Quantitative Analysis of Atmospheric OH by Active Titration

在过去的十几年中，大气科学家已经确定，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)被紫外线分解成一个3能氧原子，然后与一个水分子反应时，就会产生羟基自由基（一对）。据信，羟基自由基是将许多不溶于水的还原性化合物（如甲烷、二氧化硫、一氧化碳、二氧化氮、氨等）转化为可冷凝和/或水溶性氧化物质的主要物质。正是氢氧根的氧化作用使得这些化合物通过沉淀或小的固体和液体颗粒的逐渐沉淀从大气中被清除。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发展起来的。迄今为止，还不可能评价对流层光化学循环理论的完整性。为了验证这一理论，需要直接测量大气中氢氧根的含量，以及它与那些被认为对氢氧根产生和破坏负有责任的化合物之间的关系。在过去的几年里，探测周围大气中的羟基自由基一直是大气化学家的主要目标。测量这一物种的困难是由于它在大气中的浓度极低。理论预测氢氧根的浓度比它的母体化合物臭氧（其本身在对流层的浓度不到千万分之一）小100万到1亿倍。目前可用的测量氢氧根的技术还不够灵敏，无法在大气中进行必要的测量，以研究这一重要物种的化学性质。在这个项目中，将尝试使用活性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的反应在一两天的时间尺度上被破坏；分散剂气体本质上是惰性的，用于__________确定混合物释放到大气中的分散速率。滴定剂和分散剂气体浓度之比的时间变化率可以与局部OH浓度有关。用电子俘获气相色谱仪测量几种用作滴定剂的不饱和卤代烃和某些用作分散剂的饱和卤代烃及其浓度。

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.