Quantitative Analysis of Atmospheric OH by Active Titration

在过去的十几年里，大气科学家们已经确定，羟基自由基(OH)显然在控制低层大气(对流层)的微量化学成分方面起着关键作用。当臭氧分子(O)被紫外光分解形成3个高能氧原子，然后与水分子反应时，就会产生羟基自由基(一对)。人们认为，羟基是主要的物种，负责将许多不溶于水的还原化合物(如甲烷、二氧化硫、一氧化碳、二氧化氮、氨等)转化为可凝聚和/或可溶于水的氧化物种。正是这种羟基的氧化，使得这些化合物可以通过沉淀或小的固体和液体颗粒的逐渐沉淀从大气中清除。我们对大气光化学氧化过程的了解主要是通过实验室和计算机模拟研究来发展的。迄今为止，还不可能评估对流层光化学循环理论的完整性。为了验证这一理论，需要直接在大气中测量OH及其与被认为对其产生和销毁负责的化合物的关系。近几年来，大气中羟基自由基的检测一直是大气化学家的主要目标。测量这种物种的困难是因为它在大气中的浓度非常低。理论预测，OH的浓度将比其母化合物臭氧小100万到1亿倍(臭氧本身的对流层浓度不到千万分之一)。目前可用的测量羟基的技术不够灵敏，无法在大气中进行必要的测量来研究这种重要物种的化学。在本项目中，将尝试使用主动滴定技术对对流层边界层羟基自由基浓度进行实验测定。该方法包括释放两类气体的混合物：滴定气体，主要通过与OH的反应在一到两天的时间尺度上被破坏；而分散气体本质上是惰性的，用来_，一旦混合物释放到大气中，就决定了它的扩散速度。滴定剂和分散剂气体浓度之比的时间变化率可与局部羟基浓度有关。作为滴定剂的几种不饱和卤代烃和作为分散剂的某些饱和卤代烃将被测量，并用电子捕获色谱仪测量它们的浓度。

In the past dozen years atmospheric scientists have determined that the hydroxyl radical (OH) apparently plays a pivotal role in controlling the trace chemical composition of the lower atmosphere (troposphere). Hydroxyl radicals (a pair) are produced when an ozone molecule (O ) is decomposed by ultraviolet light to form an 3 energetic oxygen atom which then reacts with a water molecule. It is believed that the hydroxyl radical is the primary species responsible for converting a number of water insoluble reduced chemical compounds (e.g. methane, sulfur dioxide, carbon monoxide, nitrogen dioxide, ammonia and others) into condensable and/or water soluble oxidized species. It is this oxidation by OH which permits these compounds to be cleansed from the atmosphere by precipitation or the gradual settling of small solid and liquid particles. Our understanding of the atmospheric photochemical oxidation process has been developed primarily by laboratory and computer modeling studies. To date it has not been possible to evaluate the completeness of the theory of tropospheric photochemical cycles. To test this theory requires making direct atmospheric measurements of OH and its relationships to the compounds believed to be responsible for its creation and destruction. The detection of the hydroxyl radical in the ambient atmosphere has been a primary objective of atmospheric chemists for the past several years. The difficulty in measuring this species results from its exceedingly low concentration in the atmosphere. Theory predicts OH to be in concentrations one million to a hundred million times smaller than ozone, its parent compound (which itself has a tropospheric concentration of less than one part in ten million). The techniques presently available for measuring OH are not sufficiently sensitive to make the necessary measuremetns in the atmosphere to study the chemistry of this important species. In this project an experimental determination of tropospheric boundary layer hydroxyl radical concentrations using an active titration technique will be attempted. The method involves release of a mixture of two classes of gases: titrant gases which are _______ destroyed predominantly by reaction with OH on a time scale of a day or two; and dispersant gases which are inert essentially and used to __________ determine the dispersion rate of the mixture once it is released into the atmosphere. The time rate of change of the ratio of the titrant and dispersant gas concentrations can be related to the local OH concentration. Several unsaturated halocarbons as titrants and certain saturated halocarbons as dispersants will be measured and their concentrations measured with an electron-capture gas chromatograph.