The Study Of Atmospheric HO₂ Radical Detection Technique And Field Measurement

With the acceleration of social development and urbanization,many factors such as industrial emissions,agricultural production,and human life have led to high concentrations of precursors such as NOx and VOCs,and atmospheric compound pollution characterized by PM2.5 and O3 has become increasingly serious.One of the important reasons is the increasing of atmospheric oxidation.In the entire atmospheric chemical reaction process,OH radicals are the most important oxidants in the atmosphere.Except for the primary source of HONO and O3 photolysis,HO2 radicals are the key secondary source of OH.HO2 radicals play a catalytic role in the photochemical reaction of the troposphere and are an important medium for the removal of primary pollutants and the generation of secondary pollutants.The formation of many secondary pollutants is closely related to it.The source and sink of HO2 radicals directly affect the generation of atmospheric O3.Therefore,the accurate measurement of the HO2 radicals in the atmosphere plays an important role in the study of the cause of secondary pollution.The main work and conclusions of this paper are summarized as follows:1)In order to detect HO2 radicals based on the gas expansion laser induced fluorescence technology(FAGE),a set of HO2 radical detection system was designed and built,and the important components and performances of the system were discussed.Aiming at the problem of the temperature drift of the wavelength of the laser,a miniaturized reference cell using high-temperature pyrolysis of water vapor to generate a high concentration of OH radicals was designed and built,and the laser wavelength was locked at the position of the maximum OH fluorescence.The jitter of the fluorescence signal in the miniaturized reference cell is less than 2%and it is convenient to carry in the field observation.In view of the influence of gas sampling rate,solar stray light interference in the field observation,cavity pressure and temperature on the intensity of the OH radical fluorescence signal,the detection sensitivity of the system is improved by optimizing the diameter of nozzle,cavity pressure and expansion distance.Aiming at the problem of uneven mixing of HO2 and NO in the low-pressure fluorescence detection system,a NO loop injector was designed to realize the high efficiency conversion of HO2 to OH radicals with greater than 95%.2)In order to achieve the accurate calibration of the HO2 detection instrument during the field measurement,a set of turbulent calibration system based on the simultaneous photolysis of water and oxygen was designed and built to generate known concentration of HOx radicals which have high activity and no standard materials.The turbulent calibration system solved the problem of the uneven distribution of ozone concentration under laminar flow which need to measure ozone profile factors to correct the calibration result.In order to accurately calculate the concentration of HOx radicals generated in the turbulent calibration system,the absorption cross-sections of oxygen and water vapor at 185 nm under turbulent conditions,the rapid acquisition of ozone concentration and the wall loss of HOx radicals are discussed.The results show that the uncertainty of HOx radical concentration generated in the turbulent calibration system is about 8%.The built portable turbulent calibration system is applied to the HO2 detection system and compared with the calibration results of the laminar flow calibration system.The results show that the turbulent calibration system has good accuracy,and is suitable for the system calibration during field measurement with complex environmental conditions.3)Aiming at a series of interferences such as NO,particulate matter and RO2 radicals in the atmospheric HO2 measurement,a quantitative study of the interferences in the HO2 detection system was carried out.The results show that the interferences of NO and particulate matter during the HO2 measurement is negligible.For different types of RO2 radicals(RO2 radicals derived from the reaction of propane,ethylene,isoprene,methanol with OH radicals),when the HO2 conversion efficiency is lower than 20%,the interference of RO2 radicals in the atmosphere is less than 6%and can be ignored.The HO2 detection limit of the system can reach 4.8×105 cm-3,with 20%HO2 conversion efficiency,which meets the detection limit of atmospheric HO2 concentration.Finally,the HO2 detection system was improved and integrated in the structure of low-pressure fluorescence detection system and software acquisition,which a FAGE-HO2 system suitable for the accurate field measurement of HO2 radicals was built.The HO2 measurement system was initially applied to the autumn field measurement in Shenzhen to verify its stability and accuracy.4)A comprehensive field measurement of atmospheric HOx radicals in Changzhou Gehu Lake was carried out.The concentrations of HO2 radicals were accurately obtained and combined with photochemical parameters such as j value,NOx and VOC to analyze its concentration changing characteristics.The box model based on the mechanism of RACM2-LIM1 was used to simulate the concentration of HOx radicals.On the whole,the simulated concentrations of HOx radical are basically consistent with the observed concentrations.According to the formation process of ozone,the source and sink of HO2 radicals are analyzed.The results show that the reaction of RO2+NO and OH+CO is the main generation way of HO2 radicals,and the reaction of HO2+NO dominates the removal way of HO2 radicals.According to the statistical results of the generation and removal rate of each channel in the atmospheric reaction of Changzhou Gehu Lake,it is about 5.73 ppb of ozone generated per hour in the local area.