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

我们设计并建造了用于测量环境OH自由基浓度的真空室和真空系统。为了消除O_3化学干扰，采用低压激光诱导荧光技术检测OH自由基。建立了以二极管激光器泵浦的脉冲Nd：YAG激光器的二次谐波为泵浦源的可调谐染料激光器的激光系统。染料激光器的输出被加倍，以激发OH自由基到第一双重激发态的单振子能级（A^2 SIGMA ^+ v“=0）。用透镜系统采集OH自由基的LIF信号，经308 nm干涉滤波后，用MCP选通光电倍增管检测。采用光子计数技术提高信噪比。为了确定OH自由基的绝对浓度，采用两种技术对LIF仪器进行校准。一种是采用化学滴定法控制自由基浓度，采用反应H + NO_2*OH + NO。利用该反应，并假设该反应足够快且OH自由基的化学损失可忽略不计，控制OH自由基的浓度。采用的另一种方法是光吸收技术。多程吸收（基本路径=1米; 100倍）和水蒸气的真空紫外光解被用来校准仪器。根据这些独立的技术，仪器的灵敏度被确定为5 * 10^ /cps mW ^<-9>/（OH cm ^）和9 * 10^<-9>/cps mW^<-1>/（OH cm^<-3>）。这种良好的一致性表明我们的系统具有足够的精度。在当前条件下，检测限（LOD）估计为7 * 10^8个自由基cm^2<-3>。结果表明，该仪器具有足够的测量环境HO_2自由基的电位。

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.