrument,respectively.Based on the simulation results,the IBBCEAS experimental setup is built and the mechanical structure of the instrument is improved several times to obtain a high-precision collimated optical path and a highly stable detection device.（2）The IBBCEAS system uses an LED with a central wavelength of 660 nm as the light source.To avoid the shift of the center wavelength caused by the temperature increase of the LED,a thermostat device is designed to work with the optimized optical cavity,and the device realizes the temperature regulation of the LED wick temperature by PID controller.The temperature is set at 20°C and the ambient temperature is between 10 and 22°C.The wick temperature fluctuates within±0.6°C after 24 hours of continuous measurement.（3）An optimized water vapor iterative algorithm is proposed to remove water vapor interference,using the measured spectra（containing reference and absorption）and the calibrated HITRAN reflectance as inputs to the iterative calculation,which converts them into high-resolution spectra.Combining the line-by-line parameters from the HITRAN database with the measured pressure and temperature in the cavity,the high-resolution spectra containing the water vapor absorption are calculated.The absorption coefficient at high resolution is calculated using the high-resolution spectrum,the water vapor absorption cross section and the initial water vapor concentration,which is convolved with the instrument function of the spectrometer to obtain a low-resolution absorption spectrum of water vapor.Finally,the error of the fitted water vapor concentration is evaluated to see if it reaches the desired value and to determine if it needs to be calculated iteratively to remove water vapor interference.The results show that the IBBCEAS technique based on LED light sources can accurately obtain the magnitude of atmospheric NO₃ radical concentration,and these studies are important in mastering the atmospheric nitrogen cycle,assessing atmospheric pollution,studying atmospheric photochemical reactions and oxidation capacity,and analyzing atmospheric cycling processes.By deeply understanding and accurately measuring the behavior of NO₃ radicals,we can better understand the chemical reaction processes in the atmospheric environment and provide a scientific basis for improving atmospheric quality and protecting the environment.Figure[56]Table[4]Reference[90]...