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（VOC）-是大气的关键成分。它们可能是空气污染物，本身对人类健康有害，有些是温室气体。挥发性有机化合物的大气化学处理导致二次污染物的形成，如臭氧和二次有机气溶胶-这对健康产生不利影响，破坏植被（全球作物产量减少5 - 15%）并影响气候。对大气中挥发性有机化合物收支的定量理解是大气科学的许多方面的基础。然而，对挥发性有机化合物收支进行定量是一个具有挑战性的目标，因为大量的大气挥发性有机化合物被排放，每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超过10^5个单独的化学物种。对于生物挥发性有机化合物（BVOC）来说，情况尤其如此，BVOC往往是更大，化学上更复杂的分子，并且在全球范围内主导着非甲烷VOC排放。传统的方法是测量单个物质，但很快就会遇到化学复杂性的障碍，无法评估总的VOC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。另一种方法是测量所有VOC的综合性质-例如它们的化学反应性，给定的大气氧化剂与所有存在的VOC反应的速率。这决定了所有挥发性有机化合物的反应潜力--包括那些已确定的和那些未测量的--提供了一个与二次污染物形成直接相关的指标。这种方法已经成功地用于OH自由基，OH反应性的测量表明，试图通过传统方法测量每个单独的物质可能会低估VOC预算高达90%。虽然OH自由基在白天主导许多VOC的氧化，但对于烯烃物质（例如大多数生物源VOC），与臭氧的反应也很重要-在夜间占主导地位，并且对于较大的BVOC，单萜烯和倍半萜烯，其在白天与OH一样重要，这是用常规方法测量最具挑战性的。因此，测量臭氧总反应性有可能为了解大气中存在的活性BVOCs的总预算以及目前被严重低估的程度提供新的见解-这一假设吸引了越来越多的支持，最近的一系列测量结果。我们会以个别挥发性有机化合物标准，以及实验室内植物样本的复杂挥发性有机化合物混合物，测试系统的表现，并会证明其适用性，以评估整棵树木因应环境压力而产生的BVOC排放量的变化。后一个目标将通过在位于新南威尔士州里士满的霍克斯伯里森林实验（HFE）场地的国际独特的整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随相对湿度、温度和CO2丰度变化的变化（400 ppm [即目前]与640 ppm）。在本项目实施期间，只能进行有限的实验，但这些实验将为了解植物总BVOC排放量对环境变化的响应提供独特的见解，为该方法的未来开发奠定基础。项目完成后，将达到技术准备水平（TRL）4 -受控环境中的基本验证。在这一概念验证工作（即本提案之外）之后，我们确定了该技术初步现场部署的机会，以在当前条件下和预期的未来CO2水平下，从成熟的橡树林中首次测量环境空气中的总BVOC臭氧反应性。

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.