Total Ozone Reactivity: A new measurement of volatile organic compounds in the atmosphere

气态碳氢化合物——挥发性有机化合物（VOCs）——是关键的大气成分。它们可能是空气污染物，本身就对人体健康有害，有些是温室气体。挥发性有机化合物的大气化学处理导致臭氧和二次有机气溶胶等二次污染物的形成，从而对健康产生不利影响，破坏植被（使全球作物产量减少5 - 15%）并影响气候。对大气VOC预算的定量理解是大气科学许多方面的基础。然而，量化挥发性有机化合物预算是一个具有挑战性的目标，因为大气中排放的挥发性有机化合物非常多，每一种挥发性有机化合物都会产生一系列降解产物——从较大的挥发性有机化合物中产生的单个化学物质的数量超过10^5个数量级。生物源性挥发性有机化合物（BVOCs）尤其如此，它们往往更大，化学分子更复杂，在全球非甲烷挥发性有机化合物排放中占主导地位。测量单个物种的传统方法很快就遇到了化学复杂性的障碍，无法评估VOC的总预算——因此，我们无法完全量化VOC氧化形成二次污染物的总潜力。另一种方法是测量存在的所有挥发性有机化合物的综合特性，例如它们的化学反应性，即给定的大气氧化剂与存在的所有挥发性有机化合物发生反应的速率。这决定了所有挥发性有机化合物（包括已识别的和未测量的）的反应电位，提供了与二次污染物形成直接相关的度量。这种方法已经成功地对OH自由基进行了试验，并且对OH反应性的测量表明，试图用传统方法测量每个单独的物种可能会低估高达90%的VOC预算。虽然羟基自由基在白天主导着许多挥发性有机化合物的氧化，但对于烯烃物种（如大多数生物源性挥发性有机化合物），与臭氧的反应也很重要——在夜间占主导地位，对于较大的BVOCs，单萜和倍半萜，它们在白天与羟基一样重要，这是最具挑战性的测量方法。因此，对臭氧总反应性的测量有可能提供新的见解，了解大气中存在的反应性BVOCs的总预算，以及目前被严重低估的程度——这一假设越来越受到最近一系列测量结果的支持。在这个项目中，我们将在实验室进行可行性研究的基础上，开发一个臭氧反应性仪器的原型；我们将以个别挥发性有机化合物标准和实验室围栏内植物样本的复杂挥发性有机化合物混合物测试系统的性能，并将证明其适用性，以评估在环境压力下整棵树的挥发性有机化合物排放量的变化。后一个目标将通过在新南威尔士州里士满的霍克斯伯里森林实验（HFE）站点的国际独特的整棵树室进行测量来实现，在那里，我们将测量桉树的总臭氧反应性变化，作为RH、温度和二氧化碳丰度变化的函数（400ppm[即现在]vs 640ppm）。在本项目期间，只能进行有限的实验，但这些实验将提供一个独特的视角，了解植被BVOC总排放量对环境变化的响应，为今后利用该方法奠定基础。项目的完成将达到技术准备水平（TRL） 4 -在受控环境中的基本验证。在这项概念验证工作之后（即在本提案之外），我们已经确定了初步现场部署该技术的机会，在当前和预期的未来二氧化碳水平条件下，从成熟的橡树林地进行环境空气中总BVOC臭氧反应性的首次测量。

Gaseous hydrocarbons - volatile organic compounds (VOCs) - are key atmospheric components. They may be air pollutants, harmful to human health in their own right, and some are greenhouse gases. Atmospheric chemical processing of VOCs leads to the formation of secondary pollutants such as ozone and secondary organic aerosol - which adversely affect health, damage vegetation (reducing crop yields by 5 - 15% globally) and affect climate. A quantitative understanding the atmospheric VOC budget underpins many aspects of atmospheric science.However, quantifying the VOC budget is a challenging goal, as very many atmospheric VOCs are emitted, each of which produces a cascade of degradation products - numbering over order-of-10^5 individual chemical species from larger VOCs. This is particularly the case for biogenic VOCs (BVOCs) which tend to be larger, more chemically complex molecules, and which dominate non-methane VOC emissions globally. Traditional approaches, in which individual species are measured, quickly run up against this barrier of chemical complexity and cannot assess the total VOC budget - consequently, we are unable to fully quantify the total potential for secondary pollutant formation from VOC oxidation.An alternative approach is to measure an integrated property of all VOCs present - such as their chemical reactivity, the rate at which a given atmospheric oxidant reacts with all VOCs present. This determines the reactive potential of all VOCs - both those identified and those unmeasured - providing a metric directly related to secondary pollutant formation. This approach has been successfully trialled for OH radicals, and measures of the OH reactivity have shown that attempting to measure each individual species by conventional approaches may underestimate the VOC budget by up to 90%. While OH radicals dominate oxidation of many VOCs during the day, for alkene species (such as the majority of biogenic VOCs) reaction with ozone is also important - dominant at night, and as important as OH during the day for the larger BVOCs, mono- and sesquiterpenes, which are the most challenging to measure with conventional approaches. Therefore, measurement of the total ozone reactivity has potential to provide new insight into the total budget of reactive BVOCs present in the atmosphere, and the extent to which it is currently substantially underestimated - a hypothesis attracting growing support from a range of recent measurements.Within this project, we will develop a prototype ozone reactivity instrument, building upon a feasibility study carried out in our laboratory; we will test the system performance with individual VOC standards, and with complex VOC mixtures from plant specimens in laboratory enclosures, and we will demonstrate its applicability to assess the change in BVOC emissions from whole trees in response to environmental stress. This latter objective will be achieved through measurements at the internationally unique whole tree chambers at the Hawkesbury Forest Experiment (HFE) site in Richmond, NSW, where we will measure changes in total ozone reactivity from eucalyptus trees as a function of changing RH, temperature and CO2 abundance (400 ppm [i.e. present day] vs 640 ppm). Within the duration of this project, only limited experiments may be undertaken - but these will provide a unique insight into the response of total BVOC emissions from vegetation to environmental change, underpinning future exploitation of the approach.Completion of the project will achieve technology readiness level (TRL) 4 - basic validation in a controlled environment. Following this proof-of-concept work (i.e. outside this proposal), we have identified an opportunity for initial field deployment of the technique, to perform the first measurements of total BVOC ozone reactivity in ambient air, from a mature Oak woodland under conditions of present day and anticipated future CO2 levels.