Long Absorption-path-based Faraday Rotation Spectroscopy And Its Application In Atmospheric Chemistry

The atmospheric chemical cycle of hydroxide radicals(HOx=OH+HO2)and nitrogen oxides(NOx=NO+NO2)is of great significance to the research on the mechanism of atmospheric compound pollution.The measurements of HOx radiclas and HOx can provide key scientific data for verification atmospheric model and atmospheric pollution prevention and control.However,there are still many difficulties and challenges in the high-precision and accurate measurements.Faraday rotation spectroscopy,which based on Faraday effect of paramagnetic molecules in the longitudinal magnetic filed,can effectively reduces the laser noise and eliminates absorption interferences of diamagnetic molecules.The highly detection sensitivity,good selectivity and none chemical interference make it very suitable for the measurements of key molecules such as HOx and NOx in atmospheric chemistry.In this work,research on Faraday rotation spectroscopy was carried out.The detection sensitivity was improved by increasing the optical absorption path length and suppressing the system noise.This improved Faraday rotation spectroscopy was applied to the OH and NO measurements in atmospheric chemistry research,and provided new potential techniques for photochemical smog chamber measurements and field observation.The research of the optical multipass cell was carried out to provide a long otpical path solution for highly sensitivity Faraday rotation spectroscopy.Several optical multipass cells were designed and developed,which include an asitgmatic cell with 200 m pathlength,a Herriott cell for laser flash photolysis with?60 m pathlength,an improved spherical mirror cell with more than 200 m pathlength and a Chernin cell with a portable and stable adjustment mechanism.We developed a supercongducting-magnet-based Faraday rotation spectrometer for in-situ measurement of OH in a quartz atmospheric simulation chamber at 2.8?m.The absorption line Q(3/2)(2?3/2,v=1?0)at 3568.52 cm-1 with the strongest line strength and g factor in infrared was chosed for the detection.The uniformity of the filed strength is better than 3%in a cylindrical region with 100 cm long and 40 cm in diameter of the superconducting magnet.By using a Chernin cell with a path length of 108 m and 45°-method with balance detection,a detection limit of 1.6×106 molecul/cm3(1?,4 s)was achieved.The in-situ measurement of NO in the quartz atmospheric simulation chamber at 3.55 ?m was performed on the above superconducting-magnet-based Faraday spectrometer.The absorption line Q3/2(3/2)(2?3/2Q(3/2),v=1?0)at 1875.81 cm-1 was chosen for the measurement.By using 90°-method,a detection limit of 1.15 ppbv(1?,1 s)was achieved with an absorption pathlength of 108 m.This velue can be further improved to 0.43 ppbv by increasing the acquisition time to 15 s.A compact Faraday rotation spectrometer that simulates atmospheric nozzle sampling for OH detection at 2.8 ?m was developed.By using an improved spherical mirror cell and balance detection,a detection limit of 7.07×106 molecul/cm3(1?,1s)was obtained with an absorption pathlength of 76 m.This provided an important reference for the filed application of Faraday rotation spectrometer for OH detection.