深圳市秋季大气OH自由基总反应性测量及其构成分析

Measurement and partition analysis of atmospheric OH reactivity in autumn in Shenzhen

羟基自由基(·OH)总反应性(kOH)是大气中所有·OH反应物的浓度与其·OH反应速率常数乘积的总和,对kOH的直接测量有助于识别未知的·OH反应物种及提升·OH收支分析的准确度.因此,本研究建立了一套基于激光光解-激光诱导荧光技术的kOH在线测量系统(LP-LIF),利用紫外脉冲激光在流动管内光解臭氧产生·OH,采用激光诱导荧光技术实时测量其与采样进入流动管的活性气体反应而导致的·OH浓度衰减,通过对该衰减进行指数拟合得到采样气的kOH.经实验室测试,LP-LIF系统对kOH的测量灵敏度为1.2 s^-1,时间分辨率5 min.应用该系统对2018年秋季深圳地区的大气kOH进行为期1个月的连续测量,结合同步观测的·OH反应物浓度数据发现,kOH观测值在10~30 s^-1之间,主要来自一氧化碳(14%)、氮氧化物(26%)和一次排放的挥发性有机物(24%).此外,由未测量的·OH反应物贡献的kOH平均约23%,且在夜间和早晚高峰时段贡献较高,推测其主要来自溶剂涂料、石化工业及LPG机动车排放.

The reactivity of hydroxy radical(·OH), namely kOH, is the sum of products between the reactant concentrations and their reaction rate constants with OH for all ·OH reactants in the atmosphere. Direct measurements on kOH is helpful for identifying unknown ·OH reactant and for improving the accuracy of ·OH budget analysis. In this study, we developed an online kOH measurement system(LP-LIF) based on laser flash photolysis-laser induced fluorescence technique. The system uses a pulsed UV laser photolyzing ozone to generate ·OH radical in a flow tube. By using the laser induced fluorescence technique to record the ·OH concentration decay which is caused by the reaction of ·OH with the sampled reactive gases in the flow tube, and by fitting the decay curve with an exponential function, the kOH of the sample gas can be derived. The measurement sensitivity at 5 min time resolution is 1.2 s^-1, determined from laboratory test experiments. The LP-LIF system was applied in a 1-month field observation campaign in Shenzhen in autumn 2018. Simultaneous measurements were performed for ·OH reactants including carbon monoxide(CO), nitrogen oxides(NOx), volatile organic compounds(VOCs), etc.. The observed kOH by LP-LIF is between 10 to 30 s^-1, mainly coming from CO(14%), NOx(26%), and primarily emitted VOCs(24%). Moreover, it is found that on average around 23% kOH is from unmeasured ·OH reactants. Since the contribution becomes higher during night and during morning and evening rush hours, we speculate that the missing kOH is originated from emissions of solvents, paints, petrochemical industry, and LPG vehicles.