Research On NO₃ Radical/N₂O₅ Measurement System And Its Application Based On Cavity Ring-down Spectroscopy Technology

Nocturnal atmospheric chemistry is essential in the chemical cycle of the entire atmospheric environment.As the main species of nocturnal atmospheric environmental chemical cycle,nitrate radical(NO3)and dinitrogen pentoxide(N2O5)control the removal and transformation of nitrogen oxides(NOx)and volatile organic compounds(VOCs),which contribute importantly to the generation of night-time nitrates and secondary organic aerosols.Being one of the important contents of the current research on atmospheric complex pollution in China.Because of the trace level and high chemical activity of NO3 in the atmosphere,there are still great challenges in accurate measurement.Based on the optical cavity ring-down spectroscopy technique,a set of NO3 and N2O5 measurement system was built in this work,and the strict testing and calibrations were carried out in the laboratory in order to meet its field observation applications.The main results achieved are as follows:(1)Laser light with a center wavelength of 662±1 nm is generated by a diode laser.Two sets of high-fold mirrors with reflectivity higher than 99.999%consist of two optical resonant cavities.One cavity directly measures the NO3;the other one measures the sum of NO3 and N2O5,where N2O5 is converted to NO3 free radicals by heating.Two cavities enables the real-time and synchronous measurements of NO3 and N2O5.An automatic NO3 background measurement module through its dynamic titration with NO is established to offset the interference caused by the absorption of other gases.A fully automatic membrane filter changer has been developed through identifying the surface cleanliness of the membrane using real-time laser radiation and the following detection of the light attenuation,so as to determine when to activate the switch of changer.The fully automatic filter changer can not only prevent particulate matter from entering the measurement system,which can cause the pollution of the optical path or gas path,but also minimize the loss of NO3 on the particulate matter accumulated on the filter.The two-stage heating and temperature control device is designed to ensure the effective decomposition of N2O5 and maintain the stability of the thermal equilibrium between NO3 and N2O5.The laser source,optical cavity system,gas path,and temperature control module are integrated together,and the measurement processes can be automatically achieved through an independent programming software which improves the stability of the measurement system and the applicability of the field measurement.(2)Parameter characterization and uncertainty evaluation were carried out for the NO3/N2O5 measurement system.Considering the effect of temperature variations on laser spectral drift and NO3 absorption cross section,the data acquisition and processing module was optimized and an effective correction method was established.The effective absorption cavity length was determined using ozone gas in given concentrations and absorption cross section.Allan Variance analysis on the long-term stability of the experimental equipment was carried out to determine the deviation of laser drift caused by the different accumulation time,and to determine the error in the fitting of the time constant under the instability of the light source.The NO3 loss before measurement was determined using N2O5 standard gas.As a results,the loss of NO3 was 9%on the filter with 2 ?m pore size,was 14%during the gas transport.The time resolution of the system can be lower than 1 s,and the detection limit is 2 ppt,which can meet the needs on high-sensitivity field measurement of NO3 and N2O5.Overall,its performance has reached the international advanced level.(3)The concentration of NO3 and N2O5 are closely related to the distribution of their precursors(like O3 and NO2),so it is of great significance to obtain the vertical variation characteristics of NO3 and N2O5 for nocturnal chemistry researches in the urban boundary layer.In the summer of 2018,the vertical observations of NO2 and O3 were carried out on the Tianjin Meteorological Tower.This work lays a background support for the subsequent vertical observations of NO3 and N2O5 concentrations.