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

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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.