Development of LIF instrument for Measuring Tropospheric Hydorxyl Radicals and Verification of Photochemical Theory

我们设计并建造了真空室和真空系统，用于测量环境中的OH自由基浓度。采用低压激光诱导荧光技术从臭氧化学干扰中检测OH自由基。建立了半导体激光器泵浦的二次倍频脉冲激光泵浦可调谐染料激光器系统。将染料激光的输出倍增，将OH自由基激发成第一双重态(A^2SIGMA^+v‘’=0)的单振动能级。用透镜系统采集OH自由基的LiF信号，经308 nm干涉滤光片后，用MCP选通光电倍增管检测。采用光子计数技术提高S/氮比。为了确定OH自由基的绝对浓度，对LIF仪器进行了两种技术的校准。一种是控制自由基浓度的化学滴定方法，反应采用H+NO2*OH+NO。利用该反应，假设反应速度足够快，羟基的化学损失可以忽略不计，从而控制了羟基的浓度。另一种方法是光吸收技术。利用多通道吸收(基光程=1m；100次)和真空紫外光分解水蒸气对仪器进行了标定。根据这些独立的技术，仪器的灵敏度被确定为5×10^-9；和9*10^-9；/cps mw^-1&gt；/(OH cm^-3&gt；)。这一良好的协议表明我们的系统有足够的精度。在目前条件下，检测下限估计为7×10^8个基团cm~(-3)。结果表明，该仪器具有足够的测量环境HO2自由基的潜力。

We designed and constructed vacuum chamber and vacuum system for measuring ambient OH radical concentration. To detect OH radical free from O_3 chemical interference the Low Pressure Laser Induced Fluorescence technique was employed. The lasers system of tunable dye laser pumped by 2rd harmonic of pulsed Nd : YAG laser which was pumped by diode laser was constructed. The out-put of dye laser was doubled to excite OH radical into single ro-vibronic level of the first doublet excited state (A^2SIGMA^+v''=0). LIF singnals of OH radicals were collected by lens system and detected by MCP gated PMT after interference fillter of 308 nm. Photon counting technique was employed to improve S/N ratio. To determine the absolute concentration of OH radicals, two techniques were employed for calbration for the LIF instrument. One is chemical titration method to control radical concentrations ; the reaction H + NO_2*OH + NO was used. Using this reaction and assuming that this reaction is fast enough and chemical loss of OH radical is negligible, concentration of OH raical was controled. The other method employed was optical absorption technique. Multi-pass absorption (base path=1 m ; 100 times) and VUV photolysis of water vapor were used to calbrate the instrument. From these independent techniques, sensitivity of the instrument was determined to be 5 * 10^<-9> and 9 * 10^<-9>/cps mW^<-1>/ (OH cm^<-3>). This good agreement indicates our system has enough precision. Limit of detection (LOD) was estimated to be 7 * 10^8 radicals cm^<-3> under present condition. It is found that this instrument has enough potential for measuring ambient HO_2 radical.