Measurement of Halogen Species by Resonance Fluorescence

大气臭氧水平对人类健康和环境至关重要，原因如下：在平流层，臭氧层吸收有害的太阳紫外线辐射。在对流层中，臭氧是一种污染物，对人体健康、植被和物质有害，是一种温室气体，但也是关键大气氧化剂OH的主要来源，它启动了对大气中大多数化合物的去除，包括污染物和甲烷等温室气体。因此，了解可能影响大气中臭氧和OH丰度的过程具有相当大的社会重要性。已知含有卤素原子碘和溴的化合物在海洋环境中通过生物和物理过程释放到大气中。一旦进入气相，组成卤素原子可能通过光解作用释放，导致对流层组成的一系列影响：催化臭氧破坏，改变HOx和NOx的比率和丰度，在碘化合物的情况下，形成可能生长并导致CCN浓度的新颗粒，从而潜在地影响辐射平衡和气候。因此，了解低层大气中卤素种类的来源和分布，并量化它们对大气成分的潜在影响是很重要的。该项目的主要目的是研制一种测量低层大气中卤素（碘和溴）含量的新仪器。共振荧光技术将用于探测卤素原子；这样就可以检测到周围的I和Br原子、光可降解的含碘和含溴物质（通过宽带紫外可见光分解导致其组成的卤素原子释放而检测到）和BrO自由基（通过与添加的NO反应化学转化为Br来检测）。通过以前的NERC奖励资助，开发了用于检测碘物种的原型仪器，并于2007年夏季成功完成了现场试验部署，证明了该概念和基本测量能力的可行性。该项目是前一个奖项的延续，将把原型开发成一个实用的现场系统，提高性能并扩展对溴物种的探测能力。该项目的第二个目标是应用该系统在即将到来的东太平洋研究巡航中测量碘化合物，并在东北大西洋的一个沿海地点进行溴物种的点测量。前一种测量将评估卫星观测确定的高卤素活度地区的边界层碘活度，并将限制低层大气气相碘的总含量，从而确定可能形成新粒子的可冷凝碘种类的浓度。后一项实验将包括首次测量大气中可光性溴含量，将确定梅斯黑德溴活性的空间分布（沿海与公海）（以前仅使用平均空间异质性的长径吸收仪器进行测量），并提供全面优化仪器能力的现场测试/演示。

Atmospheric ozone levels are of critical importance to human health and the environment for a number of reasons: In the stratosphere, the ozone layer absorbs harmful solar UV radiation. In the troposphere, ozone is a pollutant, harmful to human health, vegetation and materials, and is a greenhouse gas, but is also the major source of the key atmospheric oxidant OH, which initiates the removal of most compounds emitted to the atmosphere, including pollutants and greenhouse gases such as methane. Understanding the processes which may affect the atmospheric abundance of ozone and OH is therefore of considerable societal importance. Compounds containing the halogen atoms iodine and bromine, are known to be released to the atmosphere in marine environments through biological and physical processes. Once in the gas phase, the constituent halogen atoms may be released through photolysis, leading to a number of effects upon tropospheric composition: Catalytic ozone destruction, alteration of HOx and NOx ratios and abundance, and in the case of iodine compounds the formation of new particles which may grow and contribute to CCN concentrations, hence potentially affecting radiative balance and climate. It is therefore important to understand the sources and distribution of halogen species in the lower atmosphere, and to quantify their potential effects upon atmospheric composition. The main aim of this project is to develop a new instrument for the measurement of halogen (iodine and bromine) content in the lower atmosphere. The technique of resonance fluorescence will be used to detect halogen atoms; this then permits the detection of ambient I and Br atoms, photolabile iodine- and bromine-containing species (detected following broadband UV-visible photolysis leading to the release of their constituent halogen atoms), and BrO radicals (detected through chemical conversion to Br by reaction with added NO). A prototype instrument for the detection of iodine species, funded through a previous NERC award, was developed and a trial field deployment successfully completed in summer 2007, demonstrating the viability of the concept and the basic measurement capability. This project, a continuation of the previous award, will develop the prototype into a practical field system, improving the performance and extending the detection capability to bromine species. A second aim of the project is to apply the system to measure iodine compounds on a forthcoming research cruise in the Eastern Pacific Ocean, and to perform point measurements of bromine species at a coastal site in the North-East Atlantic. The former measurements will assess the boundary layer iodine activity in an area identified by satellite observations to be of high halogen activity, and will constrain the total gas-phase iodine content of the lower atmosphere, hence determining the concentration of condensable iodine species which are potentially available to form new particles. The latter experiment, which will include the first measurements of photolabile bromine content in the atmosphere, will determine the spatial distribution (coastal vs. open ocean) of bromine activity at Mace Head (previously only measured using long-path absorption instruments which average spatial heterogeneity), and also provide a field test / demonstration of the full, optimised instrument capability.