Development Of Measuring Instrument Of The Atmospheric Total OH Radical Reactivity And Its Application

The hydroxyl radical(OH)is an important atmospheric oxidant that plays a key role in the formation of tropospheric ozone and secondary organic aerosols and controls the lifetime of atmospheric trace gases that contribute to global climate change.A complete understanding of OH sources and sinks is therefore essential to understand these processes.The total hydroxyl radical(OH)reactivity,the inverse of the lifetime of OH,is the sum of the concentration of all gases reacted with OH radical and its reaction rate coefficient.It is also an important index of OH radical chemistry and can be used to investigate OH sink.At present,the OH reactivity has been used to study the OH radical sink budgets.However,some trace active components cannot be measured so far,which makes the calculation of OH radical reactivity based on trace gas concentration incomplete.Therefore,direct measurements of actual OH radical reactivity in the atmosphere can better determine the comprehensiveness of the measurements used in the model to predict air quality and ozone production,and,in conjunction with measurements of OH radical concentrations,to assess our understanding of OH production rates.Meanwhile,the important chemical reaction processes involved in OH radical are the key of atmospheric chemistry,and the study of free radical reaction kinetics is crucial for the understanding of these key chemical reaction processes.In this paper,the high sensitivity OH radical reactivity detection instr?ment has been built and its application were studied.The main work and innovation of this paper are as follows:1.Using a tunable diode laser with a center wavelength of 2.8 ?m as the light source and combining with a 266 nm flash light source,a new laser-flash photolysis-Faraday rotation spectrometer(LFP-FRS)instr?ment was established.Based on the principle of Faraday magnetic rotation,the OH radical was produced by photodissociating O3 in the sample air mixed with water through the Nd:Yag laser of 266 nm wavelength.The OH radical decay in the sample was directly measured by the absorption of 2.8?m middle infrared magnetic rotation spectr?m.The decay curve of OH radical was fitted based on the pseudo first-order hypothesis,and the measured OH radical reactivity was obtained.2.By calibrating a series of optical parameters of the measuring instr ? ment,the detection limits of OH radical concentration and reactivity were determined to be 4×106 molecule/cm3(56 s)and 0.09 s-1(112 s),respectively,under the condition that the overlap optical path between the ultraviolet photodissociation laser(266 nm)beam and the detection laser was 25 meters.The whole process was controlled by LabVIEW program.The results show that the flash photolysis-magnetic rotation spectrum can be used to directly measure the total reactivity of OH and the OH radical reaction kinetics.The laboratory study on the determination of OH reactivity in different trace gas was carried out,and the feasibility of the instr?ment was verified though comparing with the literature values,and it was successfully applied to the determination of OH radical reactivity in zero air.3.Studies of the reactions rate between OH radical and alkanes/carbon monoxide/alkenes were performed with LFP-FRS under different pressures.The rate constants of the reactions between OH radical and alkanes/carbon monoxide/alkenes were determined under pseudo-first order conditions though directly measuring the OH decay.Our results were well comparable with previous literature reported values.The reaction process has been simulated by implementing several existed reaction channels into a chemical kinetic model,which provides a basis for studying the important atmospheric chemical mechanism and reaction process of OH radical and verifies the feasibility of operating the instrument under different pressure conditions.4.The change of total OH reactivity in namtso area was studied by field observation of the qinghai-tibet plateau from April to July of 2019.In this background area,the reactivities measured varied from 1 s-1 to 6 s-1,reaching a mean maximum value of about 6s-1 in the early morning and evening,and a mean minim?m value of about 1 s-1 at noon.In the same year,the detection of OH reactivity was also carried out in Hefei Science Island.The measured OH reactivity mean maxim?m value of about 40 s-1 in the morning in this urban,and exhibited a significant diurnal profile.The OH reactivity dominated is mainly affected by anthropogenic pollutants,such as carbon monoxide and nitrogen oxides,and the alkene emitted by plants.The comparison and analysis between the measurement and the calculated OH reactivity will be further strengthened.